Wetland-Dryland Vegetational Dynamics in the Pennsylvanian Ice Age Tropics

Abstract

Premise of research. The Late Paleozoic Ice Age was the last extensive pre-Pleistocene ice age. It includes many climate changes of different intensities, permitting examination of many and varied biotic responses. The tropical Pennsylvanian Subperiod, usually visualized as one vast wetland coal forest, in fact also was dominated, periodically, by seasonally dry vegetation that, in turn, covered most of the central and western Pangean supercontinent. Equatorial wetland and dryland biomes oscillated during single glacial-interglacial cycles. This recognition changes understanding of the Coal Age tropics; examination of their spatiotemporal patterns indicates that these vegetation types responded differently to global environmental disturbances and long-term trends and points to potentially different underlying controls on evolutionary histories of their component lineages. Methodology. This study is based on the published literature and on examination of geological exposures and fossil floras, mainly from North America but also from other parts of the world. Pivotal results. Wetlands and seasonally dry habitats were equally part of the Coal Age Pangean tropics. They dominated these landscapes at different times, under different climatic regimes. Wetland vegetation was likely forced into refugia during seasonally dry (subhumid to semiarid) parts of glacial-interglacial cycles; it reemerged/reassembled during wet (humid to perhumid) periods. Seasonally dry vegetation resided permanently in areas of western and central Pangea and in microhabitats within the Central Pangean Mountains. Both floras had lower biodiversity than modern floras in similar habitats. The wetland flora species pool was more phylogenetically disparate than any modern vegetation. In contrast, seasonally dry floras were dominated by seed plants. These biomes responded differently to acute environmental changes and chronic long-term aridification. Conclusions. From ecological or evolutionary perspectives, the present-day world is but one possibility. Lower-diversity worlds such as the late Paleozoic, with a rich spectrum of environmental variations, offer insights into relationships between organisms and environments that expand understanding of these phenomena and enlarge our sense of what is possible or probable as we look to the future.

Full text

123
Int. J. Plant Sci. 175(2):123–164. 2014.
䉷2013 by The University of Chicago. All rights reserved.
1058-5893/2014/17502-0001$15.00 DOI: 10.1086/675235
WETLAND-DRYLAND VEGETATIONAL DYNAMICS IN
THE PENNSYLVANIAN ICE AGE TROPICS
William A. DiMichele
1,
*
*Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA
Editor: Patrick S. Herendeen
Premise of research. The Late Paleozoic Ice Age was the last extensive pre-Pleistocene ice age. It includes
many climate changes of different intensities, permitting examination of many and varied biotic responses.
The tropical Pennsylvanian Subperiod, usually visualized as one vast wetland coal forest, in fact also was
dominated, periodically, by seasonally dry vegetation that, in turn, covered most of the central and western
Pangean supercontinent. Equatorial wetland and dryland biomes oscillated during single glacial-interglacial
cycles. This recognition changes understanding of the Coal Age tropics; examination of their spatiotemporal
patterns indicates that these vegetation types responded differently to global environmental disturbances and
long-term trends and points to potentially different underlying controls on evolutionary histories of their
component lineages.
Methodology. This study is based on the published literature and on examination of geological exposures
and fossil floras, mainly from North America but also from other parts of the world.
Pivotal results. Wetlands and seasonally dry habitats were equally part of the Coal Age Pangean tropics.
They dominated these landscapes at different times, under different climatic regimes. Wetland vegetation was
likely forced into refugia during seasonally dry (subhumid to semiarid) parts of glacial-interglacial cycles; it
reemerged/reassembled during wet (humid to perhumid) periods. Seasonally dry vegetation resided permanently
in areas of western and central Pangea and in microhabitats within the Central Pangean Mountains. Both
floras had lower biodiversity than modern floras in similar habitats. The wetland flora species pool was more
phylogenetically disparate than any modern vegetation. In contrast, seasonally dry floras were dominated by
seed plants. These biomes responded differently to acute environmental changes and chronic long-term
aridification.
Conclusions. From ecological or evolutionary perspectives, the present-day world is but one possibility.
Lower-diversity worlds such as the late Paleozoic, with a rich spectrum of environmental variations, offer
insights into relationships between organisms and environments that expand understanding of these phenomena
and enlarge our sense of what is possible or probable as we look to the future.
Keywords: Pennsylvanian, tropics, upland, Coal Age, paleoecology, climate, biodiversity.
Introduction
The Pennsylvanian Subperiod of the Carboniferous is, per-
haps, best known colloquially as the Coal Age and envisioned
as a time when the tropical land surface was covered in vast
forested wetlands, many sitting on meters-deep peat deposits.
Thanks to more than 250 yr of collecting in coal mines and
from natural exposures, plant fossils from this time period
populate the drawers of nearly every major museum in Europe
and North America, many private collections, and displays in
schools and private homes. No image of the deep past may be
more iconic in the public mind than that of a dark, forbidding
forest filled with strange plants, primitive amphibians, spiders
1
E-mail: dimichel@si.edu.
Manuscript received October 2013; revised manuscript received December2013;
electronically published December 31, 2013.
the size of dinner plates and dragonflies with meter-wide wing-
spans. So much has been collected and published about Coal
Age plants and animals that one might reasonably conclude
that specialists have reached a broad general understanding of
the organisms and the ecology of the time, vividly portrayed
in dioramas. However, this is not so.
Over the past 40 yr the foci of Pennsylvanian tropical pa-
leobotany, while continuing with traditional systematic and
evolutionary studies, have multiplied, expanding particularly
into the geological sciences. Greater appreciation of the im-
portance of the late Paleozoic as an ice age has spurred interest
in climate as a dynamic variable; of changing atmospheric
composition; and of the covariant changes in sea level, climate,
and sedimentation patterns. Paleobotanists have brought to
their work much greater appreciation of the effects of large-
scale, long-term geological factors, such as mountain building
and changes in continental positions through time. Further-
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124 INTERNATIONAL JOURNAL OF PLANT SCIENCES
more, biological understanding has come to include much
more about physiology and development, changing and enliv-
ening the ways in which we understand the plants themselves.
There is insufficient space in this review to go into depth in
all these areas. Nonetheless, several basic patterns are emerging
that illustrate our vastly changed appreciation of the dynamics
of this time period as well as the basic ecology of different
kinds of Pennsylvanian-age tropical vegetation.
1. The Pennsylvanian was a time of dozens of glacial ad-
vances and retreats, leading to tens of millions of years of
climate and sea level oscillations. There is a hierarchy of such
oscillations, beginning with 10
5
-yr-duration glacial-interglacial
cycles superimposed on more widely spaced 10
6
-yr periodic
intervals of intense global warming and nearly complete global
deglaciation, with these then further superimposed on a long-
term multimillion-year trend of tropical aridification.
2. Polar and high-elevation 10
5
-yr cyclic changes in ice vol-
ume (glacial-interglacial cycles) were accompanied by changes
in climate, sea level, and sedimentation that were strongly ex-
pressed in the tropics. In great contrast to what is understood,
or at least broadly promulgated, about the tropics of our mod-
ern ice age, the wettest times in the Pennsylvanian tropics seem
to have corresponded to glacial maxima and the driest times
to interglacial periods.
3. The concept of an invariably ever-wet tropical realm is
incorrect. Reflecting the strong climatic oscillations on glacial-
interglacial timescales of 10
5
yr, vast areas of the Pennsylvanian
tropics alternately harbored wetland and dryland floras. The
preservational potential of these two kinds of vegetation was
vastly different; strong selection against the preservation of
dryland vegetation makes its sporadic occurrences appear
anomalous. This apparent peculiarity of seasonally dry vege-
tation in coal-bearing strata is belied, however, by increasing
appreciation of paleosols in and sequence stratigraphic archi-
tectures of these strata.
4. Vegetation characteristic of seasonally dry climates may
have dominated the lowland tropics for as long as or longer
than wetlands during any given 10
5
-yr glacial-interglacial cy-
cle. From a biogeographic viewpoint, resident populations of
plants that preferred seasonal aridity appear to have lived per-
manently in the western and central regions of the Pangean
tropics and possibly within microhabitats in parts of the Cen-
tral Pangean Mountains. The evolutionary and ecological im-
plications of these dynamics and the associated patterns are
profound.
5. The densest of tropical vegetation, from the wettest time
periods, may have been much more open than previously en-
visioned, produced much less moisture feedback to the atmo-
sphere through evapotranspiration than modern tropical for-
ests, and had patterns of tree growth and turnover unlike any
we know today. In this sense, there may have been no true
tropical “rain forests” during the Pennsylvanian, at least not
as we understand them and their water cycle in the modern
flowering-plant-dominated world.
6. The biodiversity of the Pennsylvanian, and the late Pa-
leozoic in general, was much lower than that of today in all
habitat types. It might be expected, therefore, that certain as-
pects of ecosystem structure and function were simpler than
or different from today and that many species may have en-
compassed broader resource spectra than typical of most ex-
tant plants.
All told, this is a world about which a great deal is known,
perhaps more than any other period in deep-time, terrestrial
earth history. Though much remains to be learned, wedo know
that this time has many parallels with our modern world. The
physical earth dynamics are quite similar to those of today.
Vegetational responses to those environmental cycles provide
us with insights into the behavior of biological systems under
the kinds of background conditions and likely disruptive in-
fluences faced at the present time. Like few other times in the
past, the Late Paleozoic Ice Age (LPIA) is a part of deep-time
history to which we can turn if we want to calibrate, test, or
more fully develop models of how the glacial-interglacial earth
and its biota function.
Environmental Framework of the
Pennsylvanian Glacial World
The Late Paleozoic Ice Age
During the late Paleozoic, the terrestrial surface of the Earth
was largely aggregated into the megacontinent of Pangea (fig.
1). A string of eastern microcontinents and islands circum-
scribed an eastern ocean, Tethys, and surrounding Pangea was
the vast stretch of the Panthalassic Ocean. The Pangean land-
mass is subdivided into four regions based on both latitude
and paleogeography. The north and south temperate latitudes
fall within Angara and Gondwana, respectively. The eastern
tropical microcontinents comprise Cathaysia. The principal
tropical continental landmass is Euramerica, encompassing es-
sentially what today are the northernmost parts of South
America and Africa, most of North America, and Europe. Due
to the way in which the continents were sutured together, a
significant mountain range, the Central Pangean Mountains,
ran roughly NE-SW through the center of Euramerica. Though
reconstructions show this mountain range as synchronously
present throughout its length, it appears, rather, to have been
uplifted and eroded earlier in its eastern regions than in the
west (Krohe 1996; Hatcher 2002; Hnat et al. 2012). It is sub-
divided into several subregions; here only the Variscans and
Appalachians are referred to.
The LPIA comprises the most significant set of boundary
conditions for the physical and biotic dynamics and patterns
of the Pennsylvanian and Early Permian (Fielding et al. 2008).
The earliest development of major volumes of ice took place
at the end of the Devonian and continued into the earliest
Mississippian (Brezinski et al. 2008; Isaacson et al. 2008). This
was followed by an interval of intense drying and warming at
tropical and paratropical latitudes (Cecil 2004). Ice reappeared
in the middle Mississippian and continued into the Middle
Permian (Montan˜ ez et al. 2007; Fielding et al. 2008; Bishop
et al. 2009; Isbell et al. 2012; Montan˜ ez and Poulsen 2013),
a period of as much as 65 million years in duration, during
which time moisture returned to the equatorial region. Figure
2 outlines the geological stratigraphic terms used in this article.
The LPIA is complex, which is no surprise, given its long
history. Recent studies indicate that LPIA ice was not organized
into a massive south-polar ice cap but consisted instead of
numerous ice centers (Isbell et al. 2003a, 2003b; Fielding et
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DIMICHELE—DYNAMICS OF COAL AGE TROPICAL VEGETATION 125
Fig. 1 Pennsylvanian world and major geographic features. Note that current research does not support the existence of a large Southern
Hemisphere ice sheet but rather a number of smaller ice centers. Base map courtesy of Ron Blakey, Northern Arizona University.
Fig. 2 Stratigraphic names used in the Pennsylvanian and tem-
porally adjacent geological intervals. For details see Davydov et al.
(2012). Relative abundance of arborescent lycopsids (green) and mar-
attialean tree ferns (yellow) in wetlands and cordaitaleans (brown)and
conifers (red) in seasonally dry environments. Chesterian-Virgilian p
names used in North American stratigraphy. Namurian-Autunian p
European names used for terrestrial strata. 1 pWestphalian-Ste-
phanian as defined by some stratigraphers. 2 pTraditional Westpha-
lian-Stephanian boundary as defined by the 1935 Herleen Congress
(see Falcon-Lang et al. 2012). It is essential, when reading the liter-
ature, to understand which of these boundaries is being used. Serpu-
kovhian-Asselian pinternational stratigraphic units.
al. 2008). The LPIA also was marked by considerable fluc-
tuations in ice volume and the location and number of ice
centers, which have been documented most extensively in the
Southern Hemisphere (Montan˜ ez and Poulsen 2013 provide a
summary and integration of current knowledge of the LPIA,
including extensive literature citations). In the tropics, these
ice volume changes are reflected in a hierarchy of global sea
level fluctuations of different magnitudes, from glacial-inter-
glacial cycles, on 10
5
timescales (Heckel 2008; Horton et al.
2012), to longer-term trends during which glacial and non-
glacial phases alternated on timescales of 10
6
yr (Birgenheier
et al. 2009), reflecting periods of more extensive global warm-
ing and cooling (Haq and Schutter 2008; Heckel 2008; Rygel
et al. 2008). These patterns are further superimposed on a
long-term trend of warming and aridification in the tropics
(for summary see Tabor and Poulsen 2008). The more distant
driver of these changes, on all scales, appears to have been
atmospheric CO
2
concentration (Peyser and Poulsen 2008;
Horton et al. 2012; Montan˜ ez and Poulsen 2013), which was,
on average, at one of the lowest concentrations of the Phan-
erozoic (Berner and Kothavala 2001; Royer et al. 2007).
Tropical Cyclic Sedimentation: Background
The Pennsylvanian portion of the LPIA left a distinctive
lithological signature on the continental shelves of tectonically
active, equatorial depositional basins (figs. 3, 4). Described as
cyclothems for the repeated cyclic pattern of different lithol-
ogies, representing different kinds of original depositional en-
vironments (for historical summaries see Langenheim and Nel-
son 1992; Archer 2009), these signature deposits were first
recognized in the Illinois Basin (Udden 1912), described in
detail by Wanless and Weller (1932) and attributed to polar
glaciation by Wanless and Shepard (1936). A cyclothem is a
record of covarying changes in sea level, climate, and sedi-
mentation patterns (Cecil et al. 1985, 2003b; Tandon and Gib-
ling 1994; Heckel 1990, 2008; Cecil and Dulong 2003), rep-
resenting cycles of approximately Milankovich durations of
100,000 and 400,000 yr (Heckel 1986; Pointon et al. 2012).
Recent studies have demonstrated that specific cyclothems, rep-
resenting individual glacial-interglacial cycles, can be corre-
lated across much of the expanse of the Euramerican equatorial
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126 INTERNATIONAL JOURNAL OF PLANT SCIENCES
Fig. 3 Cyclothem as developed on the interior Pangean continental
craton. A, Terrestrial and marine strata, with transitions between. Cli-
mate covaries with sea level (SL) change, both reflecting changes in
ice volume. Vegetation reflects the prevailing climate. Dry-climate flo-
ras predominate from late sea level highstand to early to middle low-
stand. Wetland floras predominate from middle to late lowstand into
the early phases of transgression. B, Relationship between sea level,
sedimentation, position of the terrestrial water table, and distribution
of vegetation types. Note that sea level rise is marked by repeated
meltwater pulses. Diagram modified from Rosenau et al. (2013b), with
permission of the Society for Sedimentary Geology.
Fig. 4 Cyclothems in the field. Both late Middle Pennsylvanian
(Desmoinesian) age. A, Springfield coal and associated rocks, Indiana.
B, Croweburg coal and associated rocks, Kansas. Note in Bthe pres-
ence of a gray shale tidal sediments, part of an areally restricted wedge
present only locally over the coal, in areas proximate to contempo-
raneous river channels, following conversion to estuaries during sea
level rise. Bcourtesy of Blaine Cecil.
portion of Pangea based on dating with marine fossils, par-
ticularly condonts and fusilinids, and on radiometric methods
(Heckel et al. 2007; Eros et al. 2012). Specific individual cy-
clothems have been traced from coal basins across large por-
tions of the Euramerican continent into western Pangea, in-
cluding paratropical regions (Ritter et al. 2002; Cecil et al.
2003b; Greb et al. 2003; Bishop et al. 2010). The ability to
identify these cycles over a wide geographic expanse is an
expectation realized, given that the controls are thought to
represent global events.
Spatial variation in cyclothem expression. Classic mixed
terrestrial and marine cyclothems (fig. 4) developed in North
America mainly in low-gradient, flat cratonic settings where
epicontinental seas alternated with coastal terrestrial fluvial-
deltaic sedimentation (Watney et al. 1989). One or the other
of these phases can predominate, depending on distance from
the continental shelf. As Heckel (2008) has noted, cyclothems
reflecting baseline 100,000-yr glacial-interglacial cycles are
well represented in the American Midcontinent. Farther east,
into the Eastern Interior (Illinois) and Appalachian Basins, ma-
rine incursions from the west were progressively less common,
and both the number of cycles and the extent of marine influ-
ence diminished, resulting in fewer recognizably distinct
100,000-yr cycles. Larger 400,000-yr megacycles, into which
the 100,000-yr cycles are grouped, are nearly always recog-
nizable throughout the entire region (Heckel 2008; Belt et al.
2011; Falcon-Lang et al. 2011a).
Yet farther to the east, in central Pangea, the nature of gla-
cial-interglacial cyclicity, on the finest scales, becomes increas-
ingly difficult to recognize because of the complexity of the
landscape in and around the Variscan portion of the Central
Pangean Mountains and the overprint of syndepositional tec-
tonics in some portions (Stollhofen et al. 1999; Oplusˇtil 2005;
Oplusˇ til and Cleal 2007; Cleal et al. 2011). Even so, changes
in climate and sedimentation patterns can be recognized to
varying degrees in these areas and related to glacial-interglacial
controls at various spatiotemporal scales (Oyarzun et al. 1999;
Stollhoffen et al. 1999; Roscher and Schneider 2006; Bertier
et al. 2008; Gastaldo et al. 2009; van Hoof et al. 2012; Oplusˇtil
et al. 2013b). Cycles of 10
5
yr are again recognizable in the
Donets Basin of the Ukraine, in the eastern part of central
Pangea, and can be correlated with those of the Midcontinent
of North America (Eros et al. 2012).
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DIMICHELE—DYNAMICS OF COAL AGE TROPICAL VEGETATION 127
Drivers of cyclic sedimentation patterns. Although it is
generally assumed that the proximate driver of cyclothem de-
position was changing sea level, issues have been raised about
the degree of sea level fluctuation that could be caused by the
melting of continental ice in the Southern Hemisphere (Isbell
et al. 2003b). Models of sea level fluctuation that call for 70–
100 m of change within the average cyclothem, developed
partly to account for the genesis of black shales (Heckel 1977,
1991b), have been challenged for many years (Zangerl and
Richardson 1963; Coveney et al. 1991; Reischenbacher et al.
2013), and actual maximum water depths reached during in-
terglacials may have been closer to 30 m (Isbell et al. 2003b).
Further discussion of the origin of marine black shales is out-
side the scope of this review.
There is a considerable amount of discussion about the re-
lationship between climate cycles and sea level cycles, as ex-
pressed in equatorial cyclothems. These have been studied re-
cently through the lens of sequence stratigraphy (Gastaldo et
al. 1993; Miller and West 1998; Catuneanu et al. 2009; Meyers
and Milton 2011; Patzkowsky and Holland 2012), which, to
paraphrase Catuneanu et al. (2009), is a method of describing
different rock types, representing specific environments of de-
position (facies), in terms of their stacking patterns in time and
through space. These patterns will reflect changes in sea level,
climate, and sedimentation, within the framework of the tec-
tonic structure of a given depositional basin. It is uncontro-
versial to say that a mixed terrestrial-marine cyclothem rep-
resents the deposits formed during a single glacial-interglacial
cycle. It reflects directional changes in sea level, linked mainly
to fluctuation in the volume of glacial ice. If allowance is made
for sufficient tectonically created space to bury these deposits
in the long-term, so-called accommodation space, most of the
elements of such a cycle will be preserved in the stratigraphic
record. The correlation of individual cyclothems across large
geographic areas, referred to above, suggests that accommo-
dation space was produced continuously, on average, in the
major basins.
During any such individual cycle, climate also changes sys-
tematically (Cecil et al. 1985, 2003b; Tandon and Gibling
1994; Olszewski and Patzkowsky 2003; Falcon-Lang 2004;
Feldman et al. 2005; DiMichele et al. 2009a; Horton et al.
2012), strongly covarying with sea level changes. Within the
pages of the articles cited above and others, however, thereader
will find disagreement regarding the sea level position of key
climate indicators, particularly paleosols (seasonally dry) and
especially coals (wet with minimal seasonality): the principal
points of contention regarding coals are that they formed dur-
ing lowstand, during sea level rise and transgression, or at
highstand, thus covering nearly all of the possibilities.
Tropical Cyclothems and Climate Cycles
In this article, I take the position that the weight of evidence,
both empirical and from climate models, suggests that themost
continuously wet (humid to perhumid climate) parts of Middle
and Late Pennsylvanian glacial-interglacial cycles were to be
found during mid- to late lowstand, at times of maximum
glacial ice (Cecil et al. 2003b; Peyser and Poulsen 2008; Horton
et al. 2010, 2012; Montan˜ ez and Poulsen 2013; fig. 3). Why
is it important to establish this in an article ultimately focused
on the spatiotemporal distribution of wetland and seasonally
dry floras? In a description frequently attributed to the bio-
geographer Ko¨ ppen (1936), plants are crystallized, visible cli-
mate. The spatial locations and dynamics of these various flo-
ras, as will be discussed below, have been and continue to be
issues of some considerable controversy. Thus, it is crucially
important, first, to document the evidence for strongly devel-
oped climate cycles and, second, to show that the kinds of
beds in which plant fossils are found reflect those climates in
a consistent and understandable way. From reading the con-
flicting views in the literature, one might think it is chaos out
there. Instead, I would argue that these different interpretations
can be reconciled, though I will not attempt to do so here. For
consistency, I present a unitary viewpoint from the perspective
of the principle of total evidence (Good 1967), the interpre-
tation that provides the maximum fit of all the available data;
I will attempt to justify each of the assertions made.
It is important to note that coal/peat is an indicator of rain-
fall. Extended periods of rainfall can raise local water tables
to the surface quickly, on human timescales, as indicated by
many extreme rainfall events in otherwise seasonally dry cli-
mates (Konrad 2001). It is very unlikely that, as is often as-
serted, the formation of peat swamps of vast extent and low
mineral content was driven by rising sea level and coastal pa-
ludification. This is indicated strongly by the fact that there is
little or no coal/peat in the highly seasonally dry to arid regions
of western Pangea at exactly the same time such peat is present
and widespread in the west-central and central Pangean coal
basins, yet sea level rise and fall of a cyclothemic nature oc-
curred in both places (Ritter et al. 2002; Bishop et al. 2010;
Elrick and Scott 2010; Nelson et al. 2013b,2013c). Hypo-
thetically, peat can be present at any part of a sea level cycle—
cyclothem—if a mechanism can be identified to make the cli-
mate sufficiently wet at that point in space and time (110 mo
a year with rainfall exceeding evapotranspiration in the trop-
ics; Cecil and Dulong 2003). For example, most modern peat-
forming environments similar to those of the Pennsylvanian
are forming today, at or near sea level highstand, in tropical,
high-rainfall environments, such as Sumatra (Anderson 1983;
Cecil et al. 1985; Esterle and Ferm 1994). As modern ana-
logues, peats forming in coastal lagoons or along shorelines
in seasonal climates (e.g., Snuggedy Swamp of South Carolina
[Staub and Cohen 1979], Changuinola peat of Panama [Cohen
et al. 1989], Okefenokee Swamp of Georgia [Cohen 1984])
are not ideal analogues for most Pennsylvanian coals because
they are (a) too high in mineral matter, so that after diagenesis
and lithification they would yield organic-rich mudrocks rather
than coals, and (b) too limited in areal extent to be comparable
environmentally to Pennsylvanian coal beds, both in terms of
their present-day extent and accounting for the entire extent
and volume of the deposits that have formed from them over
the past 10,000 yr.
A maximum glaciation, sea level lowstand position for most
economic (low mineral matter content) coals is supported by
a number of factors: sea level dynamics across an extremely
flat depositional surface, subcoal paleosols indicative of a
change from early rainfall seasonality to nearly continuous
rainfall late in soil history, gradational contact between pa-
leosols and overlying coals (histosols), and the presence of a
marine transgression/ravinement erosional surface and sub-
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128 INTERNATIONAL JOURNAL OF PLANT SCIENCES
sequently deposited marine rocks immediately above major
coal beds. These factors are discussed in more detail below.
1. The Pennsylvanian craton was an extremely flat surface
with gradients of perhaps only centimeters or kilometers over
distances of thousands of kilometers (Watney et al. 1989).
Thus, changes in the coverage of these surfaces by seawater,
during both transgression and regression, were likely to have
been rapid over great areal extents. This militates strongly
against peat formation during transgression, particularly when
it is considered that the ice-melting phase of an interglacial is
much more rapid and pulse-like than the ice formation phase
and thus accompanied by rapid sea level rises caused by melt-
water pulses (for discussion of such effects during the Penn-
sylvanian, see Archer et al. 2014). In addition, changes in cli-
mate would have had effects over large areas due to the nearly
uninterrupted nature of the flat topography.
2. During falling stages of sea level, when cratonic exposure
would have occurred rapidly due to its flat aspect, and during
early sea level lowstand, much of the cratonic surface was
subject to pedogenesis (Joeckel 1994; Cecil et al. 2003b; Fal-
con-Lang et al. 2009). The soils (now paleosols) that formed
during these intervals (figs. 4, 5) mark surfaces of nondepos-
ition or limited deposition. They have characteristics that in-
dicate climatic seasonality of varying degrees (Joeckel 1995,
1999; Feldman et al. 2005; Catena and Hembree 2012; Ro-
senau et al. 2013a, 2013b). These empirical observations also
suggest seasonally dry climates during the falling stages of sea
level and during the early phases of lowstand (fig. 5). Climate
models independently predict strongly seasonal equatorial-
Pangean climates in these same phases of a sea level sequence
(Horton et al. 2012).
3. The paleosols formed during regression and early low-
stand show evidence of changing climatic conditions during
their formation (i.e., they are polygenetic). Almost without
exception, they vary from indicative of seasonal dryness in
their formative stages to indicative of increasing moisture input
under humid climate during their later stages. It was during
the latter phases that various clays and minerals, including
iron, were translocated down the soil profile, resulting in gley-
ing, indicating both that the soil was still well drained and
that weakly acidic conditions, resulting from organic matter
accumulation, had begun (converting low-solubility ferric to
more soluble ferrous iron; Miller and West 1993; Miller et al.
1996; Tabor and Montan˜ ez 2004; Driese and Ober 2005; Ro-
senau et al. 2013a; fig. 5). Subcoal paleosols frequently tran-
sition to coal beds, indicating that water tables rose to the
surface of the soil, most likely driven by increased rainfall,
which suppressed decay and permitted the accumulation of
organic matter (Cecil et al. 2003b; Rosenau et al. 2013a,
2013b). It is unlikely that rising sea level would cause a rise
in base level of sufficient geographic extent, i.e., across most
of the vast Pangean interior continental shelves, to initiate peat
formation simultaneously across the region (see point 4; coals
are not notably time transgressive), nor can rising sea level
account for late-stage soil gleying because it provides no mech-
anism to translocate soluble iron downprofile or out of the
soil entirely.
4. Where it has been possible to determine it, Pennsylvanian
coal beds appear to represent peat bodies that were contem-
poraneous throughout their distributions (this is true whether
they formed as one temporally unbroken swamp or as several
stacked peat bodies separated by hiatuses; Jerrett et al. 2011).
That is, they are not directionally time-transgressive deposits,
significantly older at the shelf edge and progressively younger
inland; they did not form in narrow coastal bands being
pushed continuously inland by rising sea level ahead of other
narrow bands of marine muds and limestones. This time equiv-
alence is indicated by such features as ash-fall partings in sev-
eral coal beds: the Fire Clay coal of the Southern Appalachian
Basin (Lyons et al. 1992; Greb et al. 1999b), several coals in
the mid-Pennsylvanian of the Czech Republic (Oplusˇtil et al.
2007), and a Pennsylvanian-Permian coal from the Wuda Dis-
trict in China (Pfefferkorn and Wang 2007; Wang et al. 2012).
Also indicative of widespread age synchrony are mineral part-
ings continuously recognizable throughout coal beds: the
“binder” beds, which separate petrographically and palyno-
logically distinct benches of the Pittsburgh coal of the Central
Appalachian Basin (Gresley 1894; Eble et al. 2006), or by the
blue band of the Herrin coal of the Illinois Basin, which may
carry into equivalent coal beds in the Midcontinent (Greb et
al. 2003).
5. At this point, it is important to remember that most Penn-
sylvanian coals formed as histosols, which are organic-rich
soils. Thus, they reflect very specific kinds of climatic condi-
tions (humid to perhumid, in the tropics, by analogy with
modern tropical peats; Anderson 1983; Cecil and Dulong
2003) and formed in situ. In the case of Pennsylvanian-age
economic coal beds, the parent peats were quite low in mineral
matter; otherwise, following diagenesis they would have
formed organic-rich shales rather than coals. Thus, peat
swamps either were protected from mineral matter input or
formed during times when mineral matter transport was lim-
ited. Under conditions suitable for tropical peat formation,
rainfall exceeds evapotranspiration and runoff nearly year-
round (humid to perhumid climates; Cecil and Dulong 2003).
Under such conditions, plant density on the land surface is
high throughout a drainage basin, regardless of elevation, as
is subsequent rooting, which binds sediment and limits the
entry of mineral matter into stream and river drainages, in-
cluding those beyond the limits of peat formation (Cecil et al.
1993, 2003a).
6. Major coals of middle and late Pennsylvanian age from
American coal basins are overlain by a surface formed by ma-
rine transgression, known as a ravinement surface because of
its erosional base (Demaris et al. 1983; Demko and Gastaldo
1996; Liu and Gastaldo 1992; DeMaris 2000). Immediately
above this surface may be shell hash or thin limestones of
marine origin. More generally, however, theravinement surface
is overlain by thin, marine, sheety, black shale, which is over-
lain by open marine limestone (fig. 4). Under such circum-
stances, the coal must have formed prior to transgression, with
flooding of the craton by marine water. Such coals are difficult
to place at highstand, given they are overlain by open marine
deposits, often of considerable thickness.
Recent climate models, both conceptual (Cecil et al. 2003b)
and numerical (Peyser and Poulsen 2008; Horton et al. 2010,
2012), suggest that in the tropics of Pangea, glacial maxima/
sea level lowstands were the wettest parts of glacial interglacial
cycles, varying regionally in amount. Furthermore, they sug-
gest that during interglacials there were no large, ever-wet areas
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Fig. 5 Coals (paleohistosols) and underlying paleovertisols, illustrating climatic contrasts and transitions between these two types of soil
deposits. Paleosols indicated by double-headed arrows. Coals labeled with letters. A, Cohn coal (CC), Late Pennsylvanian (Missourian), Illinois.
B, Harlan coal (HC), Late Pennsylvanian (Missourian), West Virginia. AMZ pAmes Marine Zone, the last major marine transgression of the
late Paleozoic documented in the Appalachian basin. Note two paleosols, both vertisols with calcic horizons and, in 2, calcified root casts. C,
Danville (No. 7) coal (D7C), Middle Pennsylvanian (Desmoinesian), Indiana. Paleosol shows iron staining demonstrating incomplete gleying
and clear horizonation. D, Unnamed Staunton Formation coal (USFC), Middle Pennsylvanian (Desmoinsian), Indiana. Note strong expression
of vertical slickensided surfaces in the paleosol. E, Herrin (No. 6) coal (H6C), Middle Pennsylvanian (Desmoinesian), Indiana. In this location,
the Herrin coal is very near the edge of its development and is only a few centimeters thick. It is underlain by a vertic paleosol with calcium
carbonate nodules. F, Core through paleosol below the Herrin (No. 6) coal, Indiana. Core segments are 61 cm long. Note strong gleying at the
top of the soil profile, indicative of late-phase high levels of soil moisture. This is superimposed on earlier phases of soil development that indicate
much seasonally drier climatic conditions, including nodular calcium carbonate, at ∼90-cm depth at white bracket and angular vertic surfaces
throughout the upper 2 m of the profile. Near base of profile, at arrowhead, on left, the soil is red.
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130 INTERNATIONAL JOURNAL OF PLANT SCIENCES
for coal swamp and other wetland vegetation to migrate to.
Horton et al. (2012) did find evidence of high volumes of
strongly seasonally distributed rainfall during the early parts
of interglacials and into early highstand. In combination with
highstand deltaic sedimentation (much like that found around
the world today; Stanley and Warne 1994), this may help ac-
count for possible highstand coals. Such coals are rarely of
more than very local economic thickness and quality (low ash
and sulfur) and lack the subcoal paleosols indicative of strong
climate contrasts. These features are consistent with their for-
mation in localized wet habitats or under rainfall conditions
less than ideal for peat preservation and as intrinsic elements
in coastal settings, such as highstand to regressive-phase deltas.
On longer timescales, at coarser levels of resolution, de-
creases in ice volume and increases in global sea level are cor-
related with increased seasonality of rainfall (Tabor et al. 2002;
Poulsen et al. 2007; Tabor and Poulsen 2008). These patterns
of change in climate and ice volume appear to be strongly
correlated with and probably driven by changes in atmospheric
CO
2
concentration (Montan˜ ez and Poulsen 2013). Further-
more, there is evidence that the Pennsylvanian tropics at times,
or perhaps at parts of glacial-interglacial cycles, may have been
cooler than those of the modern world (Soreghan et al. 2008;
Tabor et al. 2013a). These findings are in conformance with
the surmise of Fredericksen (1972) that Pennsylvanian wet-
lands may have been cooler than generally envisioned.
Late Paleozoic Ice Age–Quaternary Conflicts
It is important to acknowledge that there are conflicts be-
tween the interpretation presented here—that is, that it was
wettest in the equatorial regions of Pangea during glacial max-
ima through late glacial stages and driest during interglacials
through early glacial phases—and the patterns found in the
equatorial regions of the Pleistocene through Recent tropics.
This is a subject that would require considerably more space
to discuss than is possible here. Several things should be con-
sidered, however. In the present ice age, there has been a great
deal of ice in the north-polar region. There is only limited
evidence for high-latitude northern ice in the LPIA. This alone
would cause major differences in patterns of atmospheric cir-
culation (Mu¨ ller et al. 2013). Continental configurations are
quite different in the two ice ages as well: today’s dispersed,
high-aspect, continental configuration differs greatly from the
huge Pangean supercontinent, with its nearly E-W transequa-
torial mountain range and vast areas of low-elevation, flat,
interior continental shelves.
Finally, there are important differences in the plant com-
position of the two time periods that may have important
consequences for the tropical hydrological cycle. The presence
of angiosperms is the great and notable difference, and studies
by Boyce and colleagues (Boyce et al. 2009, 2010; Boyce and
Lee 2010; Lee and Boyce 2010) suggest that angiosperms can
translocate water at rates up to 10 times greater than any other
group of vascular plants. To the extent that these determina-
tions are correct, the angiosperms, as one of the above-cited
titles indicates, “put the rain in the rainforest.” The high rates
of evapotranspirative moisture released by angiosperm trees
create the high volume of recycled water that makes current
interglacial rainforests wet. The implication is that if these
forests were composed of any other groups of plants (such as
lycopsids, marattialean tree ferns, and pteridosperms), there
would be insufficient evapotranspiration to create this rainfall
feedback. Instead, late Paleozoic wetlands would have to be
made wet by primary, atmospheric-circulation-driven rainfall.
Thus, models that wish to reconstruct late Paleozoic wetlands
must not assume landscapes populated by plants with angio-
sperm-type physiologies and the associated recycling of water.
Pennsylvanian Lowland Vegetation
There were two principle species pools, or biomes, that al-
ternately dominated the Pennsylvanian lowlands, plus some
subsidiary floras that may have been edaphic specialists. The
better known of these floras was composed of plants that were
obligately tied to wet substrates and possibly to high levels of
atmospheric humidity (Scott 1977; Gastaldo 1987; Hilton and
Cleal 2007; Galtier 2008; King et al. 2011; Wagner and Castro
2011; see summaries in Eble and Grady 1990; DiMichele and
Phillips 1994). The other flora consisted of plants that were
tolerant of moisture deficits of varying degrees (Leary and Pfef-
ferkorn 1977; Galtier et al. 1992; Falcon-Lang et al. 2009;
Plotnick et al. 2009; Bashforth et al. 2014). These floras os-
cillated in dominance through time on glacial-interglacial tem-
poral scales (Falcon-Lang 2003c, 2004; Falcon-Lang and
DiMichele 2010), reflecting the climatic variations that oc-
curred in concert with glacial-interglacial cycles and their
global effects on climate, sea level, and other factors.
Each major species pool can be subdivided further spatially.
Under humid climates, for example, there is a distinct segre-
gation in the dominance patterns and compositional aspects
of floras from peat substrate swamps, mineral substrate
swamps, and terra firma mineral soils. Within any of these
species subpools, distinct, if often gradational, communities
can be recognized, reflecting the combined effects of local en-
vironmental conditions, dispersal limitation, and incumbency/
competition. Similarly, the species pool of seasonally dry cli-
mates consisted of a number of compositionally overlapping
assemblages. These differ both quantitatively and qualitatively
with regard to the dominant elements, which presumably re-
flects the duration of moisture deficits but also the effects of
various biotic factors and smaller-scale physical conditions.
In addition to spatial complexity, there are temporal patterns
of change in these distinct species pools. For the most part,
these changes, at the level of species and generic composition,
occurred gradually over time, driven by evolutionary change
(within ecological context). At times, however, the changes in
species pool composition were extensive and appear to have
occurred rapidly, at rates and to degrees that suggest large-
scale regime shifts (Phillips et al. 1974; DiMichele et al. 2009a).
Such changes are more common among wetland floras than
within the group of seasonally dry floras, for reasons discussed
below.
A note must be made here on the matter of diversity, which
will be addressed later as well. Based on modern tropical an-
alogues (Gentry 1982; Valencia et al. 1994; Condit et al. 1996,
2002, 2005; Wright 2002; Hubbell et al. 2008; Lamarre et al.
2012), habitat quality is higher in terra firma lowland envi-
ronments than in swamps (Prance 1979, 1989; Terborgh et al.
2002, fig. 1.2; Koponen et al. 2004; Lopez and Kursar 2007).
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DIMICHELE—DYNAMICS OF COAL AGE TROPICAL VEGETATION 131
Fig. 6 Reconstructions of two coal swamps. A, Middle Pennsyl-
vanian swamp dominated by lycopsids (Lepidophloios), tree ferns, and
pteridosperms, with open architecture, lacking a closed canopy.B,Late
Pennsylvanian swamp dominated by tree ferns, which form a nearly
closed canopy, and emergent lycopsid trees (Sigillaria), with pterido-
sperms in the understory. Reconstructions by Mary Parrish, Smith-
sonian Institution. Reprinted from DiMichele et al. (2009a).
That is, they have higher nutrient availability, better oxygen-
ation of soils, and lower levels of physical stress. Consequently,
terra firma settings are usually of higher diversity, often con-
siderably higher than swamps in the same region, under almost
any climate regime as wet as or wetter than semiarid. It there-
fore seems a legitimate starting hypothesis to expect terra firma
lowland humid-climate environments to have been more di-
verse than swamps during the Pennsylvanian. Studies of Penn-
sylvanian floras, however, as described below, suggest that any
such differences at that time were sufficiently muted as to be
nearly indetectable, compared to the modern (and, almost cer-
tainly, the post–Late Cretaceous) world. There appear to have
been large differences in diversity between humid-climate terra
firma floras and tropical dry floras of the Pennsylvanian. Sea-
sonally dry floras are not nearly as well known and charac-
terized as wetland assemblages (for preservational reasons),
which limits comparison to a somewhat anecdotal level. But
comparisons of local floras, made for paleoecological or strat-
igraphic studies (Gastaldo et al. 2004; Libertı´n et al. 2009;
Bashforth et al. 2010, 2011, 2014; King et al. 2011; Cleal et
al. 2012; van Hoof et al. 2012), suggest that floras of humid
climates were much richer in species and genera, and even in
the numbers of higher-taxonomic evolutionary lineages rep-
resented, than were seasonally dry floras under subhumid to
semiarid climate regimes.
Wetland Floral Composition
The Pennsylvanian tropical wetland flora is possibly the
best-known fossil flora of the pre-Quaternary. Thus, this re-
view need only be brief. Descriptions of the macroflora, both
compression-impression and anatomically preserved, can be
found in numerous publications dating to the early 1800s,
when formal paleobotanical taxonomic nomenclature was es-
tablished (von Sternberg 1820). The palynoflora is equally well
known, primarily from studies of coal and associated rocks
(Peppers 1996). There were two major species pools that com-
prised this flora, and these succeeded each other in time. From
the late Mississippian to the end of the Middle Pennsylvanian,
peat-forming swamps were heavily populated by lycopsids and
pteridosperms, with cordaitaleans as significant elements at
times. Other, mineral soil, wetlands were dominated by pteri-
dosperms and sphenopsids with lycopsids in less abundance.
Beginning in the late Middle Pennsylvanian, tree ferns began
to increase in abundance (Pfefferkorn and Thomson 1982) in
all types of wetland environments (figs. 2, 6A). Near the Mid-
dle–Late Pennsylvanian boundary, there was a major floristic
turnover (Phillips et al. 1974; Falcon-Lang et al. 2011a), with
tree ferns becoming the dominant elements in peat-forming
swamps and codominants in many other wetland settings (figs.
2, 6B). The timing of this turnover varied slightly across Eur-
america (Cleal et al. 2009; Wagner and A
´lvarez-Va´ zquez
2010), but the results, in terms of dominance-diversity pat-
terns, were the same everywhere. Many of the lineages, in-
cluding some of the lycopsids, dominant in Middle Pennsyl-
vanian assemblages, survived well into the Permian in
Cathaysia (Hilton and Cleal 2007), representing a species pool
from which dispersal back into Euramerica seemingly did not
occur. Figures 7 and 8 represent a compilation of examples of
the Pennsylvanian wetland plants from major American coal
basins. These are field photographs of specimens, primarily
from underground and surface coal mine exposures, which
hopefully will provide somewhat more context than more re-
fined photos of selected specimens.
Pennsylvanian tropical floras of all habitats were signifi-
cantly less diverse than modern tropical rainforests. A com-
parison of Cretaceous-Tertiary and Pennsylvanian-Permian pa-
leobotanical samples, drawn from single excavations of ∼1m
2
and 1/2 m deep (Wing and DiMichele 1995), revealed no sig-
nificant differences at that scale, which modern studies suggest
samples ∼0.1–0.5 ha of standing vegetation (Burnham 1993).
Most samples comprised around 10 species. However, when
transects made laterally within single beds were compared, the
younger angiosperm-rich assemblages proved to be much more
diverse than those of the Paleozoic, indicating an intrinsic dif-
ference in landscape-level diversity, reflected in global species
diversity compilations (Niklas et al. 1980). At larger spatial
scales, the most diverse Pennsylvanian flora from one instant
in time is the Mazon Creek flora, of late Middle Pennsylvanian
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132
Fig. 7 Wetland plants from the roof shales of the Springfield (No. 5) and Herrin (No. 6) coals of the Illinois Basin. All photos were taken
in-mine, in areas where the coal had been removed, exposing the overlying shales. The author apologizes for the lack of scales in some photos;
in many cases specimens were out of reach in the coal roof, precluding inclusion of a scale. A,Neuropteris flexuosa; pteridosperm foliage. B,
Neuropteris flexuosa; pteridosperm foliage. C,Laveineopteris rarinervis; pteridosperm foliage. D,Macroneuropteris scheuchzeri; pteridosperm
foliage (single pinnule of much larger compound leaf). E, Pteridosperm stem with petioles of leaves still in attachment. F,Odontopteris sp.;
pteridosperm foliage. G,Pecopteris sp.; tree fern foliage. H,Sphenophyllum emarginatum; ground cover sphenopsid. I,Calamites stems; sphe-
nopsids. Note characteristic jointing of stem. J,Asterophyllities equisetiformis; calamitealean foliage. K,Lepidodendron fallen main trunk; a
giant lycopsid tree. Scale in feet and tens of feet. L,Synchysidendron lateral branches; deciduous lateral branch axes of a large lycopsid tree,
with attached leaves. M,Lepidodendron sp.; branched axis from crown of this large lepidodendrid lycopsid. N,Lepidostrobus; cone of a large
lepidodendrid tree. O,Asolanus camptotaenia; bark of moderate-sized lepidodendrid tree.
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133
Fig. 8 Wetland trees, field photographs. All are from roof shales of Middle Pennsylvanian (Desmoinesian)–age coals. A, Large Sigillaria
stump cast above thin coaly layer. B, Exhumed Sigillaria tree stump. C, Base of identified lycopsid tree buried in the roof shale, a feature
colloquially known as a kettle bottom. D, Tree trunk of lycopsid partially exposed in roof shale by small roof fall. E, Prostrate trunk of a
marattialean tree fern. Arrows point to scars left following leaf abscission. Longitudinal striations are adventitious roots of the root mantle.
Metal roof bolt at top of photograph is 15.25 cm on a side. F, Terminal portion of a Cordaites crown branch bearing a tuft of long grasslike
Poacordaites leaves. Metal roof bolt for scale.
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134 INTERNATIONAL JOURNAL OF PLANT SCIENCES
Fig. 9 Wetland plants reconstructed. A, Two growth forms of
lepidodendrid lycopsids, plus immature stages. The four trees to the
left are growth stages of forms with a monocarpic crown (present only
in the final reproductive phases of growth), typical of the Lepido-
dendraceae (Lepidodendron or Lepidophloios). To right is a pair of
polycarpic trees; these produced cones on deciduous lateral branches,
giving them an extended period of reproduction, which begins as soon
as lateral branch production is initiated. These branches were shed
from the tree following maturation of the strobili, which must have
created a large litter field. B, Marattialean tree fern, Psaronius. Note
large fronds and thick mantle of adventitious roots, which provide
both support and water transport to the crown. C,Calamites of a
small form with limited wood development. Notable features are its
rhizomatous growth habit, leading to dense stands of plants, and its
whorled organization of appendicular organs. D, Medullosan pteri-
dosperms, reconstructed as a dense stand that gain support from lean-
ing on one another. Large seeds are borne in the fronds. E, Small,
scrambling cordaitalean of the Cordaixylon type. Reconstructions by
Mary Parrish, Smithsonian Institution.
age (Wittry 2006). This flora is drawn from an entire drainage
basin, including coastal, inland swamp, and low-elevation wet
soil habitats, and is compositionally conservative, having been
identified across a distance of 11000 km (Moore et al. 2014);
when accounting is made for taxonomic uncertainties, syn-
onymy, and multiple names for organs of the same plant, a
reasonable estimate of diversity appears to be ∼100 species
(Pfefferkorn 1979). A broader study of European basins (Cleal
et al. 2012) concluded that Pennsylvanian wetland landscapes
harbored 40–100 species at any point in space and time, reach-
ing perhaps 120 species at the regional level of 110
5
km
2
, but
that there was no significant difference in species diversity be-
tween modern flooded coastal swamps and Pennsylvanian wet-
lands in general (not just swamps). This latter finding is par-
ticularly significant. It partially reflects the strong physical
selective factors that swamp habitats present (and the Penn-
sylvanian lowlands harbored vast swamps compared to the
modern tropics), which lead to strong ecotonal boundaries
between swamps and surrounding terra firma habitats, even
if the prevailing climate is humid. Such swamp boundary eco-
tones are asymmetrical (DiMichele et al. 1987) in the sense
that wetland plants do not compete well with terra firma–
centered species, restricting the former to the semiflooded sites
they can tolerate. In contrast, the terra firma species cannot
grow under flooded conditions and so are excluded from these
habitats. Nonetheless, the Cleal et al. (2012) study includes
floras drawn from more than swamp settings, suggesting that
the Pennsylvanian humid-climate lowlands were, in general,
much lower in diversity than those of equivalent environments
in the modern world.
The dominant plant groups in wetlands comprise four dis-
tinct evolutionary lineages: lycopsids, ferns and sphenopsids,
all free-sporing heterosporous or homosporous plants, and
seed-producing plants of several types. This means that all the
known major body plan groups of plants extant at that time
were dominant players in wetland ecosystems (DiMichele and
Phillips 1996a). The Paleozoic pattern is notably different from
that of today, where seed plants and, in some instances, ferns
are the principal significant elements of wetland floras (Cronk
and Fennessy 2001) and of nearly all terrestrial ecosystems
(Bond 1989).
The principal lycopsids were several genera of trees most
closely related to the modern qullworts (Isoetes and Stylites;
Bateman 1994), including such recognizable names as Lepi-
dodendron and Sigillaria, but also many others, such as Par-
alycopodites,Bergeria,Diaphorodendron,Synchysidendron,
Hizemodendron,Lepidophloios,Sublepidophloios, and Om-
phalophloios (Wagner 1989; Bateman et al. 1992; Oplusˇtil et
al. 2010; Thomas et al. 2013; DiMichele et al. 2013a;A
´lvarez-
Va´ zquez and Wagner, forthcoming). These trees, supported by
a cylinder of bark rather than a core of wood, are among the
most unusual in all of earth history (Andrews and Murdy
1958). Some of the larger species reached heights of more than
30 m (Thomas and Watson 1976), whereas others were of
more modest size. The trees comprised three dominant growth
architectures (Bateman et al. 1992; DiMichele et al. 2013a):
those that grew as unbranched poles, producing cones peri-
odically on short, trunk-borne peduncles (mainly Sigillaria);
those of monocarpic habit, in which a dichotomously branched
crown, bearing the reproductive organs, was produced only
in the final phases of growth (primarily Lepidodendraceae; fig.
9A,left); and those that produced deciduous lateral branches
on the trunk and in the final-phase crown, thus throughout
the life of the tree (the ancestral condition found in several
families; fig. 9A,right). With rare exceptions, these plants were
confined to soils with high moisture content and, most likely,
standing water throughout much of the year (DiMichele and
Phillips 1985), a role for which they had numerous morpho-
logical adaptations (Phillips and DiMichele 1992) and spe-
cialized physiologies (Green 2010). In peat-forming environ-
ments, most lycopsid trees were abundant to dominant in the
wetter parts of swamps with periodic standing water, so-called
rheotrophic swamps, but others may have been centered in
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DIMICHELE—DYNAMICS OF COAL AGE TROPICAL VEGETATION 135
highly nutrient-depleted, highly oligotrophic parts of raised or
ombrotrophic swamps. Regardless of habitat preference, all
genera appear to have been widespread across the Euramerican
part of Pangea in the Pennsylvanian (Thomas 2007), and many
extended into the Cathaysian portions of Pangea during the
Permian (Hilton and Cleal 2007; Wang et al. 2009).
Wetland ferns fall into two broad groups based on their
growth form: the scrambling, climbing, ground cover forms
(Phillips 1974) and the tree forms (Morgan 1959). The low-
growing forms and vines comprise the zygopterid (Phillips and
Galtier 2005) and filicalean (Brousmiche 1983; Rothwell 1987;
Phillips and Galtier 2011) ferns and were widespread in wet-
lands (DiMichele and Phillips 2002). The dominant Euramer-
ican Pennsylvanian-age tree ferns were primarily the Marat-
tiales (fig. 9B), but tree forms also existed in other evolutionary
lineages of ferns (Sahni 1929; Pfefferkorn 1976). The Marat-
tiales are a group that is still extant today and of primarily
tropical distribution. In this diverse group (Lesnikowska 1989;
Millay 1997; Liu et al. 2000; He et al. 2013), the stems (Psa-
ronius,Caulopteris,Megaphyton) were supported by a mantle
of adventitious roots (Ehret and Phillips 1977). The root man-
tle hosted a large array of climbing and epiphytic plants
(Ro¨ ßler 2000), making Psaronius an important ecosystem en-
gineer. In addition, the adventitious roots, and in some species
the stems also, were composed mainly of air spaces, creating
inexpensive trees, from the viewpoint of carbon allocated to
structure (Baker and DiMichele 1997). Cheap construction
was accompanied by large fronds, probably borne in an um-
brella-form crown, on which were produced vast numbers of
reproductive organs and prodigious numbers of spores. The
dispensability and large numbers of these spores permitted the
marattialean ferns to locate and colonize remote, often isolated
areas of wet substrate, allowing them to persist in areas varying
from large swamps to small, isolated wet areas embedded
within seasonally dry landscapes, well into the drier parts of
Permian Pangea (DiMichele et al. 2006b).
The sphenopsids are composed of two large taxonomic
groups that have different growth architectures and ecologies.
The sphenophylls were small, scrambling, thicket-forming, and
climbing plants (Batenburg 1982; Galtier and Daviero 1999;
Bashforth and Zodrow 2007) that colonized exposed sub-
strates under generally wet climatic regimes. The more wide-
spread group was the calamitaleans, close relatives of the ex-
tant Equisetales (fig. 9C), a group that likely includes
considerably more taxonomic, structural, and ecological di-
versity than is understood at present (Ro¨ ßler and Noll 2006).
These plants are widely known from stem casts preserved in
upright position but also as compressed stems, often in
streamside and lakeside habitats (for a large review of literature
see DiMichele and Falcon-Lang 2012). They also are well doc-
umented from petrified material in coal-swamp settings (An-
drews and Agashe 1965; Spatz et al. 1998) and clastic wetlands
(Ro¨ ßler and Noll 2006; Mencl et al. 2013), from which stem
sizes are often considerably larger than those known from
compression preservation. Most calamitaleans were rhizo-
matous, the only group of ecologically dominant, large-size
wetland plants with this habit, which enabled them to persist
through or recover from repeated inundation by flood-borne
sediments (Gastaldo 1992; DiMichele et al. 2009b). The strong
ties of these plants to riparian corridors also permitted them,
like the marattialean ferns, to locate and persist in wet areas
of otherwise seasonally dry environments (Falcon-Lang
2003b). Wetland seed plants consisted of several distinct evo-
lutionary lineages with markedly different architectures. The
principal groups were the pteridosperms Medullosales, Lygi-
nopteridales, and Callistophytales and the conifer sister group
Cordaitales.
From a biomass perspective, the most important of the pteri-
dosperms were the medullosans, a group that has received,
perhaps, more attention from paleobotanists than any other
group of Pennsylvanian plants—thus, there is a large literature
that is altogether taxonomic, morphologic, floristic, and bio-
stratigraphic. A highly diverse and ecologically differentiated
group of plants based on studies of compression foliage, they
also comprise a variety of growth architectures, including li-
anas and small trees (fig. 9D) of various configurations (Pfef-
ferkorn et al. 1984; Dunn et al. 2003; DiMichele et al. 2006a),
a great many of which were likely ground cover or subcanopy
plants (Wilson and Fischer 2011; Raymond et al. 2013). Found
in both clastic and peat substrates (Cleal and Shute 2012),
much greater diversity has been described in the former, and
no species have yet been described that are specific to peat
substrates. Biogeographic differentiation has been documented
in the Variscan mountain region of Euramerica (Cleal 2008b),
and the group shows little dispersal into eastern Pangea during
the Permian, even though wetlands persisted there after dis-
appearing in central and western Pangea (Hilton and Cleal
2007). Thus, although tied to wetlands, possibly the higher
nutrient parts thereof, the medullosans seemingly did not have
the ability to colonize drier habitats or were limited in dispersal
ability. The lyginopterids comprise several genera of small
woody plants, including liana and thicket-forming and scram-
bling habits, typified by various morphological climbing aids
(Krings et al. 2003), and may have formed a significant com-
ponent of the ground cover in many parts of the wetland hab-
itat (Wilson and Fischer 2011; Tenchov 2012). They are com-
mon, diverse, and widespread, known from both mineral
substrates and peat swamps, particularly in the Early and Mid-
dle Pennsylvanian, declining in the Late Pennsylvanian (Cleal
2008a). The callistophytaleans were a highly derived group of
pteridosperms (Rothwell 1981) that were widespread through-
out the Middle and Late Pennsylvanian. They were primarily
ground cover with scrambling habits or possibly lianas (Galtier
and Be´ thoux 2002). Of evolutionary significance, possibly an-
cestral to more derived seed plants, they were a minor com-
ponent of floras in both mineral and peat substrate wetlands.
The cordaitalean gymnosperms were the other major group
of seed plants in Pennsylvanian wetlands (Costanza 1985; Triv-
ett and Rothwell 1988; Trivett 1992). Sister group to the con-
ifers, the cordaitaleans were taxonomically diverse, ranged in
size and habit from small scrambling forms to large forest trees,
and occurred throughout a large range of habitats, perhaps
larger than any other group of tropical plants, ranging from
wetlands (Greb et al. 1999a; Falcon-Lang 2005; S
ˇimu˚ nek
2008; Raymond et al. 2010) to well-drained settings or sea-
sonally dry habitats of many different kinds (Falcon-Lang and
Scott 2000; Falcon-Lang et al. 2011b, 2011c). In wetland hab-
itats, the group is most often portrayed as mangroves, based
on an interpretation of root morphology presented in an in-
fluential article (Cridland 1964). Recent analyses, however,
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136 INTERNATIONAL JOURNAL OF PLANT SCIENCES
Fig. 10 Dryland plants reconstructed. A, Large cordaitalean tree
with crown of large strap-shaped leaves. These trees may have ex-
ceeded 50 m in height. Note smaller scrambling form (B), for size
comparison. C, Walchian conifer of large stature, characterized by
plagotropic branching. D, Walchian conifer of small stature. This may
have been the size of mature adult plants at some periods in thehistory
of these plants; at other times, this size would have been characteristic
of a juvenile (modified from Hernandez-Castillo et al. 2003). Recon-
structions by Mary Parrish, Smithsonian Institution.
Fig. 11 Dryland landscapes. A, Cordaitalean forest dominated by
large trees in dense stands. Seed ferns and ferns in the understory and
as ground cover. B, Walchian conifer open woodland. Ground cover
of small pteridosperms or ferns (although we know the foliage of these
plants, we know very little about their growth architectures). Cal-
amitaleans line a stream in the background. Reconstructions by Mary
Parrish, Smithsonian Institution.
have questioned this hypothesis (Raymond et al. 2010), and
a stilt-rooted Rhizophora-mangrove-like growth habit seems
not empirically supportable. Recent cuticular analyses (S
ˇi-
mu˚ nek and Florjan 2013) suggest that the species known from
peat-forming wetlands were distinct from those found in sea-
sonally dry habitats. Diversity in the group is not well under-
stood, however, because of the fragmentary preservation and
limited number of characters on which to differentiate taxa,
though cuticular analysis of foliage appears to indicate large
numbers of species (S
ˇimu˚ nek 2000, 2007; S
ˇimu˚ nek and Florjan
2013). High diversity is supported not only by the wide dis-
tribution of the plants but also by the enormous range of
variability in their growth habits (fig. 10A,10B), from scram-
bling forms (Rothwell and Warner 1984), to small trees (Crid-
land 1964), to large, woody trees of tall stature (Falcon-Lang
and Scott 2000; Falcon-Lang and Bashforth 2004, 2005; S
ˇi-
mu˚ nek et al. 2009; Ce´ sari and Hu¨ nicken 2013). The broad
range of habitats encompassed by species of this lineage also
suggests and is consistent with high species diversity (S
ˇimu˚ nek
2000, 2007, 2008); cordaitaleans were found throughout the
Pennsylvanian tropical realm and were important in Cathya-
sian eastern Pangean floras during the Permian (Hilton et al.
2009). The group also extended into the high latitudes, par-
ticularly in the Permian, and so was of global distribution
(Meyen 1987; Ce´sari and Hu¨ nicken 2013).
Dryland Floral Composition
Pennsylvanian-age dry tropical floras can be difficult to char-
acterize because they often are not well preserved or are trans-
ported and also because the landscapes they occupied were
more environmentally heterogeneous than wetlands. Propor-
tionately, a relatively small part of the Pennsylvanian floristic
literature describes seasonally dry floras, often referred to as
upland or extrabasinal (Havlena 1970; Pfefferkorn 1980; Dim-
itrova et al. 2011). The fossil remains of such floras are gen-
erally relatively monotonous in terms of dominance patterns,
composed mainly of seed plants, though they appear to have
been moderately diverse, based on well-preserved assemblages.
Two qualitatively distinct kinds of seasonally dry assemblages
occurred periodically in Pennsylvanian lowlands (DiMichele
et al. 2010), with overlaps between them: cordaitalean dom-
inated, particularly in the Early and Middle Pennsylvanian,
and conifer dominated, particularly in the Late Pennsylvanian
(fig. 11). There also may have been edaphic specialist floras,
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DIMICHELE—DYNAMICS OF COAL AGE TROPICAL VEGETATION 137
often found on thin soils with poor water-holding capacities,
frequently developed on limestones. This is quite likely a con-
siderable oversimplification, given the magnification of vari-
ability in soil moisture deficits across landscapes when rainfall
is seasonal; it is challenging, however, to characterize such
vegetation based on the limited fossil record.
Seasonally dry floras from coal basins of the central and
west-central Pangean equatorial region are often represented
by allochthonous elements (transported from outside the hab-
itat of burial and preservation; Condon 1997; Gastaldo and
Degges 2007; Gibling et al. 2010), though parautochthonous
(transported but within the habitat of growth) and in situ
assemblages are known (Cridland and Morris 1963; Winston
1983; Mapes and Rothwell 1988; McComas 1988; Falcon-
Lang 2006; van Waveren et al. 2008; van Hoof et al. 2012;
Bashforth et al. 2014). From the Late Mississippian to the
Middle Pennsylvanian, such floras, where they are known,
appear to have been dominated by cordaitaleans of large stat-
ure (fig. 10A; Falcon-Lang 2003b; Falcon-Lang and Bashforth
2005; Bashforth et al. 2014). Walchian conifers appeared dur-
ing the Middle Pennsylvanian (fig. 10D), perhaps during a
change to greater rainfall seasonality at all phases of glacial-
interglacial cycles (Cecil 1990; Bashforth et al. 2014), and
became an increasingly important element of these kinds of
assemblages (Galtier et al. 1992; Rothwell et al. 1997; Her-
nandez-Castillo et al. 2001b; Blake et al. 2002; Falcon-Lang
et al. 2009; Plotnick et al. 2009). Plants of both lineages are
coniferophytes and had strongly xeromorphic morphological
features (Clement-Westerhof 1988; Rothwell et al. 1997; S
ˇi-
mu˚ nek 2000, 2007; S
ˇimu˚ nek et al. 2009). They are accom-
panied in the better-preserved assemblages by various elements
not typically found in wetland floras. These accessory taxa
differ to a large degree between the cordaitalean- and coni-
feralean-dominated assemblages. Those associated with the
older cordaitalean-rich assemblages have strong affinities with
Mississippian plants from seasonally dry settings (Wagner
2001; Bashforth et al. 2014). The Late Pennsylvanian conifer-
enriched floras include such plants as callipterids and tae-
niopterids (Cridland and Morris 1963), which have affinities
with lineages that became prominent in the Permian (Di-
Michele and Aronson 1992; Kerp 1996), although some of
the Mississippian-type elements persist into the later Pennsyl-
vanian and Permian as well (Mamay 1968, 1992; S
ˇimu˚ nek
and Bek 2003; Tidwell and Ash 2003; Wang and Chaney
2010). Nearly every seasonally dry flora also includes a few
stereotypically wetland taxa, particularly calamitaleans and
tree ferns, both of which persist well into the Permian. These
are sometimes accompanied by the aborescent lycopsid Sigil-
laria, a plant that may have had various kinds of morpholog-
ical and reproductive features permitting tolerance of drier
conditions than other arborescent lycopsids (Phillips and
DiMichele 1992; Pfefferkorn and Wang 2009), and by pteri-
dosperms in older, cordaitalean-rich assemblages possibly with
somewhat longer wet periods than those dominated by or en-
riched in conifers.
Cordaitaleans from seasonally dry settings are known from
a wide range of preservational conditions from parautochtho-
nous (Winston 1983; Falcon-Lang et al. 2009) to allochtho-
nous. The parautochthonous deposits often represent small,
channel-form accumulations in cratonic flatland settings (Cun-
ningham et al. 1993; Feldman et al. 2005; Falcon-Lang et al.
2009), with evidence from associated paleosols and sedimen-
tary features of seasonal moisture regimes. The allochthonous
deposits often occur as large accumulations of trunks in chan-
nel deposits, not infrequently as logjams (Calder 1998; Falcon-
Lang and Scott 2000; Stanesco et al. 2002; Falcon-Lang
2003b; Gibling et al. 2010). The allochthonous deposits are
found most often in proximity to documented upland areas,
from which the logs are presumed to have derived (Falcon-
Lang and Scott 2000; Falcon-Lang and Bashforth 2005). This
kind of preservation, in particular, gives the impression that
most of the cordaitaleans in such settings were quite large trees
(fig. 10A), ranging upward of 50 m in height (Falcon-Lang
and Scott 2000; Falcon-Lang and Bashforth 2004). Rare re-
ports of autochthonous cordaitalean stumps still in place
within seasonally dry paleosols fringing channels (Bashforth
et al. 2014) and in alluvial fans (Falcon-Lang 2006) confirm
large-sized trees in these habitats. Based on wood anatomy,
several different species of large, woody cordaitaleans occupied
such settings (Falcon-Lang 2003a). Cordaitalean species of
large stature may have been the ultimate K-selected plants of
their era (Bashforth et al. 2011), due to their large, woody
stems, which can be presumed to indicate long life spans and
relatively slower growth than most of their wetland counter-
parts. Dryland settings, however, also seem to have hosted
smaller cordaitaleans (fig. 10B) with sprawling growth forms
(Falcon-Lang 2007). The frequent association of cordaitalean
remains with abundant coniferophytic charcoal has led to sug-
gestions that fires were regular and important parts of the
landscapes they dominated (Falcon-Lang and Scott 2000; Scott
et al. 2010). As noted above, cuticular studies suggest that
peat-swamp cordaitaleans were taxonomically distinct from
those of more seasonally dry settings (S
ˇimu˚ nek and Florjan
2013).
Conifers, like cordaitaleans, are consistently associated with
depositional environments, including paleosols, indicative of
seasonally dry settings and with an abundance of conifero-
phytic charcoal, suggesting habitats subject to frequent fires
(McComas 1988; Rothwell et al. 1997; Ziegler et al. 2002;
Uhl et al. 2004; Falcon-Lang et al. 2009; Scott et al. 2010;
Looy 2013). Again, as with the Cordaitales, many of the earlier
conifer occurrences are from deposits formed proximate to
contemporaneous mountainous areas in Europe, the Appala-
chian region, and New Mexico (Lyons and Darrah 1989; Gal-
tier et al. 1992; Rothwell et al. 1997; Martino and Blake 2001;
Blake et al. 2002; Lerner et al. 2009; Lucas et al. 2013, pp.
56–57). Late Pennsylvanian conifers are more common com-
ponents of floras from intermontane or limnic basins in central
Europe (Kerp 1996) than they are from the broad, flat cratonic
areas of the North American coal basins, particularly from the
Midcontinent and Illinois Basin regions (Rothwell et al. 1997);
the relative rarity of conifers in both the Appalachian Basin
and near-mountainous areas of the Iberian Peninsula, both
close to active tectonism and associated elevated areas, is note-
worthy (Blake et al. 2002; Eble et al. 2009; Wagner and A
´l-
varez-Va´ zquez 2010; Bashforth et al. 2010). Hernandez-Cas-
tillo et al. (2003) reconstructed Late Pennsylvanian conifers as
diminutive, 2 m tall, plagiotropically branched trees (fig. 10D),
much like a juvenile of the extant Araucaria heterophylla,an
interpretation consistent with the small Middle Pennsylvanian
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138 INTERNATIONAL JOURNAL OF PLANT SCIENCES
Fig. 12 Middle and Late Pennsylvanian (Desmoinesian and Missourian) seasonally dry floral elements and depositional environments. Cerros
de Amado area, east of Socorro, New Mexico. A, Series of stacked, coarsening-upward deltaic lobes (arrows). Mudstones contain both transported
plant remains and mixed brackish and marine invertebrate fauna. B, Walchian conifer. C,Sphenopteridium of the Sphenopteris germanica type,
a pteridosperm with Mississippian affinities. D,Charliea manzanitana, a noeggerathialean with Early Pennsylvanian affinities. E, Callipterid,
possibly Lodevia sp., a peltasperm. This specimen was collected from rocks of Early Missourian age. F, Coastal deposit of Late Pennsylvanian
(Missourian) age, containing upright coniferalean tree trunks. Dune deposits consisting of carbonate and gypsum embed a small upright trunk
(dark area next to digging tool). Trunk base is covered but is surround by tufa (the frothy material near the bottom) and is rooted in a micritic
limestone, just visible below the tufa. G, Naturally exhumed tree base of large diameter. Roots are visible; these are embedded in a micritic
limestone visible below scattered pieces of tufa, which surrounds the tree base above the limestone rooting horizon. Scale bars in B–Ep1 cm.
coniferopsid stems identified by Galtier et al. (1992). Other
possible Late Pennsylvanian coniferopsids, from a coastal hab-
itat in New Mexico (Falcon-Lang et al. 2011b), suggest much
larger woody trees (figs. 10C,12F,12G). And, certainly, by
the Permian, these plants had attained relatively large stature
(Looy 2013). Throughout their Paleozoic history, from the
earliest occurrences (Hernandez-Castillo et al. 2009) to their
abundant occurrences in the Permian (Florin 1950; Kerp et al.
1990; Kerp 1996; Ziegler et al. 2002; DiMichele et al. 2007;
Looy and Duijnstee 2013), conifers were probably the most
consistently reliable indicators of environments with seasonal
moisture stress; Ziegler et al (2002) mapped their paleogeo-
graphic distribution and found them to be restricted to sea-
sonally dry environments of the Euramerican tropics and sub-
tropics. The consistent association with indicators of periodic
drought led White (1936) to call them “children of adversity.”
There was an array of other plants associated with season-
ally dry floras. Most are encountered much less commonly
than conifers and cordaitaleans, depending in part on the abil-
ity of their foliage to survive transport, burial, and decay. Bash-
forth et al. (2014) refer to these as enigmatic elements and
note changes in these elements in concert with changes in the
dominant cordaitaleans and conifers. Enigmatic elements in
Early and Middle Pennsylvanian cordaitalean-dominated flo-
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DIMICHELE—DYNAMICS OF COAL AGE TROPICAL VEGETATION 139
Fig. 14 Mixed seasonally dry flora from the Kinney Clay Pit, Late
Pennsylvanian (latest Missourian) of New Mexico. All specimens were
photographed in the field. A,Cordaites leaf. B, Walchian conifer
branch. C,Plagiozamites leafy axis, possibly cycadophytic affinities.
D,Charliea manzanitana foliage. E,Annularia sp., similar to Phyl-
lotheca paulinensis.F,Sphenopteridium manzanitanum, nearly com-
plete leaf. Penny is 19 mm in diameter.
Fig. 13 Lower Pennsylvanian seasonally dry plants (A–C,E). A,
Lesleya sp., University of Illinois specimen. B,Megalopteris sp., Uni-
versity of Illinois specimen. C,Charliea guntheri, Manning Canyon
Shale, Utah, a noeggerathialean. D,Charliea readii, from the Penn-
sylvanian-Permian boundary, Carrizo Arroyo, New Mexico; specimen
shown for comparison. E,Sphenopteridium sp., University of Illinois
specimen. Scale bars p1 cm. Cand Dreprinted from Tidwell and
Ash (2003), with permission of Elsevier.
ras are most noteworthy for the strong Mississippian affinities
of many of the taxa, including species of Adiantites,Rhacop-
teridium, and Sphenopteridium (Wagner 2001). Also included
among these cordaitalean associates are the possible edaphic
specialist plants Megalopteris and Lesleya (fig. 13A,13B),
which perhaps favored thin, droughty limestone soils, and pos-
sible noeggerathialeans (fig. 13C,13D), an enigmatic group
of uncertain affinity (Leary and Pfefferkorn 1977; Tidwell and
Ash 2003). Lesleya has been suggested as a possible cycad
precursor (Leary 1990) or an ancestor of the Gondwanan glos-
sopterids (Leary 1993). Bashforth et al. (2014) and Wagner
(2005) also report Dicranophyllum, a probable coniferophyte
(Barthel and Noll 1999), species of which persist into thePerm-
ian. Accompanying the rise of conifers, probably reflective of
increased seasonal drought during the drier phases of glacial-
interglacial cycles (Bashforth et al. 2014), a new suite of sea-
sonally dry subdominants appears, types more typically as-
sociated with Permian floras, many of which are strongly
xeromorphic (figs. 12, 14). These include such plants as Tae-
niopteris, a polyphyletic form taxon among which are possible
cycad precursors (Cridland and Morris 1960; Mamay 1976;
Kerp 1983; Winston 1983; Gillespie and Pfefferkorn 1986;
Axsmith et al. 2003), as well as ferns (Remy and Remy 1975).
Early members of the callipterid peltasperms, such as Dicho-
phyllum,Rhachiphyllum, and Lodevia (Cridland and Morris
1963; S
ˇimu˚ nek and Martı´nek 2009; Psˇenicˇka et al. 2011);
additional cycadophytic elements, such as Plagiozamites (Bas-
sler 1916); and coniferophytes, such as Podozamites (Mamay
and Mapes 1992), also have been reported. Noteworthy
among those persisting from the earlier Pennsylvanian and
later Mississippian are Sphenopteridium,Dicranophyllum,
and the noeggerathialean Charliea (Mamay and Mapes 1992;
Tidwell and Ash 2003; DiMichele et al. 2013b; figs. 12C,12D,
13D,14D,14F,15C,15D).
Plants typically identified as having wetland affinities are a
highly variable component of seasonally dry assemblages (fig.
16). The most consistently present are the calamitaleans, which
have been identified in wet areas surrounding lakes and
streamsides or in bars within streams (Falcon-Lang et al. 2004;
Falcon-Lang et al. 2012; Bashforth et al. 2014). Tree ferns also
are commonly found in similar settings, particularly in Middle
and Late Pennsylvanian assemblages, reflective of their coloniz-
ing abilities and rapid growth capacities. The pteridospermous
component often includes taxa not typical of or found in abun-
dance in wetland vegetation intimately associated with coals.
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140
Fig. 15 Late Pennsylvanian (Virgilian) representative outcrop and fossils, from north-central Texas. This outcrop represents a typical cyclothem
from this region of the Eastern Shelf of the Midland Basin, in which there are few marine strata. Seasonally dry flora and wetland flora are
intercalated, in beds of different lithology. A, Characteristic lithological sequence: paleoultisol, indicative of wet but well-drained conditions;
kaolinite bed in erosional contact with paleosol, possibly formed in a standing water lake. Organic shale,swamp deposit. Mudstones andsiltstones
formed in a wet floodplain setting. Blocks of sandstone at the top of the outcrop. B–D, Flora of the kaolinite bed. B, Walchian conifer. C,
Charliea or Yuania noeggerathialean leaf. D,Sphenopteridium sp. of the Sphenopteris germanica type. E–H, Flora of the organic shale bed. E,
Sigillaria brardii; lycopsid bark. F,Calamites stem; sphenopsid. G,Macroneuropteris scheuchzeri; pteridosperm and the dominant element of
the organic shale beds. H,Asterophyllities equisetiformis; calamitalean foliage. I–L, Flora of the floodplain mudstones. I,Annularia carinata;
calamitalean foliage. J,Pecopteris puertollanensis; tree fern foliage. K,Pseudomariopteris cordato-ovata; scrambling to climbing pteridosperm.
L,Neurodontopteris auriculata; pteridosperm foliage. Scale bars p1 cm.
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141
Fig. 16 Middle and Late Pennsylvanian (Desmoinesian and Missourian) wetland floral elements and depositional environments. Cerros de
Amado area, east of Socorro, New Mexico. A, Olive siltstone bearing a mixed seasonally dry and wetland allochthonous flora. B,Sigillaria
brardii.C,Sphenophyllum oblongifolium; ground cover sphenopsid. D,Neuropteris ovata; pteridosperm foliage. E,Annularia sp.; calamitalean
foliage. F,Neuropteris ovata; autochthonous accumulation in a clastic, coastal swamp habitat. This exposure crops out in an area of several
hundred hectares. It is overlain by a marine limestone that marks the Middle–Late Pennsylvanian (Desmoinesian-Missourian) boundary. G, The
deposit contains thickly interlayered stems, foliar axes, and laminate foliage consisting predominantly of N. ovata. Scale bars in Cand Dp1
cm. Pencil tip in Ep1 mm. Ruler in Gp10 cm.
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142 INTERNATIONAL JOURNAL OF PLANT SCIENCES
The lycopsid Sigillaria occurs in dryland habitats into the Early
Permian Texas redbeds (DiMichele et al. 2006b, their fig. 14C).
The presence of wetland elements in floras dominated by sea-
sonally dry vegetation should not be a surprise—wetter areas
exist within many dry landscapes, even deserts, and species
adapted for high soil moisture conditions are able to colonize
parts of these landscapes, such as stream and lake margins,
springs, and local swampy areas. What is unusual, however, is
the taxonomic similarity, or even identity, of the species in wet
patches of dry landscapes and the species of widespread, vast
wetland landscapes. Dispersal constraints were much less lim-
iting in wetlands than in drylands due to spatial habitat con-
tinuity in the former. In seasonally dry landscapes, these same
taxa would have had to disperse between small, discontinuous
patches of wetlands or remain confined to wetland riverine cor-
ridors. Perhaps the compositional similarity of wet substrate
species, regardless of background climate, mainly reflects the
significantly lower species diversity of that time, compared to
what we take for granted today. With many fewer species, each
may have had broader patterns of resource use (considering
species to be shorthand for niche). Thus, with fewer species to
do the job, one might hypothesize less competitive exclusion or
exclusion by simple incumbency among those species specialized
for any given set of physical conditions.
Biogeography-Spatial Dynamics of the Biomes
Climate models and geological evidence, as referred to above,
suggest several trends that require examination of the spatial
dynamics of the plants. First, it is clear that equatorial climate
oscillated during glacial-interglacial cycles between wetter and
drier phases, linked to fluctuations in sea level and likely driven
by changes in ice volume (figs. 3, 4). Second, there is considerable
evidence for an interval of strong global warming in the early
part of the Late Pennsylvanian (roughly the Missourian/Kasi-
movian), probably associated with nearly complete melting of
Southern Hemisphere ice (Fielding et al. 2008). And, finally,
much physical evidence indicates a long-term trend of increasing
equatorial aridity during the Pennsylvanian and into the Permian
(Tabor and Poulsen 2008) on which glacial-interglacial fluctu-
ations were superimposed; thus, through time and on average,
the drier parts of cycles got more dry and the wetter parts less
wet. In association with these climatic changes, there must have
been changes in the areal extent of wetland and seasonally dry
biomes across the Pangean tropical realm and certainly in the
central and western Euramerican parts of the supercontinent,
on which this review is focused. Perhaps this is most clearly
revealed by instances where the wetland and seasonally dry veg-
etation types occur in different beds, representing distinct li-
thologies and depositional environments, within the same out-
crops (fig. 15). Such occurrences indicate temporal and, thus,
spatial separation of these biomes.
Fate of the Wetland Flora during Dry-Climate Intervals
In light of the many strong geological indicators of repeated
climate changes in the equatorial Pangean tropics, the question
of what happened to the coal swamps when it was not wet
has rarely been asked. This is likely because the wetlands are
so prominently represented in the Pennsylvanian tropical fossil
record; this has led to the assumption that large swaths of
megawetlands were always present, somewhere on the land-
scape. In contrast, hypotheses about the location and under-
lying causal factors of dryland floral composition and location,
the so-called upland or extrabasinal associations, have been
subjects of discussion in Carboniferous paleobotany since such
floras were recognized—their rarity as elements of the fossil
record has given them an exotic aura. The lack of investigation
of the fate of wetlands during periods of dry climate reflects,
perhaps, the long-held view that the equatorial lowlands of
the Pennsylvanian were perpetually “wet”; it was, after all,
the Coal Age, and coal means wet climate (Cridland and Mor-
ris 1963; Lyons and Darrah 1989; Broutin et al. 1990; Dim-
itrova et al. 2011). Consequently, in this view, the fate of wet-
lands through any particular glacial-interglacial cycle is a
nonquestion; they were always there under an unchanging,
humid background climate.
The ever-present-wetlands model generally points to the dy-
namics of fluvial, coastal, and deltaic sedimentation to create
habitat heterogeneity (Horne et al. 1978; Ferm and Weisenfluh
1989) within which peat-forming wetlands, clastic swamps,
and wet mineral soil habitats and even local well-drained set-
tings were always somewhere present and in close proximity
to one another. In such models, the siliciclastic and carbonate
beds that separate coal seams reflect, through time, the local
spatial variation found on the landscape. It is through such
interpretations that the strata between coals can come to be
attributed to short-term flood events, separating long-lived
coal swamps (Dimitrova et al. 2011; but for a climate-based
interpretation of the same location, see Dolby et al. 2011).
Explicitly or implicitly, this model of the Pennsylvanian low-
land assumes constant background climate, one that was fa-
vorable for peat formation to be ever present on the landscape,
somewhere. Confounding data that such models generally do
not address include paleosols indicative of long periods of sea-
sonal drought (figs. 4, 5) located between many coal beds
(Tandon and Gibling 1994, 1997; Driese and Ober 2005; Ro-
senau et al. 2013a, 2013b) or changes in sedimentological
patterns linked mechanistically to background climate regimes
(Calder 1994; Batson and Gibling 2002; Cecil and Dulong
2003; Theiling et al. 2012; Oplusˇtil et al. 2013a).
Another way to look at this problem is to propose that peat/
coal formation is not a function of wet climates but only a
result of the ponding of groundwater. In the broad, flat, cra-
tonic settings such as the interior areas of west-central Pangea
(Watney et al. 1989), such ponding is most often envisioned
to be caused only by rising sea levels linked to glacial-inter-
glacial cycles (Bohacs and Suter 1997), thus creating bands of
coastal swamps that move inland ahead of the sea. As discussed
above, these models do not account for the great contrasts in
the rates of peat accumulation and the rates of sea level rise
in response to pulse-like ice melting (Archer et al. 2014), or
for why coals are not present in areas of demonstrably drier
climates, such as western equatorial Pangea, which also ex-
perienced glacially driven sea level rises and falls (Bishop et
al. 2010; Elrick and Scott 2010), or for the extensive evidence
that the vast Pennsylvanian coal swamps did not form as nar-
row bands proceeding landward as sea level rose but appear,
instead, to be blanket deposits, time-equivalent throughout
(Greb et al. 1999b, 2003; Cecil et al. 2003), thus requiring a
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DIMICHELE—DYNAMICS OF COAL AGE TROPICAL VEGETATION 143
mechanism that would generate widespread wet conditions
(i.e., climate). They also must explain why peat deposits of
Pennsylvanian coal quality do not fringe the margins of every
landmass on our modern Earth, given that sea levels have been
rising since the last glacial maximum. Less common, but also
suggested, is that wetlands formed by the ponding of ground-
water draining from uplands into the low ground. Calder
(1994) presents a compelling case for this, one still strongly
linked to climate cyclicity and its effects on sedimentation,
although with major consideration given to the effects of local
tectonism in a highly active area. There are suggestions, how-
ever, that this kind of moisture source also can promote peat
formation under uniformly seasonally dry climates (Chandler
1998). Were this possible, coals should be a common com-
ponent of strongly seasonal Pennsylvanian landscapes of the
mountainous portions of western Pangea (Cecil et al. 2003b;
Peyser and Poulsen 2008; Tabor and Poulsen 2008; Tabor et
al. 2008; Nelson et al. 2013b) and even of seasonal-climate
Permian red-bed landscapes proximate to upland areas (e.g.,
Rotliegends [Kerp and Fichter 1985; Howell and Mountney
1997; Rieke et al. 2003], Lode` ve [Schneider etal. 2006; Galtier
and Broutin 2008], north-central Texas [Chaney and Di-
Michele 2007; Nelson et al. 2013a]), which they are not. Thus,
humid climates appear, even in tectonically active areas, to be
required for peat formation in the tropics.
It is not the intention here to imply that wetlands, sensu
lato, invariably reflect humid climates. Any consideration of
the ecosystems in our modern world shows such a generality
to be unsupportable. What is explicitly intended, however, is
the recognition that vast wetlands of Pennsylvanian spatial
scales and, particularly, peat-forming wetlands both today and
in the past are indeed reflective of widespread humid to per-
humid climates in almost all instances. Peat, in this case, is
restricted to low-ash organic accumulations that might become
coals following burial and diagenesis. This assertion is under-
pinned by the high degree of sensitivity of plants to both local
habitat conditions and overall climatic conditions. Although
local conditions are the final arbiter of what grows at any spot
on the ground, these must be assessed within the context of
the larger-scale regional climate.
Furthermore, consideration should be given to the way any
local community of organisms is assembled; plants must be
able to (1) get there, (2) survive there, and (3) compete there
(Belyea and Lancaster 1999; Weiher et al. 2011). When con-
sidered in this light, dispersal limitation may be the strongest
filter in this process and may have constrained the distribution
of Pennsylvanian plant populations on many spatial scales
(Laveine 1997; Hilton and Cleal 2007). Such local community-
assembly controls are superimposed on glacial-interglacial
fluctuations and the large and growing body of evidence that
these were accompanied by and coordinated with widespread
climatic changes.
Wetland-plant distributional patterns during glacial-inter-
glacial cycles. Within the context of fluctuating climates, tied
to the dozens of Pennsylvanian glacial-interglacial cycles, there
are two competing hypotheses for the fate of wetland vege-
tation during the long, seasonally dry portions of these cycles
(Falcon-Lang and DiMichele 2010). (1) The coal swamps and
surrounding mineral substrate wetlands with high soil mois-
ture requirements moved their location as climate changed
during a glacial-interglacial cycle, tracking wet climates out of
the tropical latitudes or into tropical or paratropical areas
where wet climates persisted. (2) The dominant coal swamp
and other wetland species retreated into refugia on the sea-
sonally dry landscapes, occupying only much-reduced and
likely spatially discontinuous small inland swampy areas,
streamside and lakeside habitats, and coastal wetlands. A third
possibility allows for these two biogeographic phenomena to
occur simultaneously in latitudinally distinct parts of the trop-
ical regions. Due to uncertainties in models and conflicts in
their applications to geological strata in different paleogeo-
graphic areas, we cannot differentiate among these with cer-
tainty, though hypothesis 2 seems to be the more likely.
Hypothesis 1. In the equatorial regions of west-central
and western Pangea, there are both empirical evidence and
experimental climate model results for the episodic appearance
of widespread wetlands separated by time intervals during
which conditions suitable for extensive, regionally widespread
wetland development were limited or absent. Thus, there does
not appear to have been regional-scale spatiotemporal conti-
nuity of extensive wetland environments through all phases of
a glacial-interglacial cycle (Cecil et al. 2003b; Eros et al. 2012;
Rosenau et al. 2013a, 2013b). Thin coals, locally of minable
thickness, appear to occur during some interglacials, often
forming in deltaic settings during late highstand and sea level
fall, but these are separated from lowstand coals by strata that
record conditions physically and climatically unsuitable for
peat formation. Recent models of Pennsylvanian climate (Poul-
sen et al. 2007; Peyser and Poulsen 2008; Horton et al. 2012)
find little evidence of vast wetlands in tropical or paratropical
regions during the drier phases of glacial cycles, which in such
models are the interglacials. The modeling experiments of Hor-
ton et al. (2012), focused on changes in climate at the temporal
scale of a glacial-interglacial cycle (cyclothem scale), suggest,
or at least leave open, as discussed above, the possibility that
areas of very high though seasonally distributed rainfall oc-
curred during early interglacials in the Pangean equatorial
regions. These data and experimental models do not lend sup-
port to the possibility of the continuous existence of vast areas
of wetlands within the tropics or in paratropical areas during
interglacials.
Hypothesis 2. In contrast to the mass-migration scenario,
there is scattered but reasonably good evidence for the survival
of wetland taxa within west-central and central Pangean equa-
torial basins, in wet streamside settings, channel belts, and
localized wet areas within otherwise seasonally dry landscapes.
For example, Elrick et al. (2013) report tidal mudflat floras
during early interglacial stages, associated with the onset of
continental flooding. The dominant plants in these weakly sea-
sonal wetlands are pteridosperms, tree ferns, and the lycopsid
tree Sigillaria, suggested to presage major compositional
changes that occurred in wetlands at the Middle–Late Penn-
sylvanian boundary in response to what appears to have been
a period of anomalously dry climate (Phillips et al. 1974; Fal-
con-Lang et al. 2011a). Similar floras were found preserved in
small channels between both Middle and Late Pennsylvanian
coal beds in the Illinois Basin. One, an intermittent discharge
channel, was located between two major late Middle Penn-
sylvanian coals, collected by W. DiMichele and colleagues (fig.
17). This channel was found to contain several horizons of in
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Fig. 17 Small, flashy discharge channel between the Baker and Danville coals, Middle Pennsylvanian (Desmoinesian) of Indiana. There are
multiple horizons of rooted plants within the channel. A,Sigillaria tree base in upright position (at arrow), rooted within channel. B, Enlargement
of Sigillaria.C, Pteridosperm stems rooted in the channel (arrows), deformed by burial inflood-borne sediments. D,Macroneuropteris scheuchzeri;
pteridosperm foliage. E,Neuropteris ovata; pteridosperm foliage. F, Cf. Odontopteris sp.; pteridosperm foliage. G,Pecopteris cf. vestita; tree
fern foliage. H, Stigmarian root system of lycopsid tree (arrow), within channel sediments. I, Pteridosperm roots and what is probably Macro-
neuropteris scheuchzeri pinnule in atypical preservation. Scale bars p1 cm. Note pick handle base at bottom of H.
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DIMICHELE—DYNAMICS OF COAL AGE TROPICAL VEGETATION 145
Fig. 18 Late Middle Pennsylvanian conifers from the Illinois Basin.
A, Small channel below the Baker coal, laterally equivalent to thecalcic
paleovertisol below the coal. Channel base, marked by arrow,truncates
underlying sediments. B, Conifer found in sediments derived from the
channel (see Falcon-Lang et al. 2009). C, Cave from northern part of
the Illinois Basin, filled with Pennsylvanian sediments. D, Conifer from
the cave fill. Cand Dfrom Scott et al. (2010) used with permission
of Elsevier.
situ rooted plants, including the typical wetland elements Ma-
croneuropteris scheuchzeri,Neuropteris ovata,Lobatopteris
cf. vestita, calamitalean stems, and Sigillaria. The other channel
was a small Late Pennsylvanian (Missourian) estuarine deposit
that included Cordaites foliage as the dominant element, along
with Macroneuropteris and calamitalean stem remains, and
included strongly terrestrialized vertebrate remains, signifying
general seasonal dryness of the landscape at the time of plant
growth (Carpenter et al. 2011). None of the typically peat-
swamp-centered lycopsid species was found, however, which
is the most common—and puzzling—pattern. Sigillaria is by
far the most commonly encountered lycopsid in such nonpeat,
seasonal-climate wetland assemblages and is the dominant ly-
copsid in Euramerican wetland landscapes during the Late
Pennsylvanian, suggesting that these channel deposits may
have formed under more seasonally dry conditions than those
that prevailed during formation of the widespread peat
swamps/coals between which they occur. The Atlantic Cana-
dian coal basins have proven particularly fruitful ground for
the preservation of wetland elements in otherwise seasonally
dry cordaitalean-dominated habitats from the Early Pennsyl-
vanian (Falcon-Lang et al. 2006; Bashforth et al. 2014). Re-
ports of wetland plants surrounding water holes suggest small,
scattered refugia and, again, the only reported lycopsid being
Sigillaria rather than elements of the typical peat-forming taxa
(Falcon-Lang et al. 2004). Likewise, Sigillaria and calamita-
leans are reported from within channels and streamsides as
well as from cordaitalean-dominated vegetation (Falcon-Lang
et al. 2006). Falcon-Lang et al. (2009) report a macroflora
from a subcoal channel, formed laterally to a calcic vertisol
(seasonally dry climate), dominated by cordaitaleans and in-
cluding conifers, a seemingly typical example of an interglacial
or early glacial seasonally dry flora (fig. 18A); the palynoflora
derived from those same rocks, however, was dominated by
Lycospora granulata, the spore of the arborescent lycopsid
Lepidopholoios hallii, one of the most specialized wetland
components of peat-forming swamps, suggesting the presence
of these plants somewhere on that landscape. Similarly, Haw-
kins et al. (2013) were able to track vegetational changes
through a single cyclothem in the Pennine Basin of the United
Kingdom, and though they found distinct changes, including
significant increases in dryland taxa, they also found a persis-
tent signal of wetland plants through the entire profile.
Wetland plants in western Pangea. Wetlands, sometimes
of considerable areal extent and consisting of basically the
same flora as that typically found in siliciclastic rocks above
coal beds—mudstone roof shales—in the central Pangean coal
basins, existed throughout the western parts of Pangea during
the Pennsylvanian. In this region of Pangea, peat swamps were
of very limited development and then only during the Early
and early Middle Pennsylvanian (Morrowan and Atokan; Gor-
don 1907; Kosanke and Meyers 1986). The floras from this
time, Morrowan and Atokan, encompassed a considerable di-
versity of wetland species, including Lepidodendron, pteri-
dosperms such as Neuropteris, and various calamitaleans (Tid-
well 1967; Tidwell et al. 1992; Lucas et al. 2009). During the
later Middle Pennsylvanian (Desmoinesian) and Late Penn-
sylvanian (Tidwell 1988; Tidwell et al. 1999; DiMichele et al.
2004, 2010, 2013b; DiMichele and Chaney 2005; Lerner et
al. 2009; Lucas et al. 2013; Tabor et al. 2013a, 2013b), western
wetland floras were less widespread and, as in the eastern coal
basins, were greatly enriched in tree ferns, with a continued
presence of pteridosperms and Sigillaria as the only tree ly-
copsid (figs. 15E–15L,16B–16E).
Throughout the entire time interval, however, if examined
on average, there are numerous western Pangean floras rich
in wetland plants that also contain a significant number of
taxa characteristic of seasonally dry environments. This is
made most clear by transported assemblages, which are almost
invariably mixed (fig. 14) and may be either dominantly dry-
land or wetland taxa, with a background of species from the
other kind of vegetation (Lerner et al. 2009; DiMichele et al.
2013b). The implications of this are that, at times, these two
floras were growing in close proximity, with the seasonally dry
flora dominating the great majority of the land surface and
the wetland flora fringing coastal areas and waterways. The
almost constant presence of seasonally dry species in alloch-
thonous assemblages indicates that the overall climate was
seasonally dry. There also are examples of assemblages com-
posed entirely of wetland species, the more typical and diverse
of which are from older Pennsylvanian strata; these are, in
almost all instances, parautochthonous to autochthonous and
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146 INTERNATIONAL JOURNAL OF PLANT SCIENCES
thus not likely to reveal more than very local vegetation. Most
such floras have not been published (we have a number of
examples in the National Museum collections), but published
examples include floras rich in lepidodendrids and other wet-
land species from the Mississippian-Pennsylvanian boundary
in Arizona (Tidwell et al. 1992) and from strata of Atokan
age in New Mexico (Lucas et al. 2009) and Colorado (Jennings
1980; for a humid-climate interpretation of this deposit, see
Mack and Suttner 1977; Maples and Suttner 1990) and a late
Desmoinesian-age autochthonous pteridosperm-dominated
(Neuropteris ovata) coastal swamp deposit from New Mexico
(fig. 16F,16G), mentioned above, identified over several
square kilometers (Lucas et al. 2013).
Glacial-interglacial cycles can be detected in parts of western
Pangea during the Permian (Elrick and Scott 2010), though
their expression can be strongly affected by local tectonism
(Lucas and Krainer 2013). These cycles were accompanied by
moisture fluctuations and associated variations in sediment
transport patterns (Rueger 1996; Soreghan et al. 2002; Theil-
ing et al. 2012), similar in kind, if not in detail, to those in
the central Pangean coal basins. Such fluctuations should be
expected to have affected the extent and location of wet and
dry floras on the landscape, even in these consistently seasonal
parts of western Pangea. It seems that during the drier inter-
vals, the two floras lived in close enough proximity to produce
mixed, transported floras in nearshore marine and lagoonal
settings (Lucas et al. 2013). But during the wetter periods, the
coastal areas covered by wetland vegetation enlarged and in-
cluded small coastal swamps, and when preserved as parau-
tochthonous deposits, the seasonally dry flora is rarely detected
within these deposits as macrofossils (palynological investi-
gations are very limited; Rueger 1996; Tabor et al. 2013b).
This pattern continues into the Permian in this same region
(White 1912; DiMichele et al. 2006b). In these younger rocks,
wetland elements, represented mainly by tree ferns, occur spo-
radically, often in great abundance as dominants in parautoch-
thonous deposits. In many other instances, tree ferns occur as
allochthonous fragments. In either instance, the presence of
wetland elements reflects their persistence in these seasonally
dry Permian landscapes, likely in patches and along stream
corridors.
Fate of the Dryland Flora during Wet-Climate Intervals
The presence of nonwetland elements in the Pennsylvanian
tropics has been recognized for a long time (Gothan and Gimm
1930). Chaloner (1958) suggested the term “upland flora” to
explain the presence of nonwetland plants in basinal areas.
Because this term implicates areas of steep slopes, even if not
mountainous areas themselves, and because plants, particu-
larly in the Paleozoic, are likely to be found only in what were
net sediment accumulating areas—basins—this term carries
certain connotations about the habitats of nonwetland areas
that may not be universally true. Pfefferkorn (1980) attempted
to clarify this terminological problem by introducing the term
“extrabasinal,” thus implying only that the exotic species came
from areas outside of basins, with no judgments regarding
elevation.
Such basinal occurrences of nonwetland elements have been
understood in various ways, but two explanations prevail. (1)
The explanation most often called upon is transport into basins
from somewhere outside—that somewhere presumably drier
and that dryness generally attributed to drainage due to ele-
vation. (2) The other explanation calls for dispersal of the
dryland floras into basins—again from outside—but in this
instance from areas of seasonally dry climate, with the plants
moving into the basins as climate there changed to seasonally
dry.
Uplands and soil drainage. The upland interpretation was
(and remains?) by far the most prominent explanation for the
sporadic appearance of dryland taxa in the lowland basinal
fossil record. This is not as simple and straight forward a model
as it may, at first glance, appear to be. In the most common
formulation of this scenario, nonwetland plants are interpreted
to have been transported into the lowlands from dry-soil hab-
itats in surrounding upland areas. The dry soils in such settings
are proposed to have been created solely by drainage on hills
and slopes, not by climatic factors related to regional rainfall
patterns. Thus, the populations of seasonally dry plants are
proposed to have occupied well-drained areas at the same times
that wetland plants occupied non-well-drained bottomlands.
A generally unspoken corollary to this is that climate was
constantly wet in the Pennsylvanian tropical belt, necessary to
permit peat formation somewhere in the basinal bottomlands
at all times (Remy 1975; Broutin et al. 1990; Dimitrova et al.
2010). These models generally do not explicitly call for changes
in climate with increasing elevation, though this might be ex-
pected in mountainous areas if the mountains are high enough
and could account for background pollen rain from conifers
into lower-elevation intermontane wetlands (Broutin et al.
1990; Dimitrova et al. 2005, 2011; Falcon-Lang 2006; Bash-
forth et al. 2011; Dolby et al. 2011; Oplusˇtil et al. 2013b).
In the most extreme version, the uplands are envisioned as
small well-drained hills located within the lowland landscapes
(Cridland and Morris 1963). In this scenario, dryland vege-
tation is closely surrounded by wetland flora, leaving drought-
tolerant and obligate-wetland plants growing only meters
apart in space (fig. 19, top). As noted by Zeigler et al. (2002),
this is not the general idea that most people hold of uplands.
Nor is this scenario likely under a humid or perhumid climate,
where high rainfall would greatly mask or subdue the effects
of drainage on small lowland hills.
In most models, uplands and basinal lowlands are envi-
sioned as close together in space but separated by dozens to
thousands of meters of elevation, with the elevated areas on
the margins of the basins (fig. 19, bottom), such proximity
facilitating transport of upland plant remains into basins (Hav-
lena 1961, 1971; Lyons and Darrah 1989; Broutin et al. 1990;
Cazzulo-Klepzig et al. 2007; Dimitrova and Cleal 2007). In
another scenario, the upland plants become more recognizable
during sea level highstands, when the depositional environ-
ments were pushed inland (Chaloner 1958; Neves 1958;
Turner et al. 1994 [all palynoflora analyses]), effectively bring-
ing the point of burial and preservation to the uplands with
no need to account for climate change in association with mean
sea level rise.
When long-distance transport of the plant material can be
strongly supported empirically (such as in the cases of logjams
within streams or charcoal fragments of a different kind from
the flora in which they are embedded), the uplands are envi-
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DIMICHELE—DYNAMICS OF COAL AGE TROPICAL VEGETATION 147
Fig. 19 The upland concept. Upper drawing is a figure taken from
Cridland and Morris (1963; copyright 䉷1963, University of Kansas,
Museum of Natural History, used with permission). It shows the up-
land flora and lowland wetland flora in close spatial proximity. The
upland flora is envisioned as growing on local elevated areas within
the basinal lowland, drainage-creating conditions that differentiate the
two species pools. Lower drawing is modified from Broutin et al.
(1990). In this model, the upland flora lives under essentially the same
climate as the basinal wetland flora. Upland elements enter the basins
mainly during periodic storm events that, through happenstance, co-
incide with parts of the plant life histories when pollen and seeds or
branches are being shed (hence the label “abnormal sedimentation”).
Upland plant remains may rarely enter the lowlands by being carried
in streams. Different soil conditions, caused solely by drainage, sep-
arate the upland and lowland plants spatially. This diagram further
implies that seeming stratigraphic differences (the Stephanian vs. Au-
tunian floras) actually reflect different habitat preferences.
sioned as distant (Florin 1963; Falcon-Lang and Scott 2000;
Uhl et al. 2004; Gastaldo and Degges 2007; Gibling et al.
2010). Under such circumstances, the role of climate is reduced
in importance since transport might be expected to happen
under any climate in which there was enough water movement
to transport logs and/or plant debris long distances.
Climate control. In the other major view, nonwetland
plants lived in areas of seasonal climate, whether uplands or
extrabasinal lowlands; elevation is irrelevant to this model.
The seasonally dry plants then invaded basins, cratonic or
intermontane, during times of climate change in those basinal
locations (Elias in Moore et al. 1936; Arnold 1941; DiMichele
and Aronson 1992; Calder 1994; Gastaldo 1996; Blake et al.
2002; Ziegler et al. 2002; S
ˇimu˚ nek and Martı´nek 2009; Oplusˇ-
til et al. 2013b; Tabor et al. 2013b). This view differs from
the strictly upland view in that it explicitly includes a climatic
element in the equation and recognizes that the natural land-
scape is climatically complex, biome distributions being mainly
reflective of climatic patterns, primarily the interaction of cli-
mate and rainfall (Ko¨ ppen 1936; Walter 1985). In addition,
it places the dryland plants in the basins, not strictly on basinal
fringes. Elevation, in such a scenario, may cause changes in
the species composition within either the wetland or the sea-
sonally dry biome (S
ˇimu˚ nek and Cleal 2011) but will account
for whole-biome spatial differentiation only if it contributes
to climatic differences (which is what the plants are actually
responding to).
The climate control scenario is supported most strongly by
occurrences of conifers, which are the most reliable, abun-
dantly occurring indicators of seasonally dry conditions. Ex-
amples of conifers in places (a) far from any contemporaneous
upland areas and (b) in beds mixed within coal-bearing strat-
igraphic sequences are illustrated for Middle Pennsylvanian
occurrences in figure 18 and for Late Pennsylvanian strata in
figure 15. Such conifer occurrences include the reports of Elias
(in Moore 1936; see also Cridland and Morris 1963; Winston
1983; Feldman et al. 2005), Mapes and Rothwell (1988; see
also Cunningham et al. 1993; Hernandez-Castillo et al. 2009),
McComas (1988; see also Hernandez-Castillo et al. 2001b),
Falcon-Lang et al. (2009; fig. 18A,18B), Plotnick et al. (2009;
fig. 18C,18D), and Tabor et al. (2013b; fig. 15B). All are
preserved under peculiar circumstances, between coal beds, but
not as roof shales, representing falling stages of sea level and
early lowstand portions of glacial-interglacial cycles, and in
well-documented association with indicators of seasonal
drought. It is in these parts of the cycles where climate models
also, and independently, indicate that such seasonality is to be
expected (Horton et al. 2012). These unusually preserved de-
posits serve as a reminder that seasonally dry conditions are
most often highly unfavorable for the preservation of organic
matter (Gastaldo and Demko 2011); while we might not usu-
ally look in such strata for plant fossils, it is, nonetheless, here
that unusual floras are likely to be preserved.
Also indicative of climate are floras from intermontane ba-
sins of central Pangea, where conifers occur in parautochtho-
nous assemblages, sometimes in association with callipterid
peltasperms, including in close proximity to coal beds. Such
occurrences strongly suggest resident populations in the local
upland areas that moved into the intermontane basins during
periods of regional climatic change (S
ˇimu˚ nek and Martı´nek
2009; Oplusˇtil et al. 2013b). These macrofossil occurrences
have been well constrained by sedimentary analyses, and it is
clear that the plant remains were not subjected to long-distance
transport from the uplands but rather were derived from pop-
ulations living in the basins at the time of preservation. Such
occurrences are an important indicator of habitat heteroge-
neity, which could have been considerable in these mountain-
ous regions. A heterogeneity model, in combination with con-
sideration of the response of plants to regional climate changes
within these mountainous areas, could have important evo-
lutionary implications, as documented for modern mountain-
top islands (Hughes and Eastwood 2006; Kramer et al. 2011).
In mountainous terrain, it is possible that extensive micro-
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148 INTERNATIONAL JOURNAL OF PLANT SCIENCES
Fig. 20 Biogeographic patterns, Euramerica. Known occurrences
of seasonally dry and wetland biomes during a glacial-interglacial cy-
cle. The seasonally dry elements are shown in red. The wetland flora
is shown in green. In areas of mixed floras, where it is presumed that
seasonally dry conditions predominate, refugial pockets of wetland
flora are indicated by small green ovals. The interglacial map is Penn-
sylvanian and the glacial map Early Permian (it was necessary to use
the Early Permian map in order to show the sea in an off-continent
position during glacial maximum), although the floras shown arePenn-
sylvanian. Base maps courtesy of Ron Blakey, Northern Arizona
University.
habitat heterogeneity was present through all parts of glacial-
interglacial cycles due to variation in slope, aspect, and ele-
vation and that dryland floras oscillated in coverage area as
general climate changed. Such habitat heterogeneity is much
less likely to have been found in the extensive flat cratonic
areas of the American midcontinent (Watney et al. 1989), west
of the Appalachians.
Biogeographic patterns. The seasonal climate model is tied
more closely to the biogeography of seasonally dry floras than
is the elevation drainage model, and it (the climate model)
subsumes much of the matter of elevation, treating it as a
significant modifier of local to regional climatic patterns. Based
strictly on empirical data, referred to and discussed above,
there appear to be three areas of the Euramerican tropical and
paratropical belts in which plants adapted to seasonal drought
may have resided permanently, during all phases of glacial-
interglacial climatic fluctuations (fig. 20). In the Early and early
Middle Pennsylvanian, such areas, dominated by cordaita-
leans, are documented mainly in Atlantic Canada in central
Pangea (Falcon-Lang and Scott 2000; Falcon-Lang 2003b,
2006; Falcon-Lang and Bashforth 2004; Dimitrova et al. 2011;
Bashforth et al. 2014). In the late Middle and Late Pennsyl-
vanian, conifer-rich to conifer-dominated floras appear to have
resided in western regions of Pangea (Rueger 1996; Rothwell
et al. 1997; Falcon-Lang et al. 2011b; DiMichele et al. 2013b)
and in mountainous regions from the Appalachians through
Atlantic Canada (Lyons and Darrah 1989; Falcon-Lang et al.
2006; Dolby et al. 2011) into the Variscan highlands in central
Europe (Broutin et al. 1990; Doubinger et al. 1995; Dimitrova
and Cleal 2007; Bashforth et al. 2011). These persistent species
pools have been documented with autochthonous and alloch-
thonous macrofossils, including transported charcoal, and
with palynomorphs. They suggest resident high-elevation pop-
ulations throughout the Variscan-Appalachian mountainous
regions, perhaps outside of the regions subject to the highest
amounts of rainfall during glacial-interglacial cycles but still
within the area of moderate temperatures. The far western
Pangean floras appear to have existed at all elevations, reflect-
ing a general west-to-east gradient in moisture, particularly
from the later Middle Pennsylvanian into the Permian. As sum-
marized by Ziegler et al. (2002; fig. 5), walchian conifers are
known from many sites in Euramerica but only a few outside
of that area; thus, it is unlikely that populations of these plants
resided permanently in eastern Pangea (Angara and Cathaysia)
during drier periods.
The biogeographic distributions of seasonally dry plants are
imperfectly understood (an understatement). The likelihood of
their preservation as macrofossils is low, due both to their
position on the landscape and to the climatic conditions under
which the plants flourished (Gastaldo and Demko 2011). Con-
sequently, we see only glimpses of them, even in palynological
studies, which might be expected to capture these elements
more commonly than macrofossils. Nonetheless, it is certain
that one or more biomes, populated by plants tolerant of mild
to extreme seasonal moisture deficits, existed continuously in
the tropics along with the much better-known and better-doc-
umented wetland biome.
Climate models should play a significant role in helping us
understand the patterns of distribution of the seasonally dry
flora. Unlike the fate of the wetland biome during dry phases
of a glacial-interglacial cycle, the seasonally dry biome prob-
ably did not break up into isolated pockets or refugia during
the wetter parts of cycles. Admittedly limited but nonetheless
powerful evidence suggests that during the driest parts of gla-
cial cycles, when vertic, sometimes calcic, soils were forming
in basinal lowlands throughout nearly all of the cratonic in-
terior of equatorial Pangea, the seasonally dry flora migrated
into these areas and became the dominant vegetation in those
basins (Falcon-Lang et al. 2009, 2011b, 2011c; Plotnick et al.
2009). By extension, this flora also was likely dominant in the
low-latitude nonbasinal areas. These drier parts of cycles may
have exceeded the wetter parts in terms of the overall time
represented, which would mean that even though poorly rep-
resented in the fossil record, the seasonally dry biome may
have been the dominant vegetation type of the Pennsylvanian
tropics (Falcon-Lang et al. 2009). Additionally, if similar to
the modern world, the near-tropical regions may have been
more persistently seasonal than the equatorial tropics, meaning
that there would have nearly always been large areas of sea-
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DIMICHELE—DYNAMICS OF COAL AGE TROPICAL VEGETATION 149
sonally dry-wet climates throughout central Pangea within
which these dryland floras could have resided permanently.
Discussion
The perception of the Pennsylvanian tropics as an ever-wet
steaming jungle is a mistaken vision that is slowly loosening
its grip on terrestrial late Paleozoic paleontology, even as it
persists in the public mind. Climate modelers, sedimentolo-
gists, and stratigraphers have accommodated climatic variation
into their work. Climatic factors exert a strong and sometimes
primary control on sedimentation dynamics (Stanley and
Warne 1994; Peizhen et al. 2001) and, even through climatic
effects on erosion, greatly affect timing and rates of mountain
building (Montgomery et al. 2001; Grujic et al. 2006; Whipple
2009). The long-recognized cyclic patterns of sedimentation
in paleotropical regions were early attributed to glaciation in
the Southern Hemisphere (Coleman 1908) and were appre-
ciated by paleobotanists at the time (White 1907, 1909). Mod-
ern research on these subjects, as discussed above, has greatly
refined our understanding of Ice Age history and dynamics.
For the purposes of this article, it is most important to rec-
ognize, above all else, that there were regular and numerous
changes in climate, sea level, and sedimentation patterns,
driven proximately by changes in ice volume and its effects on
atmospheric circulation. If CO
2
were the ultimate cause of the
ice fluctuations, as models suggest, then the cause of oscilla-
tions in that variable remains opaque. The second point is that
these changes in climate were accompanied by great changes
in the spatial distribution of floras through time, a dynamic
that should play as strongly into our thinking about evolution
as it does about paleoecology. And, finally, it is the intention
here to provide a somewhat more refined understanding of the
details of where and when climatic changes of different types
occurred within glacial-interglacial cycles and what the effects
of those may have been on vegetational patterns. As a phe-
nomenon with global expression, climatic changes should have
enormous predictive power to help us understand floristic and
associated physical geological changes, even in areas where the
effects of sea level changes cannot be observed directly, such
as within mountain belts.
Taphonomy, the processes and events attendant fossilization,
is possibly the single greatest influence on perception of the
Pennsylvanian tropics as a vast, static wetland. Quite simply,
wetland vegetation is overwhelmingly more likely to be pre-
served, and beautifully so, than vegetation in seasonally dry
habitats. The strong selection against preservation of organic
matter in a dryland settings is, in general, a matter of three
factors, as discussed in detail by Gastaldo and Demko (2011).
These are amplified in important ways when considered in the
context of the LPIA. The first factor, and the most important,
is short-term burial by removal of the buried organic matter
from oxygenic decay—its burial below the vadose zone of the
water table. In periodically to perennially dry soils (forming
under dry subhumid to arid climates), this is much less likely
to happen than in wetlands, with high water tables, which can
be present under any kind of climate (though climate is likely
to dictate the areal extent of the wetlands). Thus, we should
rarely expect to find seasonally dry floras, particularly if, as
in the Pennsylvanian, they tend to occur in the falling stages
of sea level and early interglacial lowstands. These are times
of dropping water tables and landscape erosion. Wetlands, on
the other hand, are concentrated in mid- to late lowstand and
early phases of transgression during Pennsylvanian glacial cy-
cles. These are times when rising sea level creates ideal con-
ditions for intermediate-term burial below a column of sea-
water. This intermediate-term burial, if organic remains can
survive to this point, may allow sufficient time for long-term
burial due to the tectonically driven creation of accommoda-
tion space in basinal areas. Dryland floras and wetland floras,
within this framework of glacial-interglacial cycles, and their
directly and closely related changes in climate, sea level, and
sedimentation patterns have quite unequal chances of making
it into the fossil record. Thus, the wetland signature over-
whelms that of the drylands, which even though rarely and
usually poorly preserved may have been an important part of
and long resident in the Pennsylvanian lowlands (Falcon-Lang
et al. 2009).
Roof Shales versus Peat-Forming Floras:
Implications from Biogeography
The widespread occurrence of many elements of typical coal-
basin roof shale floras (Gastaldo et al. 1995) beyond the geo-
graphic extent of coal-bearing strata suggests that clastic sub-
strate floras do not provide a 1 : 1 representation of peat
substrate floras, at least in dominance-diversity structure. It
has sometimes been assumed that these two kinds of floras are
interchangeable and one is fully reflective of the other (Jennings
1986). However, examination of the composition of peat sub-
strate floras using coal balls (anatomically preserved peat
stages of the coal; see summary in Phillips et al. 1985) dem-
onstrates very different patterns of dominance from mineral
substrate floras. There are few genera that can be confidently
assigned to only one kind of substrate or the other (mineral
or peat), though there may be many such species. It is difficult
to detect species differences with confidence due to the very
different forms of preservation between coal balls and ad-
pressions (compressions and impressions; see Shute and Cleal
1986) and, hence, the different character suites on which much
of the taxonomy is based. Direct comparisons are possible and
have been made in some instances (e.g., pteridosperm foliage
species [Oestry-Stidd 1979; Mickle and Rothwell 1982; Beeler
1983; Reihman and Schabilion 1985; Cleal and Shute 2012;
Raymond et al. 2013], lycopsid stem taxa [Bateman et al.
1992; DiMichele and Bateman 1996]). However, comparisons
at a broader level show consistent differences in quantitative
aspects of composition, such as the more common occurrence
of the lycopsid Sigillaria or the greater dominance of pteri-
dosperms (King et al. 2011; Wagner and Castro 2011) in min-
eral substrate wetlands than in peat swamps. Some larger-scale
patterns, such as the qualitative rise in importance of the mar-
attialean fern clade in wetlands of the late Middle Pennsyl-
vanian (Phillips et al. 1974; Pfefferkorn and Thomson 1982;
Phillips and Peppers 1984; Peppers 1996; Dimitrova et al.
2005; Dimitrova and Cleal 2007) or the decline in wetland
cordaitales (Phillips and Peppers 1984; Raymond et al. 2010),
are detectable in both forms of preservation and appear to
occur at approximately the same time in both, despite differ-
ences in quantitative aspects (for the rise in tree ferns, cf. Pfef-
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150 INTERNATIONAL JOURNAL OF PLANT SCIENCES
ferkorn and Thomson 1984 and Phillips et al. 1985 or Peppers
1996; the timing is the same even though quantitative ex-
pression differs).
Also bearing on this matter are the discoveries of plants in
small intermontane basins in the Variscan portions of the Cen-
tral Pangean Mountains that contain unusual elements, par-
ticularly pteridosperms and ferns, related to but atypical of
roof shale assemblages (Cleal 2008b;S
ˇimu˚ nek and Cleal
2011). Such finds imply that even among the largely humid-
climate, wet substrate floras, there are differences that reflect
habitat heterogeneity. Such appearances of odd members of
various groups also have been reported from wetland-type flo-
ras in western regions of Pangea (Read 1934; Mamay and
Read 1956; Tidwell et al. 1988; Knaus and Lucas 2004;
DiMichele et al. 2013b), though many of these floras also
contain seasonally dry elements.
Dry Floras: Uplands versus Climate
The most persistent misconception about plants from sea-
sonally dry habitats is that they are unfailing indicators of
uplands. As reviewed above, this reflects the underlying sup-
position that under a common, presumably wet climate, slope
and elevation are both necessary and sufficient to create the
conditions under which these dry-habitat plants are favored
and where the lowland wetland plants cannot live. When con-
sidering the occurrences of these dry-habitat plants in the vast
expanses of the west-central Pangean interior, where there were
no mountains within thousands of kilometers, it became nec-
essary either to propose phantom uplands or to require that
small amounts of elevation were sufficient to support dry sub-
strate plants in the midst of the soggy bottomlands (Cridland
and Morris 1963). Spatiotemporal, biogeographic patterns
also conflict with upland models. Read and Mamay (1964),
for example, suggested that climate was drier in the western
United States earlier than in the east during the Pennsylvanian,
based on the small numbers of wetland floras known from the
western parts of Pangea and their concentration in the Early
Pennsylvanian, in combination with early occurrences of dry-
land plants of various kinds in the west and their later ap-
pearances in the east. Uplands play no role in such an expla-
nation because there were not systematic changes in elevation
from west to east within that time and space. Pfefferkorn
(1980) and Winston (1983) also recognized that dry and up-
land were not necessarily interchangeable concepts. Gastaldo
(1996), S
ˇimu˚ nek (2008), and Oplusˇtil et al. (2013a) all make
strong cases for climatic control of biomic dynamics, even in
intermontane regions of central Pangea, and conclude that they
reflect the same kind of climate cycles that are found in lowland
basins.
Can elevation alone (drainage), under uniformly humid cli-
mate, account for occurrences of the dryland flora? It must
be emphasized that cyclic patterns of lithological change,
though without any evidence of marine influence, can be de-
tected in intermontane regions (Cecil 2013; Oplusˇtil et al.
2013a, 2013b). These lithological sequences document
changes in prevailing climate similar to those in more classic
cyclothems of the paralic regions in west-central equatorial
Pangea—specifically, the alternation of seasonal subhumid to
semiarid climates with humid climates on the time frame of
glacial-interglacial cycles. This strongly suggests the existence
of an external allocyclic cause, paralleling that seen in marine-
influenced basins where the consequences of sea level rise and
fall are more apparent. Although tectonic effects may overprint
cyclicity, that lithological cyclicity can still be recognized is a
significant indicator of the importance of climate.
However, if one were to push aside the physical indicators
of glacially driven climate changes in tectonically active areas
with steep elevational gradients, the following questions might
be asked: Does evidence suggest that elevation alone, under a
constant, humid background climate (which is necessary to
form peat/coal any time or place other conditions permit) is
sufficient to explain occurrences in basins (presumably via
long-distance transport) of cordaitaleans, conifers, and other
taxa otherwise atypical of wetland assemblages? Is it reason-
able to assume that slopes alone will create enough soil drain-
age to result in periodic or nearly continuous moisture stress
and that such soil drainage will be the sole factor setting these
habitats and their floras apart from low-lying bottomlands,
again under invariantly humid background climate? In other
words, are drylands set apart from wetlands simply due to the
effects of soil drainage, were climate to be held constant as
humid/perhumid? Note that one must make an explicit as-
sumption about the climate mode because low-ash peat (as a
parent material to economic coal beds), which is part of these
landscapes, will not form under climates with high seasonality
of rainfall. The answer to all these questions, in brief, appears
to be no. A more subtle interpretation, however, would suggest
that elevation, if high enough to engender climatic and asso-
ciated microclimatic changes, might have such effects, though
such elevation would have to be considerable in the tropics.
There are opportunities to assess empirically the contribu-
tion of elevational heterogeneity to total diversity in Pennsyl-
vanian basinal lowlands. Several reports from within the Var-
iscan mountain belt in central Europe describe narrow,
steep-sided valleys with coal at the elevation of the valley
floors, possibly at base elevations of 1000 m or more (Oplusˇtil
2005). Clastic deposits associated with the coals contain floras
that appear to reflect wet climates. These floras are drawn
almost entirely from wetland evolutionary lineages, even if
some species and genera of pteridosperms and ferns are rarely
found in typically lowland wetland settings from the interior
Pangean and marginal-Variscan flatlands (Cleal 2008b;S
ˇi-
mu˚ nek and Cleal 2011). It has been hypothesized that the steep
walls of these narrow basins supported a flora that, though
adapted to wet climates, was specialized for growth under
conditions of less waterlogging than found in the basin floors,
where coals developed (Oplusˇ til and Cleal 2007). These as-
semblages, however, lack elements typical of the seasonally dry
flora, mainly conifers or known minor associates of the dom-
inant elements. Similarly, Wagner and A
´lvarez-Va´ zquez (2010;
and, by default, Bashforth et al. 2010) note the paucity of
conifers in Late Pennsylvanian Iberian Peninsula floras, despite
the presence of abundant well-drained mountainous habitats,
including high-elevation alluvial fans, in proximity to basinal
environments. On a smaller scale, Feldman et al. (2005) con-
trast several examples of Late Pennsylvanian valley fills from
late interglacial to early glacial time periods. Those that are
deep and reflect large drainage basins, high rainfall, and high
water throughput lack conifers. In contrast, shallow valleys
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DIMICHELE—DYNAMICS OF COAL AGE TROPICAL VEGETATION 151
reflective of small drainage basins that formed under more
strongly seasonally dry climates (corroborated by the char-
acteristics of time-equivalent paleosols and sedimentology)
contain floras dominated by xeromorphic taxa, often including
a much larger spectrum of characteristic seasonally dry plants.
Thus, again, where there is evidence of significant topography,
the dominant dryland elements are found only where and when
there is additional evidence of climatic seasonality of moisture.
Such patterns suggest that, much like the modern tropics,
under conditions of high rainfall of a nearly aseasonal to
weakly seasonal pattern, slope and drainage effects are reduced
in importance as agents of habitat differentiation (e.g., British
peat lands; Moore 1987). In modern tropical wet forests under
climatic regimes where rainfall exceeds evapotranspiration for
most of the year, accompanied by only a limited dry season
(humid to perhumid climates in the terminology of Cecil and
Dulong 2003), effects of elevational heterogeneity are sup-
pressed and found not to contribute significantly to landscape-
level (beta) biodiversity (Davidar et al. 2007). Rather, the main
control on beta diversity appears to be variation in the sea-
sonality of rainfall and hence the number and length of periods
of drought across a landscape, to which most tropical trees
are highly sensitive (Davidar et al. 2007; Engelbrecht et al.
2007). Gradients in seasonality of rainfall contribute to in-
creases in beta diversity under such conditions. Seasonal mois-
ture stress, however, will reduce site (alpha) diversity at any
given point even as it creates regional diversity gradients.
Changes in dryland flora through time—what happened to
the cordaitalean-dominated assemblages? Cordaitaleans
were the unquestionable monarchs of the seasonally dry trop-
ics from the Mississippian to the mid–Middle Pennsylvanian
(Falcon-Lang and Scott 2000; Falcon-Lang and Bashforth
2005; S
ˇimu˚ nek 2008), often accounting for 175% of the bio-
mass in many assemblages (Falcon-Lang 2003b). They were
accompanied by an interesting array of other seed plants. The
whole system gradually diminished during the later Middle
Pennsylvanian, and its frequency tapers off considerably in the
Late Pennsylvanian. S
ˇimu˚ nek (2008) notes that these floras are
not found in environments that appear to be semiarid but
rather are associated with floras and physical environments
that appear to be humid, perhaps trending to subhumid
(Oplusˇ til et al. 2005). After considering a large number of
recorded occurrences of cordaitalean-dominated seasonally
dry floras, Bashforth et al. (2014) conclude, in agreement with
the above authors, that cordaitaleans occupied seasonal but
relatively humid environments.
Conifers, on the other hand, are often found, even in their
earlier parautochthonous occurrences (Plotnick et al. 2009),
in association with settings that appear to have been more
seasonally dry than those in which cordaitaleans were domi-
nant. Conifers, via pollen occurrences, are suspected to have
appeared in the late Mississippian (Turner et al. 1994; Falcon-
Lang 2006), but they do not really become important parts of
equatorial floras until the Late Pennsylvanian, where there are
numerous occurrences throughout Euramerica, as discussed
above. Conifers also are associated with a wide array of other
kinds of seed plants, often with considerably xeromorphic fo-
liar morphologies. Many conifer-containing assemblages are
dominated or codominated by these other kinds of plants, such
as callipterids. This is in contrast to the cordaitalean assem-
blages where the minor constituents rarely are prominent el-
ements quantitatively, in terms of biomass. Where such oc-
currences do occur, they often are on substrates that suggest
strong edaphic controls, such as floras containing abundant
Megalopteris or Lesleya.
These patterns of gradual transition from cordaitalean to
coniferalean dominance as the major seasonally dry biome
correlate with a gradual drying trend in the tropics through
the late Middle Pennsylvanian and Late Pennsylvanian. These
independently determined climatic trends reveal spreading sea-
sonality in the tropical realm. This seasonality becomes ap-
parent even during the wet phases of glacial-interglacial cycles
beginning near the Desmoinesian/Atokan (Bolsovian-Asturian)
boundary, revealed by changes from ombrotrophic domed
peats to rheotrophic planar peats (Cecil 1990), associated with
increases in coal sulfur (Cecil et al. 1985; Neuzil et al. 2005)
and with changes in sandstones, indicating a transition from
wet low-seasonality climates to greater seasonality (Bertier et
al. 2008). The wetland flora also undergoes many composi-
tional and dominance-diversity changes at this same time (Pfef-
ferkorn and Thomson 1982; Phillips and Peppers 1984; Pep-
pers 1996), much more abruptly than the seasonally dry flora.
Ecological and Evolutionary Implications
Consideration of climate, but within a glacial-interglacial,
oscillating context, has many profound implications for un-
derstanding vegetational dynamics and for possible controls
on evolutionary patterns. We will touch on two here.
Periodic reassembly of the wetland biome versus spatial mi-
gration of the seasonally dry biome. Analysis of the wetland
biome, particularly that of peat-forming landscapes, indicates
a considerable amount of conservatism in species composition
and dominance-diversity structure through millions of years
and multiple glacial-interglacial cycles. This assessment has
been based on both macrofossils and microfossils recovered
from successive coal beds (Phillips et al. 1985; Willard and
Phillips 1993; DiMichele and Phillips 1996b; Peppers 1996;
Pfefferkorn et al. 2000, 2008; DiMichele et al. 2002; Willard
et al. 2007), representing the wettest parts of glacial-intergla-
cial cycles. It appears, furthermore, to be undergirded by con-
siderable phylogenetic conservatism in the resource centroids
of the major clades, resulting in phylogenetically conservative
patterns of species replacement through time (DiMichele and
Phillips 1996a), so-called niche conservatism (this is currently
an active field of study; see Ackerly 2003; Wiens 2004; Wiens
and Graham 2005; Wiens et al. 2010). Niche conservatism is
particularly important here because of the much lower diver-
sity of the late Paleozoic, in general, than of the modern. We
are used to thinking of large amounts of heterogeneity and
strongly individualistic responses of species to changing cli-
mates and physical conditions in the modern world (Jackson
2006; Mascaro et al. 2013), and no doubt this is so at some
spatiotemporal scale. However, there do seem to be biomic
bounding conditions, set by large-scale climatic effects and
physiography, and this can be quite clearly discerned in the
less cluttered world of the Paleozoic. When niche conservatism
is considered, the marked clade-by-environment patterns of
species replacement through time suggest that, as proposed by
Wiens (2004; see also Kozak and Wiens 2006), niche conser-
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152 INTERNATIONAL JOURNAL OF PLANT SCIENCES
vatism may promote speciation by restricting and fragmenting
species distributions during times when their favored habitats
become aerially restricted.
Recurrence of similar wetland dominance-diversity patterns
from one period of humid/perhumid climate to the next most
likely requires reassembly of this vegetation, given the indi-
cations that these floras became strongly constricted to refugia
during the dry portions of climate cycles. The exact pathways
of reassembly are most likely dictated first by the dispersal
capacities of the various species and second by the capacity of
these species to survive under the limited-nutrient, long-
flooded conditions found in Pennsylvanian wetlands. It may
be that this latter capacity, in particular, favored the repeated
reexpansion of the arborescent lycopsids, many of which had
large but floatation-equipped, seedlike dispersal units (Lepi-
docarpon,Achlamydocarpon; Phillips 1979) and specialized
physiologies favoring growth under highly stressed, flooded
conditions (Green 2010). That same flooding, in contrast, may
have suppressed the expansion of the marattialean tree ferns,
which were equipped, it would appear, with much higher dis-
persal capacities, via their billions of small isospores, possibly
more rapid attainment of sexual maturity (cheap construction
compared to a lycopsid tree, many of the latter having mono-
carpic growth habits as well), but also a requirement for a
suitable place for growth and sexual reproduction carried out
by a surficial gametophytic prothallus. Despite beginning to
increase in abundance in the Middle Pennsylvanian, it was not
until catastrophic environmental changes eliminated the ly-
copsid hegemony, near the Middle–Late Pennsylvanian bound-
ary, that the tree ferns rapidly expanded to dominance in
wetlands.
In contrast to these kinds of dynamics, seasonally dry floras
appear to have maintained large standing crops of trees and
associated smaller plants in many parts of the Pangean tropical
belt, even during the least favorable times for them, when
humid climates and wetlands were widespread. This is true of
both the earlier cordaitalean-dominated forests and the later
conifer-dominated woodlands, with their associated callipter-
ids, taeniopterids, and cordaitalean-forest holdovers. Evidence
of the continued existence of these plants during what were
the wetter periods in the coal basins is suggested by both the
presence of coniferophytes in western Pangea seemingly at all
times from the late Middle Pennsylvanian onward and their
presence in the mountainous regions of central Pangea, re-
vealed mainly by pollen. In the same way, cordaitaleans, which
may have been less tolerant of severe drought than the conifers
and more tied to mesic habitats, are revealed by both pollen
and macrofossils during what appear to be the wetter parts of
glacial-interglacial cycles. Interpreting the palynological record
of cordaitaleans is problematic because of the conservatism of
their pollen morphology and the great breadth of ecological
preference encompassed by the clade.
As a consequence of the large acreage evidently occupied by
the seasonally dry biomes during their respective times of trop-
ical maxima and minima, their spatiotemporal patterns appear
to be controlled largely by expansion and contraction of pop-
ulations across the area of favorable habitat. During such pe-
riods of expansion and contraction, most of the species that
made up these ecosystems likely maintained continuity of large
panmictic populations, given almost certain wind-pollinated
life histories. This is quite a different pattern from that pro-
posed for the wetlands, one of significantly greater organiza-
tional conservatism. As yet unpublished data of W. DiMichele
and colleagues, mainly from the western parts of Pangea, sug-
gest that these species assemblages did not show any note-
worthy changes during times of significant climatic change at
either the Atokan-Desmoinseisan boundary or the Middle–
Late Pennsylvanian boundary.
Furthermore, the change from cordaitalean dominance to
conifer dominance appears to have taken place gradually, as
well. Thus, one might think of these assemblages as buffered
against catastrophic changes by their large population sizes,
capacities to tolerate moisture stress (and temperature stress
(?), given their mountainous distributions in some areas), avail-
ability of migratory routes, and K-selected reproductive modes.
Evolutionary implications of plant ecologies #climate fluc-
tuations. The potential for isolation of populations, wetland
or seasonally dry, during parts ofglacial-interglacial cycles may
have been a powerful engine of evolutionary change. This will
manifest itself in different ways for populations of seasonally
dry and wetland plants. All of the dominant seasonally dry
plants were wind-pollinated seed plants, and most of the less
common forms also were seed plants. Wetlands, by contrast,
were dominated by a broad spectrum of both homosporous
and heterosporous lower vascular plants and various kinds of
primitive seed plants, which, whereas mainly wind pollinated
or dispersed, may also have been obligately tied to water for
reproduction or dispersal. Thus, these two floras possibly had
different average patterns of dispersal limitation that affected
their evolutionary patterns differently. That said, there also are
many life-history similarities among the dominant elements of
these two kinds of floras. The most prominent of these simi-
larities is the small size and dispensability of the microspore/
isospore phase of their life histories. Also similar is the gigantic,
apparently continuous, distributions of these plants during the
respective climate phases of glacial-interglacial cycles favorable
to each.
Wetland plants, as discussed by Falcon-Lang and DiMichele
(2010), ought to have been strongly influenced by the periodic
confinement to refugial wetlands during interglacials through
early glacial maxima. This is primarily due to the reduction
or elimination of many dispersal pathways, potentially re-
stricting gene flow, thus creating conditions for the evolution
of new species in small, isolated populations. Such restrictions
certainly became more severe and the isolation of populations
ever greater during the later Middle Pennsylvanian (Des-
moinesian) and early Late Pennsylvanian (Missourian), when
much evidence indicates increasing aridity in the tropical realm
(wets less wet, drys more dry), possibly tied to extensive melt-
ing of polar ice (Fielding et al. 2008; Rygel et al. 2008). In
the midst of this interval, there is a great turnover in the flora,
accompanied by the loss of most of the dominant Middle Penn-
sylvanian lycopsids (Phillips et al. 1974; Heckel 1991a), a
change in the common tree ferns (Lesnikowska 1989), and
many changes in the pteridosperm component of wetlands
(Phillips 1981; Phillips and Peppers 1984; DiMichele et al.
2006a; Wagner and A
´lvarez-Va´ zquez 2010). From a life-his-
tory viewpoint, perhaps the least likely to produce new species
during these periods of habitat contraction and isolation were
the arborescent lycopsids. Most of these heterosporous plants
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DIMICHELE—DYNAMICS OF COAL AGE TROPICAL VEGETATION 153
produced large volumes of highly dispersible microspores,
which often are abundant even in strata deposited during the
drier periods (Turner et al. 1994; Falcon-Lang et al. 2009;
Hawkins et al. 2013). The near disappearance of these plants
at the Middle–Late Pennsylvanian boundary thus may reflect
a particularly severe period or series of sequential intervals of
intense fragmentation of their populations (Phillips and Pep-
pers 1984; Heckel 1991a). Tree ferns similarly may have been
considerably insulated from founder effects during periods of
population contraction due to both their prolific production
of small isospores and the subsequent high dispensability of
these spores (Lesnikowska 1989; Millay 1997). At the other
end of the spectrum were the medullosan pteridosperms. These
plants had some of the largest pollen grains in seed plant his-
tory, which begs the question of animal pollination (Taylor
1978; Taylor and Millay 1979). Experimental analysis of this
type of pollen indicates little likelihood of wind delivery
(Schwendemann et al. 2007). In an interesting twist on this
matter, Peter Crane (personal communication) hypothesized
that the medullosans may have had life histories similar to
those of heterosporous plants, wherein their pollen was water
dispersed and swimming sperm reached ovules without deliv-
ery of pollen to a pollen chamber. Such a life history would
be very different from that of the lycopsids, because neither
the microspore nor the seed/dispersal organ is particularly
suited to wide dispersal, effectively permitting high levels of
speciation in response to isolation. In keeping with the size
proportions of their pollen grains, medullosan seeds are the
largest of the Pennsylvanian tropical flora (Sims 2012) and
may have floated, like small coconuts (Falcon-Lang 2009 has
suggested a mangrove habit for Macroneuropteris scheu-
chzeri), or required animal vectors for dispersal (many have
fleshy sarcotestas, which may have attracted animals, and thick
sclerotestas, which may have resisted crushing and passage
through the gut). Despite many synthetic studies of these
plants, however, including paleoecology, biogeography, and
phylogentic relationships (Phillips 1981; Stidd 1981; Di-
Michele et al. 2006a; Hilton and Bateman 2006; Hilton and
Cleal 2007; Cleal 2008b), there appears to be only one study,
that of Raymond and Costanza (2007), that investigates spe-
cies durations of medullosans and compares them to those of
other groups; these authors find that, indeed, medullosan spe-
cies have shorter durations than wind-pollinated wetland cor-
daitalean or callistophytalean seed plants.
There are reasons to expect that the effects of glacial-inter-
glacial climate changes on dryland flora may have been qual-
itatively different from those of the wetlands, but this too has
not been investigated, and such investigation may be more
challenging than for wetland plants because of the poor pres-
ervation of tropical dryland species during the Pennsylvanian,
limiting understanding of species-level spatiotemporal pat-
terns. Nonetheless, there are interesting patterns worth noting
that prompt some speculation. Most of the dryland plants were
wind pollinated. Studies of modern wind-pollinated plants
subject to isolation in fragmented landscapes suggest high lev-
els of gene flow created by pollen dispersal. Documentation
of this pattern in conifers (Jørgensen et al. 2002; Liepelt et al.
2002; O’Connell et al. 2007) may have the most applicability
to the late Paleozoic, where the two dominant groups in sea-
sonally dry habitats, cordaitaleans and conifers, are both con-
iferophytes in a broad evolutionary sense (Hernandez-Castillo
et al. 2001b; Rothwell et al. 2005; Looy 2007). High gene
flow might be expected to have kept speciation rates low in
these groups, regardless of fluctuating climates. However, re-
cent studies of both cordaitaleans (S
ˇimu˚ nek 2000, 2007; Ray-
mond et al. 2010) and conifers (Rothwell et al. 1997; Her-
nandez-Castillo et al. 2001b, 2009) indicate greater species
numbers than previously appreciated. This can be considered
from a life-history perspective. Both cordaitaleans and prim-
itive walchian conifers have large pollen grains of primitive
architecture compared to more advanced groups of conifers
(Gomankov 2009), generally produced in simple pollen cones
(Mapes and Rothwell 1998). Compound cones, paralleling
ovulate structures in organization, also have been found (Her-
nandez-Castillo et al. 2001a). Additionally, these early groups
have much simpler ovulate cone architectures than modern
conifers (Florin 1951; Mapes 1987; Kerp and Clement-West-
erhof 1991; Hernandez-Castillo et al. 2001b), which may have
given them much less efficient pollen capture than modern
groups with compact cones (Niklas 1985; Owens et al. 1998).
In addition, pollen morphology appears to track the patterns
of ovulate cone evolution (Gomankov 2009). Thus, one might
speculate that in these primitive coniferophytes, given the pe-
riodic isolation of their populations, particularly in the moun-
tainous regions of central and western Pangea, restriction of
gene flow may have been more likely than in modern groups.
Perhaps paralleling the patterns found in other wind-pollinated
or wind-micro/isospore-dispersed Pennsylvanian taxa, these
primitive wind-pollinated plants may have had higher rates of
speciation than those seen in modern coniferophytes subject
to periodic isolation during the present ice ages.
Conclusions
The Pennsylvanian ice age provides an excellent point of
comparison to our modern period of cool Earth climate, each
dominated environmentally by the framework of glacial-
interglacial cyclicity. Differences from today in the composition
of the terrestrial biota and the diversity of that biota make the
LPIA world a place within which to look for generalities about
ecological and evolutionary dynamics, otherwise formulated
on the basis of our thin, modern slice of time (extended back
a bit into the Pleistocene).
Long study of the late Paleozoic tropical realm leads me to
a number of (unsurprising) conclusions.
1. Environmental change, often of a catastrophic, cataclys-
mic, or just plain old really rapid nature, happens often. Such
change seems to give little in the way of warning, at least on
the spatiotemporal scales detectable by paleontological reso-
lution, suggesting the existence of biotic thresholds. The
crossing of such thresholds leads to so-called regime shifts,
which are rapid and often irreversible. Furthermore, change
of smaller-scale similarly is the rule rather than the exception.
2. The late Paleozoic tropics were characterized by a number
of distinct biomes, divisible principally into a humid to per-
humid wetland group and a subhumid to semiarid dryland
group. Each of these has a great deal of variability, reflecting
habitat preferences and the vagaries of dispersal limitations
and incumbency.
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154 INTERNATIONAL JOURNAL OF PLANT SCIENCES
3. Plants of the late Paleozoic reflected climate closely and
faithfully, much like they do today.
The Pennsylvanian Coal Age tropics were not wet. They
alternated between wet and dry with each glacial-interglacial
cycle. The degree of development of either of these climate
extremes varied from one cycle to the next, but a long-term
trend of drying and warming is suggested.
4. The evolutionary dynamics of the wetland and seasonally
dry species pools were different. The difference reflects the
periodic contraction into and expansion from refugia of the
wetlands within the tropical belt versus the persistent wide-
spread, highly connected habitats occupied by the dryland spe-
cies in western and parts of central Pangea.
Acknowledgments
I would like to extend particular thanks to Tom Phillips,
Blaine Cecil, Isabel Montan˜ ez, and John Nelson, who, over
many years, have contributed extensively to my understanding
of the late Paleozoic world—they bear no responsibility for
my misunderstandings or misinterpretations. Sincere thanks to
Mary Parrish, Paleobiology Department, NMNH, for prepa-
ration of the artwork. I also owe a great deal to Alan Archer,
Arden Bashforth, Richard Bateman, Dan Chaney, Peter Crane,
Scott Elrick, Howard Falcon-Lang, Robert Gastaldo, Martin
Gibling, Robert Hook, Hans Kerp, Cindy Looy, SpencerLucas,
Hermann Pfefferkorn, Chris Poulsen, and Neil Tabor for dis-
cussion of many and various matters and for freely sharing
their ideas. Arden Bashforth’s reading of this article is grate-
fully acknowledged. The comments of Hermann Pfefferkorn
and two anonymous reviewers improved the article signifi-
cantly. Appreciation is extended to Pat Herendeen for the in-
vitation to write this review and for his patience while it was
being completed. Much of the fieldwork that undergirds this
discussion was financed by various grants from the Smithso-
nian Institution, the National Museum of Natural History, the
National Science Foundation, and the Bureau of Land
Management.
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