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vol. 165, no. 6 the american naturalist june 2005 !
Eocene Plant Diversity at Laguna del Hunco and
Rı´o Pichileufu´ , Patagonia, Argentina
Peter Wilf,
1,
*
Kirk R. Johnson,
2,†
N. Rube´n Cu´ neo,
3,‡
M. Elliot Smith,
4,§
Bradley S. Singer,
4,k
and
Maria A. Gandolfo
5,#
1. Department of Geosciences, Pennsylvania State University,
University Park, Pennsylvania 16802;
2. Department of Earth Sciences, Denver Museum of Nature and
Science, Denver, Colorado 80205;
3. Museo Paleontolo´gico Egidio Feruglio, Trelew, Chubut 9100,
Argentina;
4. Department of Geology and Geophysics, University of
Wisconsin, Madison, Wisconsin 53706;
5. L. H. Bailey Hortorium, Department of Plant Biology, Cornell
University, Ithaca, New York 14853
Submitted October 1, 2004; Accepted February 9, 2005;
Electronically published April 7, 2005
Online enhancement: appendix.
abstract: The origins of South America’s exceptional plant diver-
sity are poorly known from the fossil record. We report on unbiased
quantitative collections of fossil floras from Laguna del Hunco (LH)
and Rı´o Pichileufu´ (RP) in Patagonia, Argentina. These sites represent
a frost-free humid biome in South American middle latitudes of the
globally warm Eocene. At LH, from 4,303 identified specimens, we
recognize 186 species of plant organs and 152 species of leaves. Ad-
justed for sample size, the LH flora is more diverse than comparable
Eocene floras known from other continents. The RP flora shares
several taxa with LH and appears to be as rich, although sampling
is preliminary. The two floras were previously considered coeval.
However,
40
Ar/
39
Ar dating of three ash-fall tuff beds in close strati-
graphic association with the RP flora indicates an age of 47.46 !
Ma, 4.5 million years younger than LH, for which one tuff is0.05
reanalyzed here as Ma. Thus, diverse floral associations51.91 !0.22
in Patagonia evolved by the Eocene, possibly in response to global
warming, and were persistent and areally extensive. This suggests
*Correspondingauthor;e-mail:pwilf@psu.edu.
†
E-mail: kjohnson@dmns.org.
‡
E-mail: rcuneo@mef.org.ar.
§
E-mail: msmith@geology.wisc.edu.
k
E-mail: bsinger@geology.wisc.edu.
#
E-mail: mag4@cornell.edu.
Am. Nat. 2005. Vol. 165, pp. 634–650. "2005 by The University of Chicago.
0003-0147/2005/16506-40660$15.00. All rights reserved.
extraordinary richness at low latitudes via the latitudinal diversity
gradient, corroborated by published palynological data from the Eo-
cene of Colombia.
Keywords: biodiversity, Eocene, geochronology, paleobotany, paleo-
climate, Patagonia.
South American floras today are marked by high diversity
and endemism. Most notably, Neotropical plant diversity
exceeds other tropical regions by factors of two to three
(Gentry 1988b;Davisetal.1997;PhillipsandMiller2002).
Elevated richness exists outside of the main equatorial belt,
including the Mata Atlaˆntica region of southeastern Brazil
(Mori and Boom 1981). Davis et al. (1997) designated 46
areas of the continent as centers of floral diversity and
endemism, including several extratropical locales such as
the central Chilean Mediterranean region and the Gran
Chaco of Brazil, Bolivia, Argentina, and Paraguay.
Because of a general lack of reliable, quantitative data
from plant macrofossils, remarkably little is known about
the origins, history, and geologic context of South Amer-
ican floral diversity, especially before the Pleistocene. Mod-
ern tropical weathering, vegetative cover, a paucity of in-
vestigators, and the obsolescence of historical reports
account for the scarcity of information from equatorial
latitudes for the Paleogene (Romero 1986; Mello et al.
2002; Burnham and Johnson 2004) and Neogene (Burn-
ham and Graham 1999). Most hypotheses for the Neo-
tropical region have emphasized geologically recent events.
Neogene uplift of the Andes is thought to have fostered
diversification (Van der Hammen and Hooghiemstra 2000;
Colinvaux and De Oliveira 2001), and a palynological
study indicates high floral richness during the Miocene in
Colombia (Hoorn 1994). The prevailing model for several
decades, that Pleistocene rain forest refugia catalyzed cur-
rent biodiversity as sites of allopatric speciation (Haffer
1969; Haffer and Prance 2001), has been strongly chal-
lenged (Nelson et al. 1990; Colinvaux et al. 1996, 2001;
Moritz et al. 2000; Kastner and Gon˜i 2003). Evidence for
timing of diversification in various lineages mostly comes
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Eocene Plant Diversity in Patagonia 635
Figure 1: Locations of Laguna del Hunco, Rı´o Pichileufu´, and cities
mentioned in text, mapped onto 50-Ma positions of modern coastlines,
Mollweide Projection. Reconstruction made using the Plate Tectonic
Reconstruction Service of the Ocean Drilling Stratigraphic Network,
http://www.odsn.de/odsn/services/paleomap/paleomap.html, based on
data from Hay et al. (1999).
from molecular clock studies, constrained using the few
fossil data that are taxonomically reliable (Richardson et
al. 2001; Bremer 2002; Davis et al. 2004, 2005).
In contrast to conventional literature, two recent in-
vestigations indicate elevated plant diversity in South
America during the early Eocene, 30 million years before
significant Andean uplift, 50 million years before the Pleis-
tocene, and on a warm and humid continent without Pleis-
tocene analog refugia. First, palynological data from Co-
lombia quantify significant, apparently in situ plant
diversification in association with global warming and in-
creasing regional precipitation across the Paleocene/Eo-
cene boundary (Jaramillo and Dilcher 2000; Jaramillo
2002). Second, and antecedent to this article, is a prelim-
inary macrofloral study from Laguna del Hunco (LH) in
the northern Patagonian region of Argentina (Wilf et al.
2003; fig. 1). During the globally warm Eocene, when trop-
ical organisms reached middle and high latitudes of both
hemispheres (Estes and Hutchison 1980; Greenwood and
Wing 1995), LH was located near the southern limit of
Neotropical floral influence (Romero 1978, 1986, 1993;
Hinojosa and Villagra´n 1997). Today, LH is a desert area
with abundant exposures of fossiliferous strata. Analyses
of quantitative, stratigraphically controlled, unbiased col-
lections suggested that when adjusted for sample size, LH
is the most diverse fossil flora of Eocene age known any-
where in the world (Wilf et al. 2003). However, sample
sizes at individual quarries were small compared to North
American floras used for comparison (∼175–315 speci-
mens vs. typically more than 500 specimens). The report
also demonstrated that data from elsewhere in Patagonia
are needed to test the pattern of elevated floral richness
in time and space.
New results are presented here from significantly ex-
panded collections at Laguna del Hunco and initial col-
lections from Rı´o Pichileufu´ (RP; fig. 1), a rich Eocene
site about 160 km NNW of LH that has received negligible
investigation since the 1930s (Berry 1938). We test the
observation of elevated floral diversity at Laguna del
Hunco using a sample size approximately triple that of
the preliminary report (Wilf et al. 2003). We use paleo-
ecological data to ask whether the LH floras are compo-
sitionally stable through time, and we refine paleoclimatic
estimates and biome interpretation. Using the first quan-
titative paleobotanical data from Rı´o Pichileufu´, we ask
whether its flora had diversity comparable to LH. An ac-
curate geochronology for the floras, conventionally as-
sumed to be coeval, is critical for addressing several eco-
logical questions, including persistence. We analyze ashfall
tuffs intimately associated with the fossil plants at RP to
determine the first high-precision ages for the flora, and
we reanalyze a tuff from LH to resolve its age more finely.
We conclude by discussing the implications of our results
for understanding past and present South American
biodiversity.
Laguna del Hunco and Rı´o Pichileufu´Floras
Overview and Previous Work
The classic fossil floras from Laguna del Hunco (“Lake of
Reeds”) and Rı´o Pichileufu´ (“Little River”) have a long
history of investigation (Berry 1925, 1938), and they fea-
ture prominently in phytogeographic and paleoclimatic
literature about the continent (Mene´ndez 1971; Arago´n
and Romero 1984; Romero 1986, 1993; Hinojosa and Vil-
lagra´n 1997) and the Southern Hemisphere in general
(Christophel 1980; Hill 1994). However, these assemblages
are historically understudied and are mostly represented
by small museum collections made from limited numbers
of quarries collected near the surface without reliable strat-
igraphic or relative abundance data.
The two floras feature very good (RP) to outstanding
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636 The American Naturalist
Figure 2: Stratigraphic section of the Tufolitas Laguna del Hunco, revised
and simplified from Wilf et al. (2003). Shown are the stratigraphic po-
sitions of fossil plant localities (LH-1 to LH-25), the samples yielding
radiometric ages (numbers in gray), including the reanalyzed age reported
here for the uppermost sample, and the paleomagnetic interpretation.
Asterisks indicate the four major sampling localities. Only 10 identifiable
fossils were found above the labeled densely sampled interval. Localities
19, 20, and 21 are stratigraphically isolated and not included in the figure.
See Wilf et al. (2003) for details of stratigraphy, lithology, and paleo-
magnetic and previous radioisotopic analyses.
(LH) preservation and are located at similar middle pa-
leolatitudes in the Patagonian region of Argentina (fig. 1).
Laguna del Hunco is a desert pond in Chubut Province,
located in the midst of the major fossil exposures. The
floras from LH are the major focus of this article because
of our extensive, recent collections; they are well exposed
and feature exceptional preservation equal to that of well-
known Eocene floras from North America such as Floris-
sant (MacGinitie 1953) and Republic (Wolfe and Wehr
1987). The Rı´o Pichileufu´floraislocatedneartheepon-
ymous stream in Rı´o Negro Province, 160 km northwest
of LH, and we report preliminary data based on initial
collections.
No significant uplift of the Southern Central Andes oc-
curred before the Miocene (Marshall and Salinas 1990);
Eocene Laguna del Hunco lay at a low elevation, and a
significant maritime influence moderated its climate. The
LH flora was deposited in tuffaceous mudstones and sand-
stones of the Tufolitas Laguna del Hunco, a lacustrine unit
of the middle Chubut River volcanic-pyroclastic complex
(Arago´n and Romero 1984; Arago´n and Mazzoni 1997)
and is inferred to represent lakeshore vegetation (Wilf et
al. 2003). The RP flora comes from volcanic lake beds of
the Ventana Formation and is probably similar in depo-
sitional setting to LH (fig. 1; Gonza´lez Dı´az 1979; Arago´n
and Romero 1984). The LH tuffs are well exposed over
an area of more than 25 km in diameter (Arago´n and
Mazzoni 1997), but fossiliferous horizons at Rı´o Pichileufu´
are restricted to a single small drainage (!300 m of map
distance separates the fossil quarries). Compressions of
flowers, fruits, seeds, and especially leaves are abundant
at both sites. At LH, fossil fish, insects (Fidalgo and Smith
1987; Petrulevicius and Nel 2003), caddis-fly cases (Genise
and Petrulevicius 2001), and pipoid frogs (Casamiquela
1961; Ba´ez and Trueb 1997) are present on the same bed-
ding planes as the plants. At RP, large-bodied ants, prob-
ably referable to Archimyrmex piatnitzkyi (Viana and
Haedo-Rossi 1957; Dlussky and Perfilieva 2003), co-occur
with plants, as do frogs (P. Wilf and K. R. Johnson, per-
sonal observation).
Fossil plants at Laguna del Hunco were discovered dur-
ing the 1920s (Clark 1923; Berry 1925), and occasional
new descriptions and taxonomic revisions have since been
published (Berry 1938; Frenguelli and Parodi 1941; Fren-
guelli 1943a,1943b;Traverso1964;RomeroandHickey
1976; Durango de Cabrera and Romero 1986; Gandolfo
et al. 1988; Romero et al. 1988; Gonza´ lez et al. 2002). The
total number of formally described entities referable to
surviving voucher specimens is about 44 species of leaves,
fruits, and seeds, nearly all in need of taxonomic revision:
only six species have been described or revised since 1945.
The age of the flora was originally reported as Miocene
(Berry 1925) and later, from
40
K/
40
Ar analyses of associated
volcanic rocks, was assigned ages ranging from late Pa-
leocene to middle Eocene (Archangelsky 1974; Mazzoni et
al. 1991).
The Rı´o Pichileufu´floraisthemostdiversefromCe-
nozoic South America, with more than 130 formally de-
scribed entities (Berry 1935a,1935b,1935c,1938);this
figure remains a useful starting estimate of richness. How-
ever, despite its diversity, very good to excellent preser-
vation, and accessibility from San Carlos de Bariloche (fig.
1), the flora has been little studied since Berry’s historic
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Eocene Plant Diversity in Patagonia 637
Table 1 : Sampling and taphonomic data for the four principal quarries at Laguna del Hunco (LH)
LH-13 LH-2 LH-4 LH-6
Meter level (fig. 2) 40.30 68.18 75.63 99.28
Excavated fossiliferous volume (m
3
) 1.4 2.0 1.8 1.9
Plant fossil density (specimens/m
3
)1,008710929535
Plant specimens:
Total 1,451 1,406 1,672 995
Identifiable 1,123 (77%) 647 (46%) 1,268 (76%) 685 (69%)
Leaves, ID’d 1036 626 1260 642
Dicot leaves, ID’d 806 609 1248 530
Dicot reproductive 30 18 7 34
Conifer leaves 218 10 6 86
Conifer reproductive 57 3 0 7
Fern leaves 5 3 5 3
Dicot leaf specimens toothed (%) 38.3 56.7 76.3 47.4
Mean area of all dicot leaves, ln (mm
2
)6.78!.92 6.83 !.91 7.05 !.85 6.91 !.95
Helicopter fruits, abundance/species 0/0 0/0 1/1 4/2
Fish fossils Absent Absent Absent Articulated
Insect accumulations Present Absent Absent Present
Insect abundance
a
15% 5% 5% 76%
Interpretation: rank distance from shoreline 2 3 4 1
a
Number of body fossils in 2002 collections divided by the number from all four major localities combined.
efforts. It has been considered approximately coeval to LH
on the basis of shared plant species (Berry 1938; Petersen
1946; Arguijo and Romero 1981; Arago´n and Romero
1984; Markgraf et al. 1996; Hinojosa and Villagra´n 1997),
but until recently, neither flora had been reliably dated.
Biostratigraphic and
40
K/
40
Ar analyses of the Ventana For-
mation from sites with uncertain stratigraphic relation-
ships to the RP flora have suggested ages ranging from
late Paleocene to early Oligocene (Gonza´lez Dı´az 1979;
Rapela et al. 1988).
In November of 1999, our group completed the first
stratigraphically controlled, quantitative collection of the
LH floras (Wilf et al. 2003). From a 170-m stratigraphic
section (fig. 2), we excavated 25 quarries and collected or
systematically tallied 1,583 identifiable macrofossil speci-
mens without collection bias (totals and analyses for 1999
collections revised slightly in this article for recently iden-
tified material). Plant diversity at LH was compared to and
exceeded seven North American midlatitude assemblages
that have been collected using similar methods. Of these,
a collection of the lacustrine Republic flora from Wash-
ington, made by K. R. Johnson and colleagues from Denver
Museum of Nature and Science locality 2130 and prelim-
inarily analyzed (Passmore et al. 2002), was the most di-
verse. Three
40
Ar/
39
Ar analyses from tuffs discovered within
the LH fossiliferous sequence, coupled with six paleo-
magnetic reversals, calibrated the fossiliferous strata to ages
near 52 Ma and near the base of magnetic polarity Chron
23 (Wilf et al. 2003; fig. 2). These results date the LH flora
to the early Eocene climatic optimum, a 1–2-million-year
interval that featured the warmest sustained high temper-
atures of the Cenozoic (Zachos et al. 2001).
Biome Characterization and Biogeography
Several reconstructions of the warm early Eocene place the
RP and LH floras within tropical rain forest areas. Ac-
cording to Morley (2000), tropical rain forest usually re-
quires a minimum mean monthly temperature above
18#C, annual precipitation above 200 cm, and a dry season
with no more than 4 months below 10 cm of rainfall per
month. In early global reconstructions, tropical rain forests
cover most lowland areas up to about 50#north and south
paleolatitude (Wolfe 1985; Frakes et al. 1992). Morley
(2000) refined the mapping of past rain forest distribu-
tions, primarily from palynological data and inference
from modern climate systems, and included desert and
grassland belts centered on latitudes 30#north and south
resulting from Hadley cell circulation. His early Eocene
reconstruction shows American tropical rain forests ex-
pressed in three latitudinal bands, separated by the dry
belts. These are the Neotropical equatorial rain forest; the
Boreotropical rain forest, located at middle Northern lat-
itudes; and the Southern Megathermal rain forest at South-
ern middle latitudes, containing the Patagonian floras dis-
cussed here. The eastern margins of continents typically
receive significant rainfall from warm offshore currents
even in the desert latitudes, and Morley shows his Neo-
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638 The American Naturalist
Table 2 : Diversity and paleoclimatic data, Laguna del Hunco (LH) flora
LH-13 LH-13 level LH-2 LH-2 level LH-4 LH-4 level LH-6 LH-6 level All
Meter level (fig. 2) 40.30 37.00, 40.30 68.18 68.18 75.63 75.63 99.28 99.28 All
Plant species:
All 81 88 75 75 64 72 74 83 186
Leaves 67 74 69 69 59 65 58 66 152
a
Dicot leaves 55 61 61 61 53 56 45 52 132
Nonleaves 14 14 6 6 5 7 16 17 34
Paleotemperature:
Species used 55 61 61 61 53 56 44 51 131
P.436 .426 .557 .557 .509 .482 .545 .569 .504
MAT (#C) 14.5 14.2 18.2 18. 2 16.7 15.9 17.8 18.6 16.6
a
j(MAT) (#C) 2.0 1.9 1.9 1.9 2.1 2.0 2.3 2.1 1.3
Paleoprecipitation:
Species used 51 57 59 59 48 51 43 49 119
MlnA, ln (mm
2
)7.177.15 7.097.097.387.407.247.207.24
MAP (cm) 110 109 104.8 105 123 125 114 112 114
a
SE !(MAP) (cm) 47.3 46.9 45.2 45.2 53.3 53.9 49.2 48.2 49.1
SE "(MAP) (cm) "33.1 "32.8 "31.6 "31.6 "37.2 "37.6 "34.3 "33.6 "34.3
Note: Levels are the minor localities grouped with major localities at the same stratigraphic levels (see fig. 2; LH-13 was grouped with LH-14, 15, and 23,
all 3.3 m below LH-13). Dicot leaf species used for mean annual temperature (MAT) and precipitation (MAP) estimates differ slightly because some species
suitable for leaf-margin analysis were too fragmentary for reliable measurement of leaf area. Gymnostoma was not used for climate estimates because of
unusual leaf morphology; sampling error, used as a minimum error estimate for MAT when 12.0#C; when , !2#C is thej(MAT) pbinomial j(MAT) !2#C
recommended minimum error estimate (Wilf 1997). of dicot leaf species with untoothed margins; natural log of species leafPpproportion MlnA pmean
areas (Wilf et al. 1998). error.SE pstandard
a
We consider these values to be the best single estimates for the flora.
tropical and Southern Megathermal Patagonian tropical
rain forests connected by a narrow strip of moist climatic
conditions along the Atlantic Coast. Morley did not pre-
sent direct evidence for Eocene tropical rain forests in
middle latitude South America, save for the presence of
diverse fossil floras with tropical affinities such as Rı´o Pi-
chileufu´. A dry or semiarid biome north of the LH and
RP floras is supported by evaporite occurrences (Ziegler
et al. 2003) and diverse evidence from the early Eocene
Gran Salitral Formation (Melchor et al. 2002).
Previous paleoclimatic analyses based specifically on the
paleobotany of the LH and RP floras have suggested some
type of moist to seasonally dry tropical or subtropical cli-
mate (Berry 1925, 1938; Arago´ n and Romero 1984; Rom-
ero 1986; Markgraf et al. 1996; Wilf et al. 2003). However,
none has indicated a tropical rain forest biome matching
the reconstructions discussed above.
Living relatives of taxa from the LH and RP floras are
dispersed widely, for the most part in tropical and tem-
perate areas of South America and Australasia; current
biogeographic knowledge of individual taxa is summarized
in the appendix in the online edition of the American
Naturalist. Gondwanic affinities of the floras are much
better understood than Neotropical relationships (e.g., Hill
and Carpenter 1991; Hill 1994). Although diverse tropical
elements are present in the fossil floras, including many
lineages that are diverse and abundant in today’s Neo-
tropics, few of these are confirmed to be Neotropical en-
demics (Durango de Cabrera and Romero 1986). Never-
theless, the proximity of the American tropics and the
presumed existence of direct interchange corridors (i.e.,
the Atlantic coast) support the argument for Neotropical
interchange with Patagonia. The warm Paleogene climate
presumably furthered the southward dispersal of Neo-
tropical elements, in concert with poleward migrations of
numerous tropical elements worldwide (e.g., Pole and
MacPhail 1996; Harrington 2004). The extant Chilean and
Argentine floras contain significant numbers of genera
with disjunct distributions in the Neotropics that may be
Paleogene relicts (Davis et al. 1997; Villagra´n and Hinojosa
1997).
The combined presence of taxa with tropical and Gond-
wanic/Antarctic affinities at LH, RP, and other Paleogene
floras in Patagonia led Romero (1978, 1986) to coin the
descriptive term “mixed paleoflora” for these associations,
which he considered to be stable vegetational units without
direct modern analog. Because both the LH and RP ma-
crofloras lack the cold-tolerant genus Nothofagus,Romero
considered them among the warmest type of mixed pa-
leoflora. However, Nothofagus fusca–type pollen was re-
cently discovered at RP, occurring at low abundance (W.
Volkheime r, p e rsonal communi c a t i o n , 2 0 0 4 ; palynologi c a l
preparations from LH so far have been unproductive).
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Eocene Plant Diversity in Patagonia 639
Figure 3: The 30 most abundant plant species at Laguna del Hunco; all localities grouped
Collections and Methods
Major new collections from LH reported here were made
in December of 2002 and are combined with the material
reported by Wilf et al. (2003; collected in 1999); all were
acquired using identical procedures, as was the RP sample.
All LH voucher specimens, more than 3,500 in total, are
housed in the Paleobotanical Collections of the Museo
Paleontolo´ gico Egidio Feruglio, in Trelew, Chubut Prov-
ince. The repository for the RP fossils is the Museo Pa-
leontolo´gico Asociacio´n Paleontolo´gica Bariloche in Bari-
loche, Rı´o Negro Province. Historic collections from LH
and RP are not included in our analyses because previous
researchers did not use an unbiased, quantitative, strati-
graphic methodology. To improve the comparison to
North American plant diversity, we have also reanalyzed
the sample from Republic, mentioned earlier, and have
incorporated additional specimens. Although an exact an-
alog to LH does not exist in North America, the Republic
flora has the most similar combination of age (49–50 Ma)
and paleoenvironmental setting to the Patagonian floras,
and it is extremely diverse (Wolfe and Wehr 1987; Pigg et
al. 2001; Wilf et al. 2003).
At LH, our major effort was intensive sampling of the
four most productive quarries identified in 1999. These
sites (LH-2, LH-4, LH-6, and LH-13; table 1; fig. 2) are
hereafter referred to as the principal or major quarries and
the remaining sites as the minor quarries. The total for
all unbiased, identifiable material is 4,303 plant specimens,
of which 3,723 (87%) come from the four major quarries
(table 1). We also collected 57 float specimens, mostly not
assignable to specific horizons; these are not included in
analyses except where mentioned specifically below.
At Rı´o Pichileufu´, we found two sites, RP-1 and RP-3,
with very good preservation and diverse floras, occupying
approximately the same stratigraphic position (we could
not correlate them precisely because of covered and locally
slumped bedding). We made a preliminary, unbiased col-
lection of 341 specimens from RP-3, the most productive
quarry, of which 252 were identifiable and 213 were iden-
tifiable dicot leaves assigned to 39 species. Three tuffs were
found in close stratigraphic proximity to the fossil floras;
tuff RP1 lies immediately on top of the RP-1 fossil layer,
tuff 2 occurs approximately 6 m below RP-1 (although its
position may be slumped), and tuff RP3 is located about
6maboveRP-3.Inaddition,asanindependentlaboratory
test, we reanalyzed a tuff from LH, 2211A, which previ-
ously yielded an age from U.S. Geological Survey labo-
ratories of Ma (Wilf et al. 2003). More than
52.13 !0.32
140
40
Ar/
39
Ar laser fusion and incremental heating analyses
of sanidine crystals from the four tuffs, using the meth-
odology of Smith et al. (2003), were undertaken at the
University of Wisconsin–Madison Rare Gas Geochronol-
ogy Laboratory. Details of radioisotopic analyses are pro-
vided in the appendix.
From our inspection of the type and referred material,
many of Berry’s identifications are clearly incorrect, are
based on poorly preserved specimens that would not merit
formal nomenclature today, or are not supported with
sufficient diagnostic characters by modern standards (see
also Dilcher 1973; Hill and Brodribb 1999). For these rea-
sons, most of Berry’s generic names are enclosed here in
quotations. Both Berry’s taxa and the large numbers of
new species found in our collections will require a major
effort toward systematic analysis and revision, now un-
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640 The American Naturalist
Figure 4: Rarefaction results for identified dicot leaves, withcomparisons
to the most diverse Eocene floras known from North America collected
using similar techniques. A,FourmajorquarriesatLagunadelHunco
combined, showing the effect of increased sampling from 1999 (reported
in Wilf et al. 2003) to 2002 collections, compared to the four census sites
from Florissant, 34 Ma (Evanoff et al. 2001), Colorado, reported by
MacGinitie (1953). B,EffectofincreasedsamplingateachoftheLaguna
del Hunco major quarries. Vertical dotted lines connect the maximum
values of the 1999 data to the current rarefaction curves for each locality.
C,Single-quarrycomparisons,includingselected95%confidenceinter-
vals, for comparison to the Republic flora (49–50 Ma, Washington, Den-
ver Museum of Nature and Science locality 2130). Republic data are
reanalyzed and updated for new specimens from Wilf et al. (2003), with
a resulting increase in rarefied diversity (the previous curve was nearly
identical to the present curve for LH-4). Rarefactions and confidence
intervals computed from the formulas of Tipper (1979). LH pLaguna
del Hunco; RP pRı´o Pichileufu´.
derway and led by M. A. Gandolfo (Gandolfo et al. 1988,
2004; Romero et al. 1988; Gonza´ lez et al. 2002).
Lack of a comprehensive reliable taxonomy is a typical
problem in floras dominated by angiosperm leaves, which
tend to be very diverse; even in floras that are relatively
well understood, it is not unusual for a significant fraction
of species to remain undiagnosed even to the family level
(e.g., Johnson 2002). However, a standard procedure ex-
ists, which relies on detailed leaf architectural analyses to
discriminate distinct leaf morphotypes that represent
probable biological species (Hickey 1979; Johnson et al.
1989; Ash et al. 1999). This approach, which we use here,
and others like it have been applied successfully to many
fossil floras to analyze ecological, diversity-related, and
paleoclimatic variables that require presence-absence or
relative abundance data for all species in a flora (Hill 1982;
Johnson et al. 1989; Burnham 1994; Wing 1998; Smith et
al. 1998; Jacobs and Herendeen 2004; Wilf and Johnson
2004) and to provide an initial framework for systematic
descriptions (Crane et al. 1990; Johnson 1996).
We follo w e d a c o nservati v e “ l u mping” a p p roach w h ere
awell-preserved,voucheredexemplarspecimenwitha
unique, reproducible suite of diagnostic architectural char-
acters and accompanying description must represent each
morphotype. The common preservation of nearly com-
plete leaves and fine venation features, especially at LH,
facilitated this methodology: the morphotypes established
from the 1999 collections, even when based on single spec-
imens, proved reliable for identifying 2002 collections in
the field and laboratory. For convenience, we will use “spe-
cies” here to indicate both formally described species and
morphotypes; comprehensive descriptions of these entities
are in separate preparation. Existing paleobotanical no-
menclature, abundance data, and voucher specimen num-
bers are provided in tables A2 and A3 in the appendix.
Mean annual temperature (MAT) and precipitation
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Eocene Plant Diversity in Patagonia 641
Figure 5: Detrended correspondence analysis of major quarries and taphonomic variables of the Laguna del Hunco (LH) flora (from table 1; most
values entered as proportions). Axes are labeled with their percentages of total variance explained. Computed using MVSP 3.1 (Kovach 2000).
(MAP) were estimated for LH, the better-sampled flora,
using the standard techniques of leaf-margin (Wolfe 1979;
Wing and Greenwood 1993; Wilf 1997) and leaf-area anal-
ysis (Wilf et al. 1998), respectively.
Ages of the Floras
All three tuff beds collected at Rı´o Pichileufu´produced
abundant sanidine phenocrysts that yielded
40
Ar/
39
Ar ages
with low analytical uncertainties of Ma for47.48 !0.10
tuff RP1, Ma for tuff 2, and Ma47.48 !0.47 47.45 !0.06
for tuff RP3 (!95% confidence about the mean). These
ages were calculated relative to 28.34 Ma for the Taylor
Creek rhyolite sanidine (Renne et al. 1998) using the decay
constants of Steiger and Ja¨ger (1977). Given the uncer-
tainties, the three
40
Ar/
39
Ar ages for RP are indistinguish-
able from one another, thereby indicating a common de-
positional age for these sediments of Ma, the47.46 !0.05
weighted mean age from all three tuffs (middle Eocene,
Lutetian). In terms of global climate, this age places the
RP flora within the initial phase of cooling subsequent to
the early Eocene climatic optimum, although temperatures
were high at this time compared to the remainder of the
Cenozoic (Zachos et al. 2001). Results from the reanalyzed
Laguna del Hunco 2211A ash gave a weighted mean age
of Ma, indistinguishable from the51.91 !0.22 52.13 !
Ma age reported by Wilf et al. (2003). Therefore, both0.32
floras are securely dated at high precision, and the RP flora
is about 4.5 million years younger than the Laguna del
Hunco flora, previously considered as nearly coeval. These
results demonstrate for the first time longevity of the di-
verse Patagonian paleofloras, discussed in more detail
below.
Floral Diversity
The total count of plant organ species from our Laguna
del Hunco collections is 186, including 152 leaves and 132
dicot leaves (table 2); these totals are more than four times
the sum of historic data. The nondicotyledonous foliar
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642 The American Naturalist
Figure 6: Cluster analyses, derived from Bray-Curtis distances, of relative abundance data from Laguna del Hunco (LH) plant species. A, Major
quarries, proportional data. B, Minor quarries with at least 40 specimens, raw data (sample size is insufficient for reliable proportional data,although
a similar clustering results). Note the stratigraphic correspondence of clusters to the major localities in A(cf. fig. 2). Computed using MVSP 3.1
(Kovach 2000).
species are eight conifers, one cycad, one Ginkgo, three
monocots, and seven ferns. The remaining species include
27 angiosperm fruits, seeds, and flowers; four or possibly
five types of coniferous cone scales, seeds, and pollen
cones; and one bryophyte. The total richness increases
slightly with float specimens, which include three addi-
tional species each of dicot leaves and dicot fruits. All four
principal localities produced comparable raw values for
plant richness, for example, 58–69 leaf species (table 2),
and the maximum occurred at LH-2. The most abundant
species at all sites combined are shown in rank order in
figure 3; thermophilic families are abundant, such as Myr-
taceae (“Myrcia”), Sapindaceae (“Schmidelia,” “Cupania”),
Fabaceae (“Cassia”), Lauraceae, and Araucariaceae. Al-
though Lomatia is common, the prevalence of “temperate”
or tropical montane groups is hard to assess because many
common species are poorly understood taxonomically.
Rarefaction analyses of dicot leaf data demonstrate high
plant diversity at LH at robust sample sizes (fig. 4). In
general, increased sampling led to dramatic increases in
species recovered, and individual rarefaction trajectories
remained the same or increased. The four principal quar-
ries combined show increasing diversity past 3,000 spec-
imens, along the trajectory of the 1999 collections (fig.
4A). In contrast, MacGinitie’s (1953) classic collections
from Florissant, Colorado, the most comparable data set
available for quarries from several stratigraphic levels
within a short-lived lacustrine sequence, do not produce
new species after about 1,000 specimens (fig. 4A). At the
principal localities, numerous additional species accu-
mulated, mostly along the previous rarefaction trajectories
(fig. 4B). The most significant change occurred at LH-13,
which showed a dramatic jump in rarefied richness and a
total increase in dicot leaves from 24 to 55 species. The
maximum raw richness occurred at LH-2, which increased
from 43 to 61 dicot leaf species.
All four principal quarries have a rarefied richness com-
parable to the extremely diverse Republic flora, and two,
LH-2 and LH-13, are more rich (fig. 4C). The difference
is significant for LH-2, based on separation of 95% con-
fidence intervals, and at 600 specimens, LH-2 has one-
third more species than Republic (61 vs. 46). This is the
case even though the reanalyzed Republic sample has
higher rarefied diversity than reported earlier (Wilf et al.
2003). Although rarefaction curves are beginning to flatten
at the four principal quarries, there is great potential for
asignificantnumberofadditionalplantspeciestobe
found at LH, considering the large area of exposure
available.
The preliminary sample from RP is as diverse on a
rarefied basis as the most diverse LH locality, LH-2 (fig.
4C), supporting the richness described in the historic
monograph (Berry 1938) and our observations of corre-
sponding type material. This, along with our geochro-
nologic results, shows exceptional plant diversity at a dif-
ferent time and location in Eocene Patagonia from LH.
Moreover, the high diversity recovered at RP, despite hav-
ing a distinctly lower quality of preservation than LH,
diminishes the possible importance of preservation bias
on rarefaction results for both floras.
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Eocene Plant Diversity in Patagonia 643
Figure 7: Percent abundance at each major quarry for the 10 most abundant species of dicot leaves at Laguna del Hunco (LH), calculated relative
to all dicot leaves from the quarry.
Stability of Floral Associations
To w h a t e x t e n t d o t h e L H a n d R P fl o r a s r e p r e s e n t a b r o a d l y
coherent, persistent floral association versus a suite of
mostly unrelated floras that share only high richness as a
common characteristic? Abundant evidence indicates the
former at LH, where the major source of variation in the
floras appears to be environment of deposition. The four
principal quarries show differences in taphonomic char-
acteristics that apparently reflect distance from shoreline
(table 1); LH-6 has the most distinct offshore signature,
seen especially in its thin, well-laminated bedding and the
presence of articulated fossil fish, symmetrically winged
(“helicopter”) dicot fruits, and relatively abundant insect
fossils (the latter two also at LH-13). Helicopter fruits and
insects are likely to be entrained in prevailing winds and
deposited far from their source, where they become better
represented in fossil deposits than material deposited solely
by gravity and water (Wilson 1980; Augspurger 1986). In
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Eocene Plant Diversity in Patagonia 645
Figure 8: Paleoclimatic results from (A) leaf-margin and (B) leaf-area analysis, Laguna del Hunco bulk flora (see table 2), plotted using the method
of Burnham et al. (2001), where successive estimates are graphed from left to right as species are added in decreasing rank abundance order.
Abundances of the species as they are added are shown in the small accessory graphs; the two abundance graphs differ slightly because some
specimens suitable for leaf-margin analysis were too fragmentary for reliable measurement of leaf area. In A, the central horizontal line shows the
best value for the sample based on all species. Symmetrical curves show 1 SD (standard deviation) about the best value based on number of species
and a binomial probability model (Wilf 1997). Temperature estimates above or below these curves indicate leaf-margin percentages that deviate
significantly from a random order of margin types taken from the entire sample. The accessory horizontal lines show !2#C from the best value,
considered to be a minimum error for leaf-margin analysis when the binomial error is less than !2#C (Wilf 1997). Bincludes estimated mean
annual precipitation and the standard error of the estimate using the regression of Wilf et al. (1998); errors are asymmetrical because they are
transformed from logarithmic data. annual temperature; annual precipitation.MAT pmean MAP pmean
adetrendedcorrespondenceanalysis(DCA)ofquarries
and taphonomic variables (fig. 5), LH-6 is alone in its
position on the first axis, and its most strongly associated
variables, in decreasing order, are the presence of fish,
winged fruits, and insects.
Clustering based on Bray-Curtis distance, which is com-
puted using relative abundance data, grouped locality LH-
6withLH-13andlocalityLH-2withLH-4(fig.6A). A
similar grouping of LH-6 and LH-13 along the first axis
results from a DCA of species relative abundance and quar-
ries (not shown). Although data from minor localities are
preliminary because of their small sample sizes, cluster
analysis based on Bray-Curtis distance also resolved the
stratigraphic groupings of six minor localities with at least
40 specimens each (fig. 6B).
The clustering of the stratigraphically lowest (LH-13)
and highest (LH-6) major and corresponding minor lo-
calities (fig. 6), combined with the prevalence of the same
dominant taxa at all localities (fig. 7), suggests that floral
differences within the LH sequence reflect paleoenviron-
ment of deposition more than compositional change
through time. These differences are mostly expressed in
varying relative abundance (figs. 6, 7) and not in com-
position. We therefore consider the LH floras to represent
a single major association that is preserved in a variety of
lacustrine environments.
The Laguna del Hunco and Rı´o Pichileufu´florashave
been considered to be similar in species composition since
Berry’s original treatments (Berry 1925, 1938; Arguijo and
Romero 1981; Markgraf et al. 1996; Hinojosa and Villagra´n
1997). Notably, Petersen (1946) listed 50 species shared
between the floras, but no surviving voucher specimens
are known that could be used to evaluate his identifica-
tions. Because the floras are shown here to be separated
by 4.5 million years, the assumption of species-level sim-
ilarity deserves a critical reevaluation, although significant
overlap of angiosperm families is indisputable (appendix).
A reevaluation of shared leaf species (table A4 in the ap-
pendix) produces a list of 10 angiosperms that is short in
comparison to the prodigious angiosperm diversity of the
two sites already discussed, although it is likely to grow
with increased collecting at RP. Significantly, only one of
the 10 most abundant dicot species at RP, “Cassia”argen-
tinensis, is among the top 10 at LH (fig. 7). Thus, the best
current support for close species similarity among the flo-
ras is in the less diverse gymnosperm fraction (table A4).
Paleoclimate
Our paleoclimatic results indicate that Eocene Laguna del
Hunco was neither warm nor humid enough to be a trop-
ical rain forest as suggested in previous global reconstruc-
tions (Wolfe 1985; Frakes et al. 1992; Morley 2000), al-
though moisture was abundant and the maritime climate
limited seasonality to a narrow range of temperatures that
precluded frost. Results for major localities and associated
sampling levels at LH are shown in table 2, along with
notes on methodology. The high numbers of species used
in the estimates help to counteract well-known biases from
undersampling, paleoenvironment, and taphonomy
(Burnham 1994; Wilf 1997; Burnham et al. 2001). For
MAT at individual sampling levels, there is less than 5#C
spread between the minimum estimate of C14.2#!2.0#
at the LH-13 level and the maximum estimate of
#C at the LH-6 level (table 2). Because there is18.6#!2.1
no evidence for significant change in floral composition
or available moisture, major shifts in temperature do not
seem likely. Accordingly, we place the greatest confidence
in the lumped estimate from all localities, #C,16.6#!2.0
based on 131 dicot leaf species. This result is within the
range (16#–17#C) of coeval sea surface temperatures an-
alyzed from several cores at bounding paleolatitudes of the
South Atlantic (Zachos et al. 1994).
Paleoprecipitation estimates for LH are nearly identical
at the various sampling levels (table 2). The 119 species
that were measurable for leaf size produce a lumped es-
timate of about 1.1 m/year. We consider this a minimum
estimate because transport into lakes generally selects
against larger leaves (Roth and Dilcher 1978). Although
several species are represented by very large leaves (area
110,000 mm
2
), their signal is diluted by numerous rare
species assumed to be undersampled for leaf size.
Sensitivity of climate estimates to sample size is tested
using the method of Burnham et al. (2001), where suc-
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646 The American Naturalist
cessive estimates are graphed as species are added in de-
creasing rank abundance order (fig. 8A). For the LH bulk
flora, MAT estimates begin to converge near 60 species,
which suggests that the estimated paleotemperature is very
unlikely to change significantly with additional sampling,
although increased resolution of climate from individual
horizons may be possible. A similar plot of MAP estimates
shows instability below about 40 species but little variation
past about 60 species (fig. 8B).
The modern climatic ranges of several higher taxa in
the LH flora are well studied, helping to define climatic
seasonality using nearest living relatives. The combined
presence of palms, cycads, at least two species of araucarian
conifers, at least five species of podocarps, and possibly
three species of Gymnostoma (Gandolfo et al. 2004), along
with numerous additional tropical elements (appendix)
and the apparent absence of Nothofagus,providesstrong
evidence for an equable climate with winter mean tem-
peratures warmer than 10#Candabundantrainfall(Chris-
tophel 1980; Romero 1986; Hill 1994; Greenwood and
Wing 1995; Hill and Brodribb 1999; Kershaw and Wagstaff
2001).
From the RP-3 locality, 23 of the 39 dicot species were
untoothed, yielding a preliminary MAT estimate of
#C. However, greater species sampling at RP19.2#!2.4
will probably change this estimate (see fig. 8). Global ma-
rine data indicate cooling in the interval from 52 to 47
Ma (Zachos et al. 2001), which can be tested in Patagonia
with increased sampling at RP.
The nontropical but moist and equable climates may
explain the long-observed “mixing” (sensu Romero 1978,
1993) of temperate and tropical elements; the absence of
drought and frost fostered the growth of tropical lineages,
while the lack of high temperatures allowed the simulta-
neous presence of Gondwanic elements (see Axelrod et al.
1991). Except for moist corridors that probably existed to
the Neotropical region, the biome containing the LH and
RP floras was bounded by cooler winters to the south and
aridity to the north. This biome could be considered an
ancient biodiversity “hot spot,” although much better doc-
umentation of floral endemism than may ever be available
from fossils would be required to test a hot spot hypothesis
by comparison to modern definitions (e.g., Mittermeier et
al. 1998).
Discussion
The Laguna del Hunco and Rı´o Pichileufu´florasrepresent
ahighlybiodiverse,floristicallyandclimaticallydistinct
region. They provide the best macrofossil evidence for an
ancient history of high plant diversity in Cenozoic South
America. The 4.5-million-year age difference between
them shows that richness was both long-lived and wide-
spread in Patagonia. Their diversity exceeds that of any
comparable Eocene flora from the middle latitudes ofwest-
ern North America, the only region with quantitative data
derived using equivalent collection strategies, and exceeds
on an absolute basis all Eocene leaf floras known to us
from outside the Americas. Even without sample size cor-
rection, organ type equivalence, or stratigraphic control in
the comparisons, they rank among the most diverse fossil
macrofloras known from any time period (e.g., Knoll et
al. 1979). Eocene floras with equal or greater species rich-
ness, such as the Clarno nut beds (Manchester 1994) and
the London Clay flora (Reid and Chandler 1933; Collinson
1983), are fruit and seed assemblages, which are produced
via fundamentally different taphonomic pathways and in-
creased temporal averaging in comparison to assemblages
dominated by leaves (Behrensmeyer et al. 2000). They also
have been collected selectively over many decades from
unknown numbers of specimens, assumed to be in the
tens to hundreds of thousands. The Castle Rock flora,
which preserves forest floor litter from the early Paleocene
of Colorado (Johnson and Ellis 2002; Ellis et al. 2003), is
the only known assemblage, dominated by angiosperm
leaves and collected using comparable methods to LH and
RP, with similar species diversity. However, it represents a
true rain forest with a significantly warmer and wetter
environment than the Patagonian floras (Ellis et al. 2003);
this climate should allow the presence of elevated plant
diversity by analogy to living forests (Phillips and Miller
2002).
Diverse floras at Patagonian middle latitudes imply great
richness at tropical latitudes of Eocene South America via
the latitudinal diversity gradient, which appears to apply
to most present and past vegetation (Crane and Lidgard
1989; Willig et al. 2003; Harrington 2004; Hillebrand
2004). Palynological data from the Paleocene-Eocene in-
terval of Colombia strongly support a hyperdiverse Eocene
tropics (Jaramillo 2002), and a latitudinal diversity gra-
dient existed for Eocene vegetation in North America
(Harrington 2004). Patagonian diversity was probably
driven both by in situ evolution and the “spillover” of
tropical taxa from extremely diverse floras of lower lati-
tudes with Eocene warming. Immigrants from low lati-
tudes consequently mixed with resident, temperate, and
endemic taxa as well as tropical migrants from other con-
tinents. An analogous effect has been observed in Eocene
palynofloras from North America (Harrington 2004).
Reliable quantitative paleontological data for plant di-
versity from other areas of South America remain rare,
and diversity fluctuated between the Eocene and Recent
(Jaramillo 2003). Any explanation for past diversity or its
connection to modern floral richness is speculative, but
we make a few general points here. South America has
possessed immense areas of low-latitude land surface since
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Eocene Plant Diversity in Patagonia 647
the Early Cretaceous and of tropical rain forest cover, dom-
inated by angiosperms, at low latitudes since at least the
late Paleocene (Morley 2000; Schettino and Scotese 2001;
Jaramillo 2002; Wing et al. 2004). Accordingly, there has
been great potential for diversification through time from
populations separated by geography, climate, or ecological
selection regime, as supported by studies in modern Ama-
zonia (Gentry 1988a;Schneideretal.1999;Moritzetal.
2000; Smith et al. 2001; Ogden and Thorpe 2002; Tuomisto
et al. 2003; Fine et al. 2004). The association of global
warming across the Paleocene-Eocene boundary and sig-
nificant in situ diversification of palynofloras in northern
South America (Rull 1999; Jaramillo 2002) suggests that
a major diversity increase occurred in Eocene South Amer-
ica, mediated by climate change. Testing this hypothesis
in Patagonia awaits quantitative analyses of suitable Pa-
leocene floras.
Acknowledgments
For generous support, we thank the National Geographic
Society (grant 7337-02), the National Science Foundation
(grants DEB-0345750, EAR-0230123, and EAR-0114055),
the University of Pennsylvania Research Foundation, the
Andrew W. Mellon Foundation, and the Petroleum Re-
search Fund (grant 35229-G2). For exceptional assistance
in the field and laboratory, we are grateful to M. Caffa, L.
Canessa, B. Cariglino, I. Escapa, C. Gonza´lez, R. Horwitt,
P. P u e r t a , E . R u i g o m e z , H . S m e k a l , X . Z h a n g , a n d t h e
Museo Paleontolo´gico Asociacio´n Paleontolo´gica Barilo-
che. Comments on drafts and reviews by E. Arago´n, R.
Horwitt, C. Jaramillo, D. Royer, and two anonymous re-
viewers greatly improved the manuscript. We are grateful
to the Nahueltripay family and INVAP (Instituto de In-
vestigaciones Aplicadas) for land access.
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