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Helodermatid lizard from the Mio−Pliocene oak−hickory
forest of Tennessee, eastern USA, and a review of
monstersaurian osteoderms
JIM I. MEAD, BLAINE W. SCHUBERT, STEVEN C. WALLACE, and SANDRA L. SWIFT
Mead, J.I., Schubert, B.W., Wallace, S.C., and Swift, S.L. 2012. Helodermatid lizard from the Mio−Pliocene oak−hickory
forest of Tennessee, eastern USA, and a review of monstersaurian osteoderms. Acta Palaeontologica Polonica 57 (1):
111–121.
The extant venomous Gila monster and beaded lizards, species of Heloderma, live today in southwestern USA and south
along the Pacific coastal region into Central America, but their fossil history is poorly understood. Here we report
helodermatid osteoderms (dermal ossicles) from the late Miocene–early Pliocene Gray Fossil Site, eastern Tennessee
USA. Twenty−three species of mammals are known from the fauna including abundant Tapirus polkensis, as well as
fishes, anurans, salamanders, turtles, Alligator, birds, and snakes. Beaded lizards belong to the Monstersauria, a clade that
includes Primaderma + Paraderma + Gobiderma + Helodermatidae (Estesia, Eurheloderma, Lowesaurus, and Helo−
derma). Osteoderms of lizards in this clade are unique within Squamata; they typically are circular to polygonal in out−
line, domed to flat−domed in cross−section, have a vermiculate surface texture, are not compound structures, and do not
have imbricate surfaces as on many scincomorph and anguid lizards. We review and characterize the osteoderms of all
members of Monstersauria. Osteoderms from the cranium, body, and limbs of Heloderma characteristically have a
ring−extension (bony flange) at least partly surrounding the dome. Its presence appears to be a key character distinct to all
species of Heloderma, consequently, we propose the presence of a ring−extension to be an apomorphy. Three osteoderms
from the Gray Fossil Site range from 1.5 to 3.0 mm in diameter, have the circular shape of helodermatid osteoderms with a
domed apical surface, and have the ring−extensions, permiting generic identification. Macrobotanical remains from the
Gray Fossil Site indicate an oak−hickory subtropical forest dominated by Quercus (oak) and Carya (hickory) with some
conifer species, an understorey including the climbing vines Sinomenium, Sargentodoxa, and Vitis. Plant and mammal re−
mains indicate a strong Asian influence.
Key words: Reptilia, Squamata, Helodermatidae, Heloderma, beaded lizards, Hemphillian, Miocene, Pliocene, Ten−
nessee, North America.
Jim I. Mead [mead@etsu.edu], Blaine W. Schubert [schubert@etsu.edu], Steven C. Wallace [wallaces@etsu.edu],
and Sandra L. Swift [Sandra.swiftone@yahoo.com], Department of Geosciences, and the Don Sundquist Center of
Excellence in Paleontology, East Tennessee State University, Johnson City, TN 37614 USA.
Received 25 August 2010, accepted 11 March 2011, available online 14 March 2011.
Introduction
The only truly venomous lizards today are the Gila monster
and beaded lizard, both in the genus Heloderma (Wiegmann
1829), in the Family Helodermatidae. These large lizards live
in southwestern USA and south along the Pacific coastal re−
gion into Central America. Their fossil history is inadequately
understood as it is for all members of the Monstersauria, a
clade including Primaderma + Paraderma + Gobiderma +
Helodermatidae (Estesia, Eurheloderma, Lowesaurus,and
Heloderma) (Norell and Gao 1997), as discussed and illus−
trated in Conrad (2008: fig. 56D; see further discussions in
McDowell and Bogert 1954; Bogert and Martín del Campo
1956; Pregill et al. 1986). Here we report helodermatid osteo
−
derms (dermal ossicles) from the late Miocene–early Pliocene
of the Gray Fossil Site, Washington County, in the Southern
Appalachian Mountains of eastern Tennessee, USA (36°N,
82°W). Osteoderms of Heloderma and other extinct monster−
saurians are distinctive within Squamata (see discussions be−
low and in Norell and Gao 1997; Gao and Norell 2000;
Nydam 2000; Conrad 2008). Moreover, there is variation in
the morphology of the osteoderms among monstersaurian
genera, through ontogeny, and in placement on different posi−
tions of the body. However, this has never been specified or
discussed in detail in the literature (see Bhullar and Smith
2008). Consequently a preliminary discussion concerning this
variation is provided below.
A recent study of both mitochondrial and nuclear DNA of
the two extant species of Heloderma (Douglas et al. 2010) re−
affirmed that Helodermatidae are monophyletic, and that it
is an ancient and conservative group. A fossil−based con
−
strained date for the origin of Helodermatidae from Douglas
et al. (2010) was approximately 106 Ma based on the ac
−
cepted earliest monstersaurian/helodermatid, Primaderma
http://dx.doi.org/10.4202/app.2010.0083
Acta Palaeontol. Pol. 57 (1): 111–121, 2012
nessovi Nydam, 2000. In addition, the DNA record implies
that an initial split of H. suspectum Cope, 1869 from a com
−
mon ancestor with H. horridum Wiegmann, 1829 was in the
early Eocene (Douglas et al. 2010).
Institutional abbreviations.—ETMNH, East Tennessee State
University and General Shale Brick Natural History Museum,
Gray, USA; ETVP, East Tennessee State University, Verte
−
brate Paleontology Laboratory, Department of Geosciences,
Johnson City, USA; KUVP, University of Kansas, Vertebrate
Paleontology, Lawrence, USA; MNHN, Muséum national
d’Histoire naturelle, Paris; TMM, Texas Memorial Museum,
Vertebrate Paleontology Laboratory, University of Texas at
Austin, USA; UCMP, University of California Museum of Pa
−
leontology, Berkeley, USA; UF, University of Florida, Florida
Museum of Natural History; USNM, United States National
Museum, Smithsonian Institution, Washington, USA.
Geological and geographical
settings
The Gray Fossil Site covers an area of about 2.5 ha and con−
tains sediments up to about 40 m thick (Wallace and Wang
2004). Less than five percent of the locality has been systemat−
ically excavated, screen−washed, and analyzed. The recovered
remains illustrate that the fauna is diverse and abundant, and
evidently not fully realized. From elsewhere in North Amer−
ica, the stratigraphic range of Teleoceras (rhino) and Plio−
narctos (tremarctine bear) constrain the age of the sediments
at the Gray Fossil Site to between 7.0 and 4.5 Ma (latest Mio−
cene–earliest Pliocene), the Hemphillian Land Mammal Age
(Wallace and Wang 2004). This age makes the Gray Fossil
Site one of the few mid−Neogene vertebrate localities in the
eastern United States (Farlow et al. 2001; Tedford et al. 2004).
A wealth of information is becoming available at the
Gray Fossil Site; at least 23 species of mammals are currently
known from the fauna (with over 80 individuals of the extinct
tapir, Tapirus polkensis), including Pristinailurus (lesser
panda), Arctomeles (Eurasian badger), in addition to fishes,
anurans, salamanders, turtles, Alligator, birds, and snakes
(Parmalee et al. 2002; Wallace and Wang 2004; Schubert
and Wallace 2006; Hulbert et al. 2009). Lizard remains are
exceedingly rare, which is perhaps not surprising in that the
reconstructed habitat is a forest surrounding a lacustrine ba
−
sin (DeSantis and Wallace 2008; see below). Even today the
temperate deciduous forest, which exists regionally near the
Gray Fossil Site, harbors only four species of lizards (Gib
−
bons et al. 2009).
Osteoderms
Terminology.—Osteoderms, especially those fused to cra
−
nial elements, are common in monstersaurians, and in some
other lizard groups. It is generally assumed that a monster
−
saurian with cranial osteoderms will also have osteoderms
over at least a portion of, if not the entire, body because this is
the ancestral condition for Anguimorpha. This is clearly the
case in extant Heloderma, but osteoderms in fossil taxa are
poorly understood. Moreover, osteoderm size and surface
texture are difficult to interpret or score as characters (Pregill
et al. 1986; Conrad 2008). Consequently, descriptions can be
subjective, and terminology varies among authors.
Osteoderms from non−monstersaurians are typically flat
(plate−like), predictably thin, and rectangular to trapezoid in
shape, as seen in Anguidae (see general descriptions in Hoff
−
stetter 1962; Meszoely and Ford 1976; Strahm and Schwartz
1977; Richter 1994; Mead et al. 1999). Anguine imbricating
osteoderms can have intricate sculpturing, some with well−
defined keels (Fejérváry−Lángh 1923; Meszoely 1970; Gau
−
thier 1982; Augé 2005). Osteoderms from the Paleogene
glyptosaurine anguids of North America and Eurasia (Estes
1983), however, are different showing a pattern of subequal,
hexagonal, and sometimes domed plates. Both the cranial and
body osteoderms are covered with tubercular mounds, some
−
times arranged in concentric patterns (Meszoely et al. 1978;
Sullivan 1979; Augé 2005), distinct from the vermiculate net−
work found in monstersaurians, especially Heloderma.
Osteoderms from the extinct Carusia are similar to those
of closely related Xenosauridae (Anguiformes, Carusioidea)
in that they cover most skull roofing elements; osteoderms
are subdivided into individual elements ornamented with a
vermiculate sculpture (Gao and Norell 1998). The row−pat−
tern of scutulation as seen on the extinct xenosaurid Exo−
stinus described by Bhullar (2010) appears unique. An indi−
vidual osteoderm can be portrayed as a “small, peaked lump
… [with a] rolling, bumpy sculpture” (Bhullar 2010: 944).
The extant, enigmatic anguimorph lizard, Shinisaurus has a
reduced number of cephalic osteoderms, which are all gener
−
ally flat, plate−like structures with irregular margins (Bever et
al. 2005).
Monstersaurian osteoderms show some consistency; most
are circular to polygonal (multilateral) in outline (osteoderms
of the tail differ, see below), are not compound structures, and
do not have imbricate (overlapping) surfaces, as in many other
lizards, especially Cordyloidea and Scinciformes (e.g., see
discussion in Richter 1994). Overall appearance of the integu
−
ment surface on monstersaurians is often described as granu
−
lar, beadlike, or tuberculate. We use the term “dome” to refer
to the individual tubercules. A keel, as in some scincomorphs
and anguids, is absent on monstersaurian osteoderms. Cross−
sectional shape of helodermatid osteoderms is typically and
uniquely domed, or a flattened dome, again giving an overall
granular appearance. However, the development of this fea
−
ture varies in earlier monstersaurians (see below). Surface tex
−
ture of individual osteoderms, which is important and can vary
from different parts of the body, has been termed vermiculate
(having wavy or worm−like, sinuous lines). This vermiculate
texture on osteoderms can be extreme, creating a pattern of
“ridges” and “pits”.
112 ACTA PALAEONTOLOGICA POLONICA 57 (1), 2012
Recent monstersaurian osteoderms.—Table 1 provides a
list of modern Heloderma horridum and H. suspectum used in
this study. Fig. 1A shows the overall exterior of the cranial and
nuchal entegument surface of a typical adult Heloderma sus−
pectum (ETVP 7096). Individual osteoderms range from 1.5
to 6.5 mm in diameter on an individual with a snout−vent lengh
of 320 mm (ETVP 7083). Typically, osteoderms covering the
cranial bones are larger, thicker, and often more polygonal in
shape compared to those from the nuchal region or rest of the
body, which usually are smaller and have a more circular
outline (Fig. 1B; see also H. horridum at http://digimorph.
org/specimens/Heloderma_horridum/). Some specimens have
smaller osteoderms on the parietal region of the cranium, be−
ing more similar to those of the nuchal than frontal and lateral
sides of the skull. A hatchling H. suspectum (ETVP 17869)
with a snout−vent length of 125 mm, did not have osteoderms
attached to any cranial elements (Fig. 2A). Notice in Fig. 2B
(ETVP 17869) the isolated, thin, wafer−like osteoderms have
numerous holes, yet already exhibit an incipient domed form,
circular−polygonal outline, and vermiculate texture on the api−
cal side of the bone (the term as used here refers to the direc−
tion away from the subintegumentary attachment), but lack a
basal platform. Fig. 2C shows a close−up of an additional juve−
nile, H. horridum (ETVP 17907), illustrating that the osteo−
derms form quickly and are already beginning to adhere to
cranial bones.
Individual osteoderms characteristically connect to oth−
ers via tissue; some cranial osteoderms abut others and
weakly ossify (Figs. 3A, 4). Osteoderms from the cranium,
body, and limbs characteristically (~70% in this study) have
a ring−extension (bony flange, cingulum) surrounding, or
partly around, the dome (Figs. 3B, 5A, B, D, E). We found no
indication in the two extant Heloderma species that presence
or absence of a ring−extension around the dome is related to
the size of the osteoderm or the ontogenetic age of the lizard.
No morphological differences were noted between osteo−
derms of the front and rear limbs. Overall size of osteoderms
varies greatly over the body regardless of the snout−vent
length of the lizard, and there is no size sorting according to
position on the body (i.e., osteoderm size is apparently ran−
dom). Surface texture of all osteoderms from Heloderma is
characteristically vermiculate, having the ridge and pit pat−
tern mentioned by Pregill et al. (1986). Incipient vermiculate
texture exists in the hatchlings as well adults (compare osteo−
derms in Figs. 1B, 2B, 3A, B). A keratinous epidermal scale
covering on live and unprocessed skeletal specimens can cre−
ate a smooth appearance to the osteoderm (dark osteoderms
in Fig. 1), but texturing of the bone exists below. Osteoderm
basal surfaces are typically flat or near−flat (Figs. 4, 5C
2
,F
2
).
Superficially, osteoderms of H. horridum and H. sus−
pectum are similar. Both species have a pattern of ridges and
pits, predominantly polygonal cranial osteoderms, and more
circular body osteoderms. The pattern of texture on the more
robust polygonal cranial osteoderms is repeated on the slightly
thiner, circular body osteoderms. However, we did notice that
regardless of size or age, the ridge pattern on H. horridum rou
−
tinely develops into acute pinnacles (spicules), providing an−
other level of granular texture to the body and skull (Fig. 3A,
B; H. horridum ETVP 7081). Ridge texture of H. suspectum
osteoderms rarely form spicules, giving a more rounded or
worn appearance. However, the presence or absence of spi−
cules does not permit a species−level identification of isolated
osteoderms. Caudal osteoderms are rectangular in outline and
vary from being rather smooth (on the lateral−to−ventral sides
of the tail), to having a dome with a slight vermiculate texture
(on the dorsal side of the tail; Fig. 3C).
Based on our study of extant Heloderma species, we are
confident that their osteoderms can be separated into general
body regions (i.e., cephalic in part, trunk, tail). Although
osteoderm morphology varies by body location, this varia−
tion is clearly narrow, meaning that cranial and post−cranial
osteoderms are much more similar to each other than to any
of the cranial osteoderms in fossil monstersaurian taxa. Fur−
ther, if extinct monstersaurians were similar to the extant in
their osteoderm patterns, we would expect their postcranial
and cranial osteoderms to be similar.
Fossil monstersaurian osteoderms.—The earliest known
monstersaurian is Primaderma nessovi from the Cretaceous
(Albian–Cenomanian) of Utah (Nydam 2000). The exterior
surface of its maxilla is covered with fused, pitted osteo−
derms, which are thinner than those of Heloderma and Para−
derma (see below), yet not as plate−like as those in anguids
(Nydam 2000: fig. 2). However, variation of Primaderma
osteoderms is not understood (no body osteoderms are
known), although based on the cranial osteoderms, they were
presumably not overly domed.
Paraderma bogerti (Estes 1964; Cretaceous, Wyoming),
had osteoderms fused to the cranial elements (Fig. 6C).
These are relatively large, polygonal in outline, pitted, sepa−
rated by a wide groove, and “resembled those of Helo−
derma”, yet are not quite as granular (Estes 1964: 133). Cra−
nial osteoderms were not domed, but flattened and plate−like
(Estes 1964: fig. 64), which is verified by a parietal fragment
(Gao and Fox 1996: figs. 34, 35), which shows the pitting but
suggests the lack of a vermiculate texture.
http://dx.doi.org/10.4202/app.2010.0083
MEAD ET AL.—NEOGENE HELODERMATID LIZARD FROM TENNESSEE 113
Table 1. Modern specimens used in this study.
Species Repository number
Heloderma horridum ETVP 7081
ETVP 7083
ETVP 17865
ETVP 17907
ETVP 17908
Heloderma suspectum ETVP 7085
ETVP 7087
ETVP 7088
ETVP 7089
ETVP 7096
ETVP 7098
ETVP 7099
ETVP 17869
Gobiderma pulchrum (Borsuk−Białynicka 1984: 39) from
the Cretaceous of Mongolia had “rounded, perforated osteo
−
derms of Heloderma type” (see also Gao and Norell 2000).
Cranial osteoderms were fused to the skull and interconnected.
Individual elements were somewhat domed yet more plate−like
(as in Primaderma and Paraderma) than those of Heloderma
(Borsuk−Białynicka 1984: fig. 11; see Gobiderma pulchrum at
http://digimorph.org/specimens/Gobiderma_pulchrum/).
Osteoderms of Estesia mongoliensis (Norell et al. 1992)
from the Cretaceous of Mongolia are inadequately known.
Originally, it was thought that Estesia did not have osteo
−
derms fused to the skull (Norell et al. 1992: table 1, charac
−
ters 47–48, fig. 13; Gao and Norell 2000). Further examina
−
tion showed a slight scar−like structure on the supratemporal
process that might imply the occurrence of at least some (al
−
beit weak) cranial osteoderms, “but it is indecisive as to
whether the osteoderms were platelike or small elements di
−
vided by grooves” (Norell and Gao 1997: 24). Based on the
apparent lack of well developed osteoderms, Estesia need
not be considered further in this study (see discussions in
Gao and Norell 2000; Conrad 2008).
Eurheloderma gallicum (Hoffstetter 1957; Fig. 6A, B; see
114 ACTA PALAEONTOLOGICA POLONICA 57 (1), 2012
5mm
1mm
Fig. 1. Extant helodermatid lizard Heloderma suspectum Cope, 1869 (ETVP 7096). A. An overall appearance of the osteoderm pattern on the cranial and
nuchal regions. B. A close−up of the osteoderms exemplifying the morphology.
also Augé 2005: fig. 181) from the middle/late Eocene of
France had granular osteoderms separated by grooves. Not all
osteoderms were fused to skeletal elements, suggesting that
fusion was likely related to ontogentic growth (Estes 1983).
Based on the presence of cranial osteoderms fused to parietal
and maxilla, body osteoderms were also probably present, but
have not been found. An isolated parietal referred to Eurhelo−
derma from the late Paleocene of Wyoming displays dermal
rugosities, but the overlying osteoderms did not fuse to the
bone, which implies a subadult individual (Pregill et al. 1986).
As noted by Pregill et al. (1986: 191), “the parietal ostoderms
are largest [relatively] in Eurheloderma gallicum, smaller
in Lowesaurus matthewi, and smaller still in Heloderma
texanum, and smallest in H. suspectum and H. horridum.” Al−
though the cranial osteoderms have a vermiculate texture, it
appears that the large osteoderms of Eurheloderma were not
domed or flat−domed, and they had a texture similar to those of
Primaderma, Paraderma,andGobiderma. Isolated osteo−
derms, although rare, are recovered in European deposit s,
suggesting that more may be learned in the future about the
dermal coverning of Eurheloderma (Augé 1995).
Lowesaurus matthewi, described as Heloderma (Gilmore
1928; Pregill et al. 1986) is known from the late Oligo−
cene–early Miocene (Orellan to Arikareean Land Mammal
Ages) of Colorado and Nebraska (see also Yatkola 1976).
Osteoderms attached to the dorsal surface of the frontal are
“shaped like flattened domes”, polygonal, separated from one
another by moderately deep grooves, and have granular tex−
ture; large ones display ridges and pits on the surface (Yatkola
1976: fig. 1; Pregill et al. 1986:183, figs. 4, 6; Fig. 6D, G).
Osteoderms of Lowesaurus are similar to those of Heloderma,
with the exception that they are larger, appear less domed, and
show no evidence of ring−extensions (Fig. 6D, G).
Heloderma texana (Stevens 1977: 6) from the early
Miocene of Texas (Arikareean Land Mammal Age) has
hexagonal− to polygonal−shaped, domed osteoderms (some
“not as flattened as those in the living species”), many with
ring−extensions, and all possess vermic ula te textures w ith
ridges and pits (see H. texana at http ://dig imorp h .or g/sp ec i−
mens/ Heloderma_texana/). These appear near−identical to
those of extant Heloderma. Although the holotype of H.
texana (TMM 40635−123) shows some abrasion, we found
some evidence of spicules formed in places. Both TMM
40635−123 and 40635−119 have ring−extensions around the
osteoderms. The ring−extension around most cranial and
body osteoderms appears to occur in all species of Helo−
derma, and so we propose that this is an apomorphy. Ste−
vens (1977) noted that osteoderms of the extinct H. texana
had deeper pits than in extant species, but we disagree: we
find that la rge H. horridum can have osteoderms with deep
pits and extreme ridges (e.g., ETVP 7081; Fig. 3A, B).
Stevens (1977) determin ed that the H. texana specimen, a
mature lizard, was 30–50% the size of ad ult extan t Helo
−
derma, thu s a d istinc tly smaller species.
Estes (1963) mentioned that a possible helodermatid verte
−
bra and femur were recovered from the Thomas Farm local
fauna of Florida (Hemingfordian Land Mammal Age, early–
middle Miocene; Tedford et al. 2004; Richard C. Hulbert per−
sonal communication, June 2010; contra Bhullar and Smith
2008). Further work produced additional helodermatid re−
mains, and Bhullar and Smith (2008) concluded that several
characters of the dentary were intermediate between Eurhelo−
derma and extant Heloderma, and therefore did not permit ge−
neric identification. A number of isolated osteoderms were re−
covered with the dentary and found to be round to polygonal
in outline; the one pictured has no ring−extension (Bhullar and
Smith 2008: fig. 2B). Apical surfaces are highly domed and
exhi
bit a complex network of ridges and pits (i.e., vermicu−
late). Where many of the ridges connect, they rise to form
“small eminences” (= spicules here) (Bhullar and Smith 2008:
291). Our analysis of fourteen additional helodermatid osteo−
derms from the locality showed that at least six (UF 255289,
255294, 255296, 255297, 255300, 255301) were domed,
moderate to heavy with vermiculate sculpturing, most with
spicules, and had ring−extensions. With these attributes, we
conclude that these osteoderms indicate that Heloderma was
http://dx.doi.org/10.4202/app.2010.0083
MEAD ET AL.—NEOGENE HELODERMATID LIZARD FROM TENNESSEE 115
5mm
1mm
5mm
Fig. 2. Extant helodermatid lizard from the USA. A. Close−up of Helo−
derma suspectum Cope, 1869 hatchling (ETVP 17869) showing that the
frontal and parietal lack fused−on osteoderms (typical of older individuals).
B. Heloderma suspectum Cope, 1869 (hatchling; ETVP 17869). Thin,
waffer−like osteoderms have numerous holes yet already show an incipient
domed form, polygonal outline, and vermiculate texture on the apical side
of the bone (top row) and lacks a basal platform (bottom osteoderm).
C. Close−up of osteoderms covering portions of the parietal and frontal ele−
ments on a juvenile Heloderma horridum Wiegmann, 1829 (ETVP 17907).
in Florida at least by the Hemingfordian, early–middle Mio
−
cene.
An additional locality in Florida has produced heloder
−
matid remains. Bryant (1991) mentioned the recovery of a
single osteoderm from Level 3 at the La Camelia Mine local
−
ity of the Willacoochee Creek Fauna (early Barstovian Land
Mammal Age; mid−Miocene). Unfortunately the osteoderm
was not figured or discussed; our analysis of the specimen
was inconclusive.
Gray Fossil Site.—Three isolated osteoderms (ETMNH
8746; Fig. 5A–C
1
), distinct in morphology to helodermatid
116 ACTA PALAEONTOLOGICA POLONICA 57 (1), 2012
5mm
1mm
1mm
Fig. 3. Extant helodermatid lizard Heloderma horridum Wiegmann, 1829 (ETVP 7081). A. Cranium showing overall pattern of osteoderms. B. Close−up of
osteoderms with an acute vermiculate pattern where spicules form on the ridges. C. Close−up of the tail vertebrae and overlaying osteoderms.
lizards, were recovered (by SLS) from screen−washed sedi
−
ments from the “Rhino Pit” excavation (TP−2−2004, 365−124
provenance) at the Gray Fossil Site. Additional fossil species
from these layers match those found in all other excavation
pits from the site. The three osteoderms are whitish in colour,
unlike the usual brown to black, indicating that they under
−
went some weathering in the upper oxidized zone. Some sal
−
amander vertebrae and Tapirus polkensis elements from the
same excavation unit are equally as oxidized and white.
The osteoderms (ETMNH 8746) are small, ranging from
1.5 to 3.0 mm in diameter, and are circular in basal aspect,
with a domed apical surface. The basal surface is largely flat,
pierced by at least one foramen, and has concentric rings (Fig.
5C
2
) that Bhullar and Smith (2008) suggested might relate to
growth. The apical, domed surface is vermiculate in texture
with a network of pits surrounded by ridges. The lack of a keel
or imbricating surfaces indicates that the osteoderms do not
come from a scincomorphan or anguid , and the other charac
−
ters indicate that they come from a monstersaurian.
The three osteoderms (ETMNH 8746) are small and cir
−
cular and so are not cranial osteoderms (generally more po
−
lygonal). In comparison with extant Heloderma, they are
postcranial, coming anywhere from the neck to the sacrum
(Fig. 5D–F
1
), but not the tail. Their size and morphology sug
−
gests they come from an individual with a snout−vent length
of 200 to 350 mm, not an immature individual (< 150 mm
snout−vent length). The vermiculate network of ridges and
pits on ETMNH 8746 are identical to those found in Helo
−
derma and Lowesaurus. Two of the osteoderms have a ring−
extension around the bone, our proposed apomorphy for
Heloderma (Fig. 5A–C
1
). Two of the three (Fig. 5A, C
1
)
show the ridge development of spicules as noted especially
in extant H. horridum, occasionally in H. suspectum, and in
the Miocene Thomas Farm specimen (discussion above). We
identify the Gray Fossil Site osteoderms as Heloderma, but
cannot go to species level without additional skeletal re
−
mains.
Discussion
Shunk et al. (2006) interpreted the depositional environment
at Gray Fossil Site as recording storm flow influxes into a
paleosinkhole lake. Abundant remains of fishes, neotenic
salamanders, aquatic turtles, and numerous Alligator speci
−
mens confirm a lacustrine environment (Schubert and
http://dx.doi.org/10.4202/app.2010.0083
MEAD ET AL.—NEOGENE HELODERMATID LIZARD FROM TENNESSEE 117
5mm
Fig. 4. Ventral view of extant helodermatid lizard Heloderma suspectum Cope, 1869, cranium (ETVP 7099) with parietal and frontal in position and adja−
cent articulated osteoderms showing basal platform, foramina, and tissue attachments between individual elements.
Wallace 2006; Boardman 2009). Multiple layers of silt and
clay, with less common layers of larger clasts, attest to a pre
−
dominantly low−eneregy aquatic environment, one with at
least some through−flow of water. Stable carbon and oxygen
isotopes from the bones of browsing mammals indicate a
moderately dense forest (C
3
dominated), yet with a grassland
(C
4
) component nearby, and with minimal seasonal varia
−
tions in temperature or precipitation (DeSantis and Wallace
2008). Rare Earth Element analysis suggest that at least the
mammals shared similar depositional environments, and so
were autochthonous (DeSantis and Wallace 2008). More
−
over, the rapid infilling of the sinkhole resulted in the
preservaton of many articulated or nearly articulated skele
−
tons, implying that it was indeed a biocoenosis.
Macroplant remains from the Gray Fossil Site include at
least 35 genera representing more than 25 families of seed
plants. These indicate an oak−hickory subtropical forest domi
−
nated by Quercus (oak) and Carya (hickory) with some form
of conifer species, an understorey of the Corylopsis (buttercup
shrub), and the climbing vines Sinomenium, Sargentodoxa,
and Vitis. Liu and Jacques (2010) described endocarps belong
−
ing to a new species of Sinomenium (Menispermaceae; S.
macrocarpum), today a woody vine confined mostly to low
−
land tropical or subtropical forests of eastern Asia (Luo et al.
2008). Gong et al. (2010) describe three new species of fossil
grapes (Vitis grayensis, V. lanatoides,andV. latisulcata), two
of which closely resemble two Eurasian Vitis species implying
a strong eastern Asian aspect to the Gray Fossil Site. In addi
−
tion, pollen recovered from sediments with vertebrates indi
−
cates tree/bush species, including Ulmus (elm), Betula (birch),
118 ACTA PALAEONTOLOGICA POLONICA 57 (1), 2012
Fig. 6. Various skeletal elements from taxa within Monstersauria illustrat
−
ing the variation in the osteoderms. A, B. Eurheloderma gallicum Hoff
−
stetter, 1957, Phosphorites du Quercy, France, “san precision de gisement”;
maxillae with attached osteoderms. A. Left maxilla, unknown specimen
number. B. Right maxilla, holotype, MNHN; from Hoffstetter 1957; see
Augé (2005) for discussion. C. Paraderma bogerti Estes, 1964, UCMP lo
−
cality V−5817; maxilla UCMP 542610 with attached osteoderms (from
Estes 1964). D, G. Lowesaurus matthewi (Gilmore, 1928) Lewis Creek,
Logan County, Colorado, Oreodon Zone, White River formation. D. Right
frontal, KUVP 49651 showing detail of large osteoderms (from Pregill et al.
1986). G. Right maxilla, UNSM 50011 with large fused osteoderms (from
Yatkola 1976). E, F. Heloderma texana Stevens, 1977, Castolon Local
Fauna, Delaho Formation, Texas. E. Cranial bone TMM 40635−119 with
fused osteoderms illustrating the ring−extension around the individual
osteoderms. F. Holotype skull TMM 40635−123.
1mm
Fig. 5. Osteoderms of the fossil and extant helodermatid lizards from the
USA. A–C. Three osteoderms of Heloderma sp. recovered from the “Rhino
Pit”, Miocene–early Pliocene Gray Fossil Site. A. ETMNH 8746a in apical
view. B. ETMNH 8746b in apical view. C. ETMNH 8746c in apical (C
1
)
and basal (C
2
) views. D–F. Three isolated osteoderms of extant Heloderma
horridum Wiegmann, 1829 showing varying degrees of a bone ring−exten
−
sion around the tubercle. D. ETVP 7083a in apical view. E. ETVP 7083b in
apical view. F. ETVP 7083c in apical (F
1
) and basal (F
2
) views.
Fraxinus (ash), Celtis (hackberry), Alnus (alder), and Salix
(willow) (Wallace and Wang 2004).
Crocodilians, especially Alligator, from the Gray Fossil
Site give additional information on the climate (Colbert et
al. 1946; Markwick 1998). Today, Alligator lives in and
survives colder climatic conditions than any other extant
crocodilian (Brisbin et al. 1982). The present northern−most
extent of A. mississippiensis (American alligator) is close to
the mean January isotherm of 7.2°C(45°F) and the mean
minimum January temperature isotherm of −1°C(34°F). It
is the latter metric that limits the present northern range of
A. mississippiensis. Adult American alligators pass periods
of excessive cold temperatures typically in deep water or
under vegetation, and extant A. sinensis (Chinese alligator)
uses burrows to avoid extreme weather. Viable populations
of Alligator are restricted largely by the greater vulnerabil
−
ity of juveniles and hatchlings than adults to low tempera
−
tures (see discussion in Thorbjarnarson and Wang 2010).
Remains of Alligator representing many different growth
stages are common at the Gray Fossil Site including the area
containing Heloderma. If they had temperature requirements
and restrictions similar to those of the extant species in North
America, then we may be able to reconstruct the local tempera−
ture regime during the Hemphillian. Using the Nearest Living
Relative model of Markwick (1998) indicates a minimum av−
erage temperature of the Gray Fossil Site during the Hemphi−
llian of at least ~22°C(71°F) in order to sustain the apparently
thriving Alligator population. Moreover, the distribution of Al−
ligator today does not include the region of the Gray Fossil Site
in part because the annual temperature range of 14–29°C
(24–84°F) goes too low.
It is not clear whether the reconstructed moderately dense
subtropical forest of the Gray Fossil Site during the Hemphil
−
lian was wet or dry. The abundant plethodontid salamander
vertebrae suggests that the local terrestrial environment was
wet enough (precipitation and/or ground litter) for these
lungless caudates (Boardman 2009). A number of the Gray
Fossil Site plant species have counter−parts in Asia that live
in tropical habitats. Stable isotope data suggest minimal vari
−
ation in temperature or precipitation (DeSantis and Wallace
2008), but there may have been dry and wet seasons (see also
discussion in Shunk et al. 2009).
Species of Heloderma today are not active at temperatures
much below 24°C(75°F; Bogert and Martín del Campo 1956).
Their classic habitat is the hot, dry subtropical Sonoran Desert
(desert−scrub) of the arid Southwest, but H. suspectum also
thrives in semidesert grasslands to woodlands. H. horridum
lives from southern Sonora south along the coastal west side
of Mexico to Guatemala, in tropical dry forests, tropical decid
−
uous forests, pine−oak woodlands, and tropical thornscrubs
(Bogert and Martín del Campo 1956; Beck 2005). While capa
−
ble of living in hot arid environments, the genus is most com
−
mon in tropical deciduous forests where it is known to climb
5–7 m up into trees (Beck 2005). It would appear that such a
helodermatid would be equally capable of surviving, if not
thriving, in the reconstructed forest of the GFS during the
Hemphillian.
The reconstructed warm climate, with either wet or dry,
subtropical or tropical habitats in North America during the
early to late Miocene extended as far north as the Beringian
platform (Wolfe 1994a, b). A tropical forest connection
between Asia and North America is also well established
(see among others, discussions in Sirkin and Owens 1998;
Reinink−Smith and Leopold 2005); however, from the east
−
ern portion of the continent, it is poorly known because of the
rarity of localities (Wallace and Wang 2004). The Pipe Creek
Sinkhole locality (Indiana; Hemphillian fauna) is interpreted
as a ponded sinkhole and the ecological reconstruction is a
warm−grassland−woodland transition, but no lizards were re
−
covered (Farlow et al. 2001). The Thomas Farm locality
(Florida; Hemingfordian fauna), although earlier than the
Gray Fossil Site, is also reconstructed as a sinkhole pond sys
−
tem with a diverse fauna (including some lizards) that inhab
−
ited a mixed dry, open country and forested ecotone (Estes
1963).
The Gray Fossil Site provides a unique view of the Mio−
Pliocene of eastern North America. Intermixed within the
oak−hickory subtropical habitat were faunal components (i)
typical of the late Hemphillian of North America, (ii) unique
components with distinct Asian affinities, and (iii) now, with
the Heloderma presented here, taxa presently restricted to arid
and tropical environments of Mexico. Clearly the onset of the
Plio−Pleistocene cooling events altered the mosaic of floral
and faunal species in the communities of the southern Appala−
chians as well as elsewhere in North America. Tihen (1964:
278–279) presented a then−merging theme, “The present
[herpetofaunal] groups inhabitating temperate North America
derive from three main sources: (a) relicts of groups that were
widespread in the early Tertiary; (b) groups entering from ‘the
north’—eventually Eurasia—between the mid−Oligocene and
mid−Pliocene; (c) groups entering from ‘the south’—Central
America … this ‘southern’ contribution is more extensive than
is usually realized.” Consequently, the Heloderma record pre
−
sented here further corroborates Tihen’s model.
Conclusions
Three lizard osteoderms were recovered from the Hemphil
−
lian−age (latest Miocene–early Pliocene) sediments at the
Gray Fossil Site in northeastern Tennessee. A review of
osteoderm morphology of extinct and extant lizards of
Monstersauria led to the discovery of a ring−extension that
can surround or partly surround the osteosderms of both the
cranium and body, character identified here as an apomorphy
for Heloderma. This genus then once inhabited an oak−hick
−
ory subtropical forest surrounding a pond environment that
contained abundant remains of fishes, neotenic salamanders,
aquatic turtles, and numerous Alligator. The forest, with
many species of climbing vines, was also inhabited by,
among others, species of lesser panda, Eurasian badger,
http://dx.doi.org/10.4202/app.2010.0083
MEAD ET AL.—NEOGENE HELODERMATID LIZARD FROM TENNESSEE 119
rhino, small bear, sabre−toothed cat, various artiodactyls,
multiple terrestrial salamanders, fossorial lizards, snakes,
and the beaded lizard Heloderma. We show that Heloderma
was present during the Miocene from at least middle
Hemingfordian to the latest Hemphillian Land Mammal
Ages in tropical to subtropical environments in southeastern
North America.
Acknowledgments
We appreciate the help of Jeff Supplee, Brian Compton, April Nye,
Shawn Haugrud, Brett Woodward, and Jeanne Zavada, all members of
ETMNH and Gray Fossil Site. We thank Christopher J. Bell (University
of Texas at Austin, USA), Randy Nydam (Midwestern University, Glen
−
dale, Arizona, USA), and Jozef Klembara (Comenius University in
Bratislava, Slovakia) for their continued assistance and discussions about
extant and fossil anguimormph lizards. We thank Richard Hulbert
(Florida Museum of Natural History, Florida, USA) for the loan of the
Thomas Farm helodermatid remains. We appreciate discussions with
Yu−Sheng Christopher Liu and Diana Ochoa−Lozano (ETMNH) about
the botanical remains recovered from the Gray Fossil Site. Helpful re
−
views and discussions were received from Marc L. Augé (Muséum na
−
tional d'Hisitoire naturelle, France), Michael Benton (University of Bris
−
tol, UK), Bhart−Anjan S. Bhullar (Harvard University, Massachusetts,
USA), and Robert Sullivan (State Museum of Pennsylvania, USA). Par−
tial funding for this project was received from National Science Founda−
tion Award 0958985 to co−authors SCW and BWS.
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