A complete late Cretaceous iguanian (Squamata, Reptilia) from the Gobi and identification of a new iguanian clade

2007·3.6 MB·English

by  Conrad, Jack L

A tamerican museum
Novitates
PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3584, 47 pp., 19 figures September 6, 2007
A Complete Late Cretaceous Iguanian
(Squamata, Reptilia) from the Gobi and
Identification of a New Iguanian Clade
JACK L. CONRAD1 AND MARK A. NORELL2
ABSTRACT
Iguania is a diverse clade with an incompletely known fossil record. Here, we describe and name
the earliest iguanian known from a complete skeleton. The specimen (IGM 3/858) comes from
Ukhaa Tolgod (Upper Cretaceous of Mongolia) and offers important insights into the
evolutionary history of iguanian osteology. The new taxon is diagnosed by a combination of
character states, including the presence of a frontoparietal fontanelle, absence of an enlarged
nuchal fossa, and unflared tooth crowns. We performed a cladistic analysis including 54 taxa
scored for 202 informative morphological characters. A strict consensus of 46 shortest recovered
trees reveals that the new taxon is a basal member of a previously unidentified clade of Cretaceous
iguanians, probably endemic to the Gobi. This clade of Gobi iguanians is nested within
a monophyletic Pleurodonta (non-acrodontan iguanians).
INTRODUCTION suggests 1,442), the iguanian fossil record is very
Iguania is a remarkably diverse squamate incomplete, especially prior to the Neogene.
clade spanning five continents and occupying The past decade has seen a dramatic increase
a variety of ecological niches and habitat types in the discovery of fossil iguanian material.
(Frost and Etheridge, 1989; Bauer, 2003; Uetz, Especially surprising is the large amount of
2006). Despite their extant diversity of form and diversity from continental Asia. Gao and Hou
the large number of species represented in (1995) described Anchaurosaurus gilmorei based
modern faunas (Frost and Etheridge [1989] on an incomplete skeleton in 1995. Later, Gao
reports 993 extant species; Uetz’s [2006] count and Norell (2000) described a wealth of new
1 Division of Paleontology, American Museum of Natural History ([email protected]).
2 Division of Paleontology, American Museum of Natural History ([email protected]).
Copyright © American Museum of Natural History 2007 ISSN 0003-0082
2 AMERICAN MUSEUM NOVITATES NO. 3584
material from the Mongolian Late Cretaceous Holotype: IGM 3/858.
relating to previously described taxa (Iso- Type Locality and Horizon: Ukhaa
dontosaurus gracilis, Mimeosaurus crassus, Phry- Tolgod, Nemegt Basin, Mongolian Gobi Desert;
nosomimus asper, Priscagama gobiensis, and Upper Cretaceous Djadokhta Formation
Polrussia mongoliensis) and several new taxa (Loope et al., 1998) (fig. 2).
(Ctenomastax parva, Temujinia ellisoni, and Known Distribution: Known only from
Zapsosaurus sceliphros) discovered as part of the type locality and horizon.
the Mongolian Academy of Science-American Diagnosis: IGM 3/858 differs from Cteno¬
Museum of Natural History expeditions. Conse¬ mastax parva and Temujinia ellisoni (each mono-
quently, our knowledge of the Cretaceous specific; hereafter, Ctenomastax and Temujinia,
iguanian fauna from the Gobi and our un¬ respectively) in possessing an apparently un¬
derstanding of iguanian morphology have in¬ calcified region of the skull roof around the
creased dramatically. pineal foramen; a parietal fontanelle (similar to
Many fossil lizards are known from Ukhaa that seen in some extant Crotaphytidae). It
Tolgod, a ‘Djadockhta-like’ locality whose differs from Temujinia and Zapsosaurus sceli¬
geology recently has been reviewed and sum¬ phros (hereafter, Zapsosaurus) in lacking the
marized (Loope et al., 1998; Gao and Norell, enlarged paired fossae for the spinalis capitis. It
2000; see also references therein). Ukhaa differs from Ctenomastax in lacking caniniform
Tolgod has been and continues to be an teeth; from Zapsosaurus by the absence of
immensely productive ‘fossil bonanza’ (Gao
strongly flared marginal tooth crowns and
and Norell, 2000: 7), yielding the remains of
the more robust shape of the retroarticular
numerous tetrapods, including mammals;
tubercle; and from Anchaurosaurus gilmorei,
many saurischian and ornithischian dinosaurs;
Isodontosaurus gracilis, and Polrussia mongolien¬
and thousands of lizard specimens. Among the
sis (each monospecific; hereafter, Anchau¬
most breathtaking of the lizard specimens
rosaurus, Isodontosaurus, and Polrussia, respec¬
collected on these recent expeditions is a com¬
tively) in possessing light dermal sculpturing on
plete iguanian (fig. 1). Although it was illus¬
the parietal and frontal and the anterolateral
trated in Gao and Norell (2000: fig. 37), we
orientation of the ectopterygoid. It differs from
describe this new taxon here for the first time.
Isodontosaurus in lacking a posteriorly spatulate
In addition to naming the new Ukhaa Tolgod
nasal process of the premaxilla, which does not
iguanian, we present a phylogenetic data matrix
make contact with the frontal on the dorsal skull
to identify its placement on the iguanian family
roof; in possessing a weakly inclined anterior
tree. The new taxon shares important charac¬
margin of the maxillary nasal process; in
teristics with some other Cretaceous iguanians
possessing a jugal that lies mostly dorsal to the
from the Gobi. Together, these early iguanians
maxilla; in possessing a supratemporal; and in
offer new insights into the evolutionary history
possessing a mediolaterally developed postfron¬
of Iguania.
tal. It differs from Polrussia in possessing a mid¬
line contact of the maxillae posteriorly to the
SYSTEMATIC PALEONTOLOGY premaxillary nasal process, an elongate supra¬
temporal, a distinct postfrontal, and an anteri¬
orly oriented ectopterygoid. It differs from both
SQUAMATA OPPEL, 1811 IGUANIA COPE, 1864 Isodontosaurus and Polrussia in possessing
PLEURODONTA COPE 1864 a forked medial margin of the postfrontal and
in lacking a dorsal process on the squamosal.
Saichangurvel davidsoni, new genus and
species
figures 1, 3-4, 6-7, 8A, 9-12, 13A, 14-18 Description
Etymology: Saichan- (Mongolian: ‘beau¬ Gao and Norell (2000: 106-107; fig. 37)
tiful’) + gurvel (Mongolian: ‘lizard’) and illustrated and briefly mentioned and IGM 3/
davidsoni after Amy Davidson, who collected 858, noting its exceptional preservation. The
and prepared the specimen. Davidson’s beau¬ entire skeleton is preserved in articulation and
tiful lizard. lacks only the right postorbitofrontal, squamo-
Fig. 1. Saichangurvel davidsoni (IGM 3/858) in dorsal view as preserved.
4 AMERICAN MUSEUM NOVITATES NO. 3584
Fig. 2. Map of Mongolia showing the location of Ukhaa Tolgod.
sal, and quadrate, the right forelimb and quarter of the length of the skull from the tip of
manus, the right femur and both hind-limb the premaxilla to the anterodorsal margin of the
zeugopodia, and part of the distal tail (fig. 1). foramen magnum. As preserved, the orbital
These portions of the skeleton were exposed region is slightly more elongate than the snout,
prior to and during the thunderstorm in which making up approximately three quarters of the
IGM 3/858 was discovered. Because the spec¬ skull length. The nares are slightly retracted,
imen remains ventrally embedded in sandstone a condition exaggerated by the fact that the
matrix, some ventral and medial elements (such snout tip has rotated slightly dorsally with
as parts of the palate and pectoral girdle) are respect to the rest of the skull (cf. figs. 3^1).
not visible. As noted by Gao and Norell (2000), The acuminate suborbital fenestra is visible in
the fantastic preservation of the specimen dorsal view through the orbit and is somewhat
indicates a rapid burial. Rapid burial is typical smaller than the supratemporal fenestra and
of many of the Ukhaa Tolgod and Bayn Dzak approximately one half the length of the orbit.
fossils (see Loope et al., 2005) and is thought to The supratemporal fenestra is posteriorly bor¬
have occurred when semi-stable sand dunes dered by the ectopterygoid and pterygoid and
catastrophically mobilized when they became anteriorly bordered by the palatine and jugal.
supersaturated with water. The teardrop-shaped supratemporal fenestra is
round anteriorly and tapers posteriorly. It is
bounded anteriorly by the postorbital and
Skull Form
postfrontal, medially by the parietal, laterally
The skull is lightly built, with large orbits and by the postorbital and squamosal, and poster¬
a complete supratemporal arch (figs. 1, 3^4). iorly by the squamosal, supratemporal, and
The antorbital snout makes up roughly one parietal.
2007 CONRAD AND NORELL: A COMPLETE GOBI IGUANIAN 5
Fig. 3. Skull of Saichangurvel davidsoni (IGM 3/858). A, photograph; and B, drawing of the skull as
preserved, in dorsal view.
Two ceratobranchials, identified as left and nasal, but extends to near the posterior margin
right ceratobranchials I, are preserved extend¬ of the external naris. The margin between the
ing posteriorly from beneath the skull (figs. 1, nasal process and the main body of the
3A). These are elongate rods extending poster¬ premaxilla is not strongly angulated; instead,
iorly to about the level of the posterior margin it curves down to the dental margin. A
of the axis. maxillary facet is present on the dorsal margin
of this curved surface, but apparently was not
exposed on the external skull surface. There
Skull Roof
are no ethmoidal foramina through the pre¬
Premaxilla (figs. 1, 3-4): The premaxilla maxilla.
is fused. It is displaced slightly out of its Maxilla (figs. 1, 3-4, 9): Both maxillae
natural articulation with the maxilla, although are well preserved, but the left side lacks the
parts of the articular surfaces remain in premaxillary process, and there is minor
contact, and the premaxillary contact with dorsal damage to the nasal process. The
the nasals remains articulated. The premaxilla maxillae are sub-triangular, with an anteriorly
retains seven visible teeth and probably con¬ placed nasal process that extends vertically
tains 10 tooth positions. The premaxillary along the side of the snout and medially near
nasal process is broadest at its base and tapers the naris. This condition also is present in
dorsally. It is only about two tooth positions Temujinia, Ctenomastax, and Crotaphytidae.
wide at its widest point but is still wider than The maxilla is sutured to the premaxilla
deep. The premaxillary nasal process is broken without a premaxilla-maxilla aperture (sensu
near its base and near its contact with the Gao and Norell, 1998; Conrad, 2006b, in
6 AMERICAN MUSEUM NOVITATES NO. 3584
Fig. 4. Reconstructed skull of Saichangurvel davidsoni (IGM 3/858). A, dorsal view; B, left lateral view.
Note that reconstructed elements appear as semiopaque shadows. No detail is given for the mandible in B,
because only the medial view is visible as the specimen is preserved (see fig. 8).
press). Although only the right premaxillary posteroventral to the premaxillary nasal pro¬
process is preserved, it is complete and shows cess in the complete articulated skull. The
that the anterior terminus was forked into shorter, anterior ramus approaches the base of
septomaxillary and anterior rami (fig. 3). The the premaxillary nasal process without actu¬
septomaxillary ramus of the premaxillary ally overlapping it. Posterodorsal to the pre¬
process projects medially. Based on the size maxillary process, the maxilla extends in
of the septomaxillary ramus and the articular a gentle slope posteriorly such that the
surfaces present on the premaxilla, as de¬ anterior margin of the nasal process is not
scribed earlier, it is clear that the septomax¬ strongly offset from the dorsal surface of the
illary rami would have met at the midline premaxillary process (fig. 4). The apex of the
2007 CONRAD AND NORELL: A COMPLETE GOBI IGUANIAN 7
Fig. 5. Drawings of dorsal skull surfaces in the area of the parietal foramen: A, Temujinia ellisoni; B,
Zapsosaurus sceliphros; C, Priscagama gobiensis. Dorsal view of skulls: D, Ctenomastax parva; E, Gambelia
wislizenii; F, Diplolaemus bibroni (REE 2506). A-C, modified after photos in Gao and Norell (2000); D,
composite illustration modified after Gao and Norell (2000: figs. 3-4). Reconstructed areas are shown as
semitransparent shadow layers. E, modified after Maisano (2003a).
nasal process overlaps the anterodorsal mar¬ it is in Leiocephalus, Oplurus, some iguanids
gin of the prefrontal and extends medially (including Dipsosaurus dorsalis), and various
onto the skull roof, where it contacts the scleroglossans. Five maxillary labial foramina
nasal. The posterior margin of the nasal are preserved, all situated anterior to the
process is slightly emarginated, accommodat¬ orbit and well separated from the dental
ing the anterior projection of the lacrimal. Its margin.
margin is posteroventrally oriented toward Nasal (figs. 3-4): The paired nasals make
the dental margin and the posterior terminus contact for approximately one half their
of the maxilla. The dental margin extends to length, but are damaged near their middle
about the level of the midpoint of the orbit. (fig. 3). Each nasal, as exposed, is sub¬
Slight damage to the posteriormost tip of the rhomboid, tapering anteriorly and posteriorly.
alveolar margin prevents identifying whether The anterolateral surface is gently curved
the ectopterygoid was exposed on the exter¬ laterally, forming part of the dorsal margin
nal surface of the skull behind the maxilla, as of the external naris. The premaxilla overlaps
AMERICAN MUSEUM NOVITATES NO. 3584
Fig. 6. A, photograph; and B, drawing of the posterodorsal view of the orbital wall and palate of
Saichangurvel davidsoni (IGM 3/858).
the nasals anteriorly, and the frontals invade (fig. 6). A single, small lacrimal foramen is
the internasal suture for more than one third present about midway up the orbital lamina of
of the length of the nasals. The nasal tapers the prefrontal. It is enclosed dorsally, medial¬
posteriorly where it dorsally overlies the ly, and ventrally by the prefrontal and
frontal in a manner suggestive of a nasal laterally by the lacrimal. The prefrontal and
lamina of the frontal. palatine are in broad contact in a straight,
Prefrontal (figs. 3-4): Each robust pre¬ slightly ventromedially oriented suture.
frontal is teardrop-shaped in dorsal view, with Lacrimal (figs. 3-4, 6): Both lacrimals are
a broad anterior process and an elongate preserved in articulation with the maxillae and
frontal process. No dermal sculpturing is jugals. The lacrimals remain in articulation
present on the prefrontal. The prefrontal boss with the prefrontals, but dorsoventral com¬
is expressed as a laterally projecting tubercle pression of the skull has shifted the prefrontals
on the anterolateral corner of the orbit, near slightly ventrally with respect to the lacrimals,
the anterior limit of the lacrimal (most visible jugals, and maxillae.
in fig. 4B). This configuration is more similar The lacrimal lies at the anterior end of the
to the condition seen in Ctenomastax jugal. The outline of the lacrimal (its dorsal
(fig. 5D), Temujinia (Gao and Norell, 2000), and ventral margins) is continuous with the
and crotaphytids (Maisano, 2003a) than mod¬ anterior part of the jugal with only a small
ern Iguana or Zapsosaurus sceliphros, in which point of maxillary invasion between the two
the prefrontal boss is less robust and more (fig. 4B). The lateral part of the jugal-lacrimal
contiguous with the rest of the skull surface. suture is nearly horizontal, and its orbital part
The frontal process extends posteriorly to near is dorsomedially oriented. Dorsally, the lacri¬
the midpoint of the orbit, but does not mal abuts the ventral surface of the prefrontal
approach the postorbitofrontal. Most of the boss. The suborbital ridge arises from the
prefrontal medial border is bounded by the point where the prefrontal makes contact with
anterior part of the frontal. The prefrontal is the lacrimal and extends posteriorly along the
blocked from contacting the jugal inside the suborbital process and up the postorbital
orbital rim by a lacrimal-palatine contact process of the jugal.
2007 CONRAD AND NORELL: A COMPLETE GOBI IGUANIAN 9
many other iguanians). A squamosal ramus of
the postorbital process is present. The squamo¬
sal ramus is flattened mediolaterally and over-
lies the lateral surface of the postorbital when in
natural articulation. The sub orbital ridge ex¬
tends from its origin on the lacrimal and curves
around the ventrolateral and posterior portions
of the orbit. The postorbital process of the jugal
lacks the anterior and posterior flanges seen in
many acrodonts, corytophanids, and some
polychrotids; thus, it is not dilated. There is
no dermal sculpturing on the jugal.
Postfrontal (figs. 3-4): Saichangurvel da¬
vidsoni retains a separate postfrontal and post¬
orbital. The right postorbital has been lost or is
incomplete and hidden by the jugal and matrix
(fig. 3). Regardless, it is out of articulation with
the postfrontal, allowing a clear view of the
postorbital facet on the postfrontal.
The postfrontal is a mediolateral bar joining
the frontal and parietal with the supratempor¬
al arch. As in Temujinia, the postfrontal is
medially forked, with a long frontal process
and a shorter parietal process, the latter fitting
into a notch on the dorsolateral surface of the
parietal. The frontal process is approximately
Fig. 7. Drawing of the braincase, posterior twice as long as the parietal process, but does
skull roof, and anterior cervical vertebrae of not approach the prefrontal. As in Temujinia
Saichangurvel davidsoni (IGM 3/858): A, postero¬ and Ctenomastax, the postfrontal is a medio¬
lateral view; B, lateral view. laterally oriented bar. It borders the orbit and
the supratemporal fenestra. Gao and Norell
(2000) noted that this postfrontal morphology
Jugal (figs. 1, 3-4, 6): Although both is unusual for iguanians and is more like that
jugals are well preserved and in articulation in scleroglossans. However, even among scler-
with the maxillae and lacrimals, they have oglossans, the mediolateral-bar morphology
been pushed somewhat out of position from of the postfrontal is relatively rare (Conrad, in
their contacts with the supratemporal arch press).
(figs. 1, 3). The left side is closer to its original Postorbital (figs. 3-4): Only the left post¬
position, but still has slid over the anterodor- orbital is preserved, and it has moved some¬
sal surface of the postorbital. The right jugal what out of natural articulation. The jugal
has turned somewhat dorsolaterally. The jugal hides the lateral surface of the postorbital, so
is broadly exposed dorsal to the maxilla in the latter’s contribution to the posterior
lateral view and would have overlapped the margin of the orbit cannot be determined.
ventrolateral surface of the postorbital and the The postorbital is triangular in dorsal view. It
squamosal. has a short orbital surface, a long lateral
The jugal is composed of a suborbital process surface, and a slightly curved contribution to
and a postorbital process forming an obtuse the supratemporal fenestra. A forked facet on
angle to each other. No posteroventral process the postfrontal clasps the medial angle of the
is present, but there is a moderately sharp point postorbital; the postorbital was laterally cov¬
where the posteroventral process occurs in ered by the jugal in a short overlapping joint.
many lizards (including Anchaurosaurus, The supratemporal process of the postorbital
Ctenomastax, Temujinia, Zapsosaurus, and extends posteriorly in a tapering process that
10 AMERICAN MUSEUM NOVITATES NO. 3584
Fig. 8. Drawings of selected pleurodontan mandibles in medial view, anterior to the right. A,
reconstruction of Saichangurvel davidsoni with semitransparent areas representing restored areas; B,
Gambelia coper, C, Hoplocercus spinosus (AMNH 93807); D, Leiocephalus carinatus. B, modified after
McGuire (1996); D, modified after Frost and Etheridge (1989). Note the variability in the splenial
morphology as it relates to the closure of Meckel’s canal.
does not reach the midpoint of the supratem- Squamosal (figs. 1, 3-4): Only the left
poral fenestra. A groove on the medial surface squamosal is preserved. It has been displaced
of the squamosal suggests that it accepted the somewhat from natural articulation with the
lateral surface of the postorbital in a shallow supratemporal and postorbital by dorsal
tongue-and-groove contact. movement of the quadrate and by dorsoven-