A new choristodere from the Cretaceous of Mongolia

2005·3.7 MB·English

by  Ksepka, Daniel T

Nowitate MUSEUM
ovitates
PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY
CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024
Number 3468, 22 pp., 11 figures March 24, 2005
A New Choristodere from the
Cretaceous of Mongolia
DANIEL T. KSEPKA,! KE-QIN GAO,? AND MARK A. NORELL?
ABSTRACT
The remains of a choristodere recently discovered at Two Volcanoes, a new locality in the
Gobi Desert of Mongolia, are described in this paper. Consisting of a fairly complete skull
and partial postcranial skeleton, this specimen represents a new species of the genus Tchoiria.
The new species differs from Tchoiria namsarai in having a much smaller number of teeth.
Several elements preserved in this specimen are unknown in 7. namsarai and thus provide
new information about the genus. Phylogenetic analysis with the addition of data from the
new specimen confirms the basal position of Tchoiria in Simoedosauridae.
INTRODUCTION ported clade of large Cretaceous and early
Tertiary forms, named Neochoristodera by
The Choristodera are a clade of aquatic
Evans and Hecht (1993).
and semi-terrestrial fossil diapsids. Twelve
?Khurendukhosaurus and ?Irenosaurus
previously described genera have been as-
are poorly known genera that appear to be-
signed to the Choristodera, but one, Laza-
long to the Neochoristodera or a more inclu-
russuchus, lacks the derived character states sive clade within the Choristodera. ?Khuren-
that diagnose the group (Gao and Fox, 1998) dukhosaurus is known from postcranial ma-
and must be placed outside of the group, terial representing a small and apparently
probably as a sister taxon. Of the remaining plesiomorphic choristodere. ?/renosaurus is
genera Champsosaurus, Simoedosaurus, Ike- known only from fragmentary postcranial
chosaurus, and Tchoiria form a well-sup- material including a humerus and vertebrae
' Division of Paleontology, American Museum of Natural History ([email protected]).
? Department of Geology, Peking University, Beijing 100871, People’s Republic of China; Research Associate,
Division of Paleontology, American Museum of Natural History ([email protected]).
3 Division of Paleontology, American Museum of Natural History ([email protected]).
Copyright © American Museum of Natural History 2005 ISSN 0003-0082
2 AMERICAN MUSEUM NOVITATES NO. 3468
(PIN 3368/2), and may be synonymous with of a very large choristodere (PIN 559/501)
another genus (Efimov and Storrs, 2000). from the Lower Cretaceous of Khamaryn
Based on the highly incomplete nature of the Khural, Mongolia. Efimov (1983) later re-
known material it is best not to speculate on described the material, and reassigned the
the relationships of these taxa until more is species to the genus /kechosaurus on the ba-
known of their morphology. sis of the rectangular tooth bases. The incom-
The remaining taxa include several small plete nature of the known material for this
forms, all seemingly plesiomorphic to the specimen makes it difficult to diagnose, and
Neochoristodera. Cteniogenys is quite small it may be better left incertae sedis until more
(about 150 mm in length, excluding the tail)
material is discovered. Efimov (1983) named
and primitive in a number of characteristics
the species Tchoiria egloni on the basis of
relative to the Neochoristodera (Evans,
some vertebrae and limb bones (PIN 3386/
1990). Another primitive genus, Monjuro-
2) from the same locality as the holotype of
suchus, reaches moderate size (~300 mm)
T. namsarai. Efimov (1988) later reassigned
and is unique in the secondary closure of the
this material, erecting the questionable genus
infratemporal fenestrae (Gao et al., 2000).
TIrenosaurus. The holotype material of [ren-
The recently discovered Shokawa represents
osaurus elongi is differentiated from Tchoir-
a new morphotype within the Choristodera
ia primarily by the lack of a prominent en-
characterized by an extremely long neck rel-
tepicondylar groove and more elongate dor-
ative to body size (Evans and Manabe,
sal vertebrae (Efimov, 1988). The differences
1999). Hyphalosaurus was originally classi-
fied as a diapsid of uncertain affinity (Gao et in the humerus may be related to ontogeny
al., 1999), but is now considered to be a (Efimov and Storrs, 2000). The neural spines
choristodere closely related to Shokawa (Gao of the dorsal vertebrae have a primitive
et al., in prep.). shape, with a concavity above the prezyga-
Pachystropheus has been reinterpreted as pophyses in lateral view, and are distinct
a primitive choristodere (Storrs and Gower, from dorsals preserved in Tchoiria. New ma-
1993). Although cladistic analysis indicates terial is needed to confirm the validity of the
that Pachystropheus may share a sister group genus ?/renosaurus.
relationship with the two long-necked gen- INSTITUTIONAL ABBREVIATIONS: AMNH,
era, near complete lack of cranial material American Museum of Natural History; GIN,
makes this relationship tentative at best Geological Institute of the Mongolian Acad-
(Evans and Manabe, 1999), and new material emy of Sciences; PIN, Paleontological Insti-
is needed to resolve the basal taxonomy of tute, Russian Academy of Sciences.
the Choristodera.
The first specimen of Tchoiria was dis-
covered in 1971 at Htiren Dukh, Mongolia, MATERIALS AND METHODS
by the Soviet—Mongolian paleontological ex-
pedition (Efimov, 1975). A partial skull and
The specimen was collected during the
partial postcranial skeleton (PIN 3386/1)
1998 field expedition of the Mongolian
were recovered from the Aptian Hiihteeg ho-
Academy of Sciences—American Museum of
rizon. The remains were assigned to the type
Natural History Paleontological Project. The
species, Tchoiria namsarai. The specimen
specimen was collected in a large indurated
described in this paper shares features diag-
nostic of Tchoiria, but differs from 7. nam- boulder at Two Volcanoes, a new locality in
sarai in certain features and is thus consid- southern Mongolia (fig. 1). Vertebrate fossils
ered a separate species of the genus. at this locality are uncommon, but some di-
Two additional species of Tchoiria have nosaur bones, a few turtles, and abundant in-
been described, but both have subsequently vertebrates have been found. Spatulate sau-
been reassigned to other genera. Efimov ropod teeth (fig. 2) collected at this site re-
(1979) named and described the species semble teeth from the Early Cretaceous Odsh
Tchoiria magnus on the basis of a partial Formation described by Osborn (1924) as
mandible and portions of postcranial material Asiatosaurus mongoliensis.
2005 KSEPKA ET AL: CRETACEOUS CHORISTODERE 3
RUSSIA
MONGOLIA
© Ulaanbaatar
= Two Volcanoes
CHINA
Beijing
o
Fig. 1. Map of Mongolia showing the collection locality.
SYSTEMATIC PALEONTOLOGY ETYMOLOGY: Named in honor of James
DIAPSIDA OSBORN, 1903 Klausen, the preparator of the holotype, in
recognition of years of dedicated volunteer
CHORISTODERA COPE, 1876
service to the American Museum of Natural
NEOCHORISTODERA EVANS AND HECHT, 1993 History.
SIMOEDOSAURIDAE LEMOINE, 1884 TYPE LOCALITY AND HORIZON: The type lo-
cality is Two Volcanoes (43°49.503’N,
TCHOIRIA EFIMOV, 1975
99°19.546’E; fig. 3). The fossil beds at Two
Tchoiria klauseni, new species Volcanoes are composed of light-colored fine
sands interbedded with coarser sands. The
HOLOTYPE: The holotype (GM 1/8) con-
age of this locality has not been firmly es-
sists of an incomplete skull and partial dis-
tablished; however, the freshwater inverte-
articulated postcranial skeleton. The follow-
brate fauna and the presence of spatulate sau-
ing elements are preserved: a skull nearly
ropod teeth suggest an Early Cretaceous Ap-
complete on the right side and fragmentary
tian age roughly equivalent to beds of the
on the left, missing the tip of the snout and
*““Hiitheg Svita’’? (Shuvalov, 2000). A more
distorted in the braincase region; partial right
well-constrained age for Two Volcanoes
dentary and splenial in articulation; five cer-
vical, six dorsal, three sacral, and four caudal awaits further work.
vertebrae or vertebral elements; several ribs DIAGNOSIS: Tchoiria klauseni can be dif-
and gastralia; right coracoid, fragment of ferentiated from the type species 7. namsarai
scapular blade; right radius, right ulna; right by the smaller number of teeth in the maxilla
ilium, partial right ischium, partial right pu- and the symphyseal portion of the dentary.
bis; right femur. There are slightly more than 34 teeth in the
- AMERICAN MUSEUM NOVITATES NO. 3468
Pig. 2: Sauropod tooth from Two Volcanoes in (a) labial and (b) lingual view and AMNH 6294, a
tooth assigned to Asiatosaurus mongoliensis in (c) labial and (d) lingual view.
maxilla of 7. klauseni while there are more The holotype skull of 7. klauseni is larger
than 60 in the maxilla of 7. namsarai. There and more robust than that of 7. namsarai
are 12 teeth in the symphyseal portion of the (PIN 3386/1), though in the absence of data
dentary in 7. klauseni and 17 teeth in the on the age or sex of these specimens it re-
symphyseal portion of the dentary in 7. nam- mains uncertain whether this difference is
saral. appropriate in diagnosing the species. The
snout is relatively wide immediately in front
DESCRIPTION of the orbits, but narrows anteriorly. The an-
terior tip of the snout was lost to erosion, so
SKULL
the exact proportions of the skull and nature
The skull resembles other neochoristoder-
of the external narial opening are uncertain.
es in general appearance in that the postor-
Examination of the nasal cavity where the
bital region is flared, the snout is elongated
snout is broken reveals that it is a laterally
and narrow, and the orbits are small and dor-
expanded oval in cross section at this point.
sally directed (fig. 4). The skull is quite flat
The nasals are fused into a single element.
dorsoventrally. Although the skull is flat-
The nasals intervene between the maxillae,
tened in all neochoristoderes, it is evident
but it is uncertain how far posteriorly they
from the orientation of the temporal bars that
extend, and whether they thin to a narrow
the degree of flatness in this specimen has
splint or a broad triangular point.
been somewhat exaggerated by postmortem
The premaxillae are not preserved. The
crushing. In dorsal view the skull is nearly
complete on the right side, except for the tip right maxilla is missing only the anterior tip
of the snout, and fragmentary on the left and the left is less complete. A row of fo-
side. The palatal region is fairly complete but ramina lines the lateral border of the maxilla
the posterior region has suffered a good deal slightly above the tooth level. The posterior
of distortion. Although many of the posterior foramina are contiguous with posteriorly run-
cranial bones are present, most have been ning grooves. There are 34 alveoli observ-
displaced from their original position. able in the right maxilla, though it is likely
2005 KSEPKA ET AL: CRETACEOUS CHORISTODERE 5
aR WW 1k eer : Te
| "25S ive\ ry
ri i +e ¥A yi}e aNBe i ! WyJ" fL Y
vas ~~ \\ Aa e° \, , 4 1 a e.e
4!
Fig. 3. Satellite photo of Two Volcanoes Locality, white arrow pointing to location where the spec-
imen was collected.
several are missing from the anterior end. In saurus (Gao and Fox, 1998). The marginal
the holotype of 7. namsarai (PIN 3386/1), teeth are conical and longitudinal striations
the number is much greater, with over 60 in are visible on several teeth. The anterior teeth
each maxilla. The alveoli are subcircular to are largest, and the teeth become progres-
ovate and fairly shallow. The marginal al- sively smaller posteriorly. There is a more
veoli of Champsosaurus are circular, while pronounced size decrease in the posterior
those of simoedosaurids are transversely ex- maxillary teeth in JT. namsarai, allowing for
panded in the anterior part of the tooth row. a greater number of teeth to be accommo-
The expansion is noticeably weaker in dated in nearly the same amount of space.
Tchoiria than in Simoedosaurus and Ikecho- Complete teeth exhibit a slight medial curve.
6 AMERICAN MUSEUM NOVITATES NO. 3468
Fig. 4. The holotype skull of Tchoiria klauseni (GM 1/8) in dorsal and ventral views. Light gray
represents preserved areas of the palatal dental batteries, dark gray represents matrix. Abbreviations are
listed in appendix 4.
Several broken teeth show that the enamel is The orbits are suboval, longer than wide,
infolded at the base. Grooves cut into the me- and much wider at the posterior margins than
dial bases of some teeth were apparently the anterior margins. They are surrounded by
caused by erupting teeth, though none are a raised rim, which is very rugose except for
preserved in situ. These grooves may exag- the lacrimal portion of the orbital border,
gerate the degree of expansion in some al- which is smoother and less elevated.
veoli. The prefrontals and frontals form the in-
2005 KSEPKA ET AL: CRETACEOUS CHORISTODERE es
Fig. 4. Continued.
terorbital bar, and the bar is about the width of Ikechosaurus has revealed that the olfac-
of the orbit halfway between the anterior and tory canal extends from this point into a
posterior orbital margins. It is arched, ele- large Cavi Nasi Proprium, divided into an
vating the medial orbital border. Ventrally, upper olfactory region and lower respiratory
the olfactory canal runs along the interorbital region (Lu et al., 1999).
bar at midline. The frontals project between The lacrimal forms the anterior border and
the prefrontals to form the dorsal border of part of the lateral border of the orbit. It is
the olfactory canal for most of its path along moderately sized and subtriangular in dorsal
the interorbital bar. The olfactory canal con- view, with a rounded anterior point. The lac-
tinues anteriorly between the ventral flanges rimal extends to overlap the jugal along the
of the prefrontals into the snout region, anterior half of the lateral border of the orbit.
where the pterygoids form a ventral border The lacrimal foramen opens between the lac-
enclosing it. Computed tomography scanning rimal, palatine, and prefrontal on the anterior
8 AMERICAN MUSEUM NOVITATES NO. 3468
Fig. 3: Posterior view of the anterior wall of left orbit. a, Two small foramina perforating lacrimal. b,
lacrimal duct opening between lacrimal, palatine, and prefrontal. Abbreviations are listed in appendix 4.
wall of the orbit (fig. 5). A crest superior to makes up the anterior border of the inferior
the lacrimal foramen separates it from two temporal fenestra.
smaller foramina that perforate the posterior The postorbitofrontal, an element derived
surface of the lacrimal. from the fusion of the postorbital and post-
The jugal forms the majority of the lateral frontal (Russell-Sigogneau and Russell,
border of the orbit. The anterior process ex- 1978), forms the posterior border of the orbit.
tends to sit in a notch on the dorsal surface It meets the dorsal process of the jugal lat-
of the maxilla and is bordered medially by erally and the frontal medially. Ventrally, at
the lacrimal. The anterior process extends as the anterolateral corner, the postorbitofrontal
far as the anterior border of the lacrimal, as overlaps the jugal, as in Simoedosaurus (Er-
in Champsosaurus and Ikechosaurus. The ickson, 1987). The postorbitofrontal has a
posterior process of the jugal joins the quad- small contact surface with the parietal pos-
ratojugal to form the inferior temporal bar. teromedially. The posterior process of the
The dorsal process is fairly prominent, unlike postorbitofrontal articulates with the squa-
that of Simoedosaurus and [kechosaurus, but mosal to form the anterior portion of the su-
similar to the condition in Champsosaurus perior temporal bar.
and Cteniogenys. It contacts the postorbito- The quadratojugal forms the posterior por-
frontal at a suture running from near the me- tion of the inferior temporal bar. The slender
dial border of the inferior temporal fenestra anterior process articulates with the posterior
to near the lateral border of the orbit, and so process of the jugal in an oblique suture with
2005 KSEPKA ET AL: CRETACEOUS CHORISTODERE 9
the quadratojugal on the lateral side. The row widens to three or four teeth across. Due
quadratojugal widens posteriorly where it to wear, it is uncertain whether the row con-
meets the squamosal. The nature of this su- tinues onto the pterygoid uninterrupted or a
ture is obscured by breakage in the area. toothless gap exists at the posterior end of
Ventrally, the quadratojugal is encrusted in the palatine. Such a gap is present in 7. nam-
matrix, but it is possible to observe the con- sarai (Efimov, 1975: fig. 1). A battery sev-
dyle for articulation with the quadrate. As a eral teeth wide is present on the pterygoid in
matter of preservation the quadrate has been alignment with the palatine tooth row, and
detached from the quadratojugal and dis- ends at the same level as the marginal tooth
placed from its original orientation. The row. A row of palatal teeth is present on each
rounded quadrate process is visible, as is the vomer, beginning posterior to an area of
condyle for the articular. smooth bone at the anterior part of the vo-
The squamosal forms the posterolateral mer. These rows widen to several teeth wide
corner of the skull. To prevent thin and frag- in front of the choanae, thin to a single tooth
ile bone from crumbling, a block of matrix row between the choanae, and then widen to
was retained in this region as the specimen a width of several tooth positions again pos-
was being prepared. The slender anterior pro- terior to the choanae. Wear makes it uncer-
cess of the squamosal forms the posterior tain if these rows are continuous with batte-
portion of the superior temporal bar, articu- ries on the pterygoids, but it seems likely
lating with the posterior process of the post- there was a toothless gap between them. The
orbitofrontal. A raised crest begins at the lev- pterygoid batteries are quite extensive, reach-
el of the posterior margin of the inferior tem- ing a width of seven teeth. The choanal
poral fenestra and runs along the medial mar- grooves separate the palatine and pterygoid
gin of the squamosal on the dorsal surface. rows, and the vomeral rows are separated
The ventral surface of the squamosal is from each other by a trough between the vo-
smooth. The contacts between the squamosal mers. The degree to which the medial com-
and the parietal, quadrate, neomorph, and plexes are separated on the pterygoids is ob-
quadratojugal are obscured by breakage and scured by wear and breakage.
the supportive matrix. A matrix-filled gap The pterygoid flange overlaps the inferior
may represent the suture between the squa- process of the ectopterygoid, which borders
mosal and parietal, in which case this suture the posterior and posteromedial edges of the
is located near the posterior margin of the maxilla. Though there are no teeth visible on
Superior temporal fenestra. However, the the transverse process of the pterygoid, it is
possibility that the gap is a crack and the worn, and teeth may have been eroded away.
suture lies farther forward cannot be ruled The quadrate process of the right pterygoid
out. is preserved. Strong ridges run along the ex-
The parietals form the posterior portion of posed ventral side, presumably for attach-
the skull roof along with the supraoccipital. ment of the pterygoideus muscle. There is no
The parietals meet the frontals at a point evidence of an internarial, but nothing is pre-
slightly anterior to the superior temporal fe- served anterior to the vomers, so its presence
nestra. The posterior processes of the parie- cannot be decidedly ruled out.
tals extend less than half of the length of the The choanae are subovular and posteriorly
superior temporal fenestra before curving lat- displaced from the external nares. They are
erally. The contacts with the squamosal, pro- bordered anteriorly and medially by the vo-
otic, and neomorph are indeterminate due to mers and laterally by the palatines. The su-
breakage. perior borders of the choanae are ridged and
The palate is covered in very small teeth. grooved. The anterior boarder of the right
These palatal teeth are arranged in several choana is at the level between the 27th and
paired complexes (fig. 4). A single row of 28th marginal tooth positions, counting from
teeth runs along a ventral ridge on each pal- the posterior-most tooth. The pterygoids are
atine beginning slightly posterior to the cho- elongate and contact the vomers near the
anae. Near the anterior border of the subor- posterior border of the choanae, though the
bital fenestra, the ridge widens and the tooth suture is not identifiable due to wear. Poste-
10 AMERICAN MUSEUM NOVITATES NO. 3468
riorly, each choana is contiguous with a the dorsal border of the foramen magnum,
groove that runs to the posterior border of contacting the exoccipitals ventrally at its
the pterygoid tooth battery. The palatal fo- posterolateral angles. A high midline crest at
ramen opens between the pterygoid and pal- the anterior apex grades into the flat dorsal
atine as in other neochoristoderes. surface posteriorly. There is a notch at mid-
The suborbital fenestra is relatively long line on the posterior border. The supraoccip-
and appears subtriangular, with a straight lat- ital remains attached to the right parietal but
eral edge and a partly round medial edge. has separated from the left parietal leaving
The medial edge appears to constrict the pal- the sutural contact surface exposed. The pa-
ate between the suborbital fossae as in rietals partially overlap the supraoccipital at
Champsosaurus. However, along the medial its anterior end. The posterior portion of the
edge thin portions of pterygoid have broken supraoccipital is exposed dorsally, as in
off, causing the foramen to appear larger and Champsosaurus, rather than being complete-
likely more rounded. The maxilla partici- ly covered by the parietals, as in Simoedo-
pates in the border of the suborbital fenestra saurus and I[kechosaurus (Brinkman and
as in Ikechosaurus (Brinkman and Dong, Dong, 1993). Dorsal exposure of the supra-
1993) and Champsosaurus. The maxilla is occipital also occurs in 7. namsarai (Efimov,
excluded from the suborbital border by the 1975). The left exoccipital is preserved at-
palatine and ectopterygoid in Simoedosaurus tached to the supraoccipital and the right is
(Russell-Sigogneau and Russell, 1978). missing. The exoccipital is an hourglass-
The shape of the interpterygoid vacuity is shaped bone forming the lateral wall of the
unclear due to distortion of the region, but foramen magnum.
the medial surface of the right pterygoid in- The bones that enclose the posterior cavity
dicates it was elongate. The vacuity appears of the braincase (sensu Fox, 1968) and form
to have extended anteriorly to a point some- the medial wall of the temporal fenestrae are
where within the middle third of the subor- partially preserved and visible in dorsal view,
bital fenestra. The pterygoids form the an- though they have been rotated from their
terior border, but the posterior border is not original position. The overlapping contact
observable. In T. namsarai the interpterygoid between the neomorph and quadrate can be
vacuity is very large and is enclosed poste- traced, but these bones are broken near the
riorly by the broadly bifurcated anterior pro- anterior portion of the contact so the location
cess of the parasphenoid. of the pterygoquadrate foramen is uncertain.
Although the occipital condyle has been The prootic and the contact between the neo-
displaced from its original position, the cran- morph and parietal are not identifiable.
iomandibular joint was located posterior to The second region of the braincase, the an-
the occipital condyle as indicated by the po- terior trough (sensu Fox, 1968), and the un-
sition of other elements. The right condyle derlying basioccipital, basisphenoid, and par-
for the articulation with the mandible has asphenoid are heavily obscured by matrix. To
been displaced forward and rotated, but its better examine this region, a fragment of
true position is discernable and confirmed by skull comprising the posterior end of the left
the quadratojugal. The occipital condyle has pterygoid attached to the above-mentioned
been disarticulated from the rest of the skull complex was left free during preparation.
along with the parasphenoid and _ basisphe- The parasphenoid has been worn away in
noid. However the position of the exoccipi- most areas, but a concretion cast preserves a
tals, supraoccipital, and parietals, in articu- portion of its outline. The degree of postero-
lation but slightly displaced, indicates the lo- lateral expansion is unclear. The rostrum of
cation of the foramen magnum and thus the the parasphenoid is also obscured as it is
occipital condyle. In 7. namsarai the cran- overlapped by the left pterygoid. However,
iomandibular joint is located on the level of the lateral edge is visible on the right side.
or slightly posterior to the occipital condyle. Though the anterior end is incomplete, it is
The braincase region is distorted and bro- clear that the rostrum is quite long and that
ken, but some observations can be made. The it widens anteriorly. The basisphenoid espe-
supraoccipital is a triangular bone that forms cially is encrusted in hard matrix and is thus