saurornitholestes robustus, n. sp. - Robert M. Sullivan

Lucas, S. G. and Sullivan, R.M., eds., 2006, Late Cretaceous vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin 35.
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SAURORNITHOLESTES ROBUSTUS, N. SP. (THEROPODA: DROMAEOSAURIDAE) FROM
THE UPPER CRETACEOUS KIRTLAND FORMATION (DE-NA-ZIN MEMBER),
SAN JUAN BASIN, NEW MEXICO
ROBERT M. SULLIVAN
Section of Paleontology and Geology, The State Museum of Pennsylvania, 300 North Street, Harrisburg, PA 17120-0024
Abstract—Saurornitholestes robustus, n. sp., is named based on new frontal material from the upper part of the
Kirtland Formation (De-na-zin Member), San Juan Basin, New Mexico. The frontal is larger, and unusually thick,
compared to specimens of Saurornitholestes langstoni. All specimens of Saurornitholestes previously collected from
the San Juan Basin, and referred to the Judithian taxon S. langstoni, are now provisionally referred to this new
species. A single tooth and a left second pedal ungual are referred to cf. Saurornitholestes robustus. The recognition
of a new species of Saurornitholestes from the Upper Cretaceous strata of the San Juan Basin, further supports the
distinct temporal position of the upper Fruitland and Kirtland vertebrate faunas, and is consistent in the recognition of
a Kirtlandian land-vertebrate “age” fauna.
INTRODUCTION
Ongoing fieldwork in the Upper Cretaceous strata of the San Juan
Basin, New Mexico, continues to add to our knowledge of dinosaur and
other fossil vertebrates during the Kirtlandian time interval (Sullivan and
Lucas, 2003, 2006). Previously, Sullivan and Lucas (2000) reported on a
water-worn left frontal of a small theropod dinosaur that compared favorably
to specimens of Saurornitholestes langstoni, with the exception that
the anterior part of the New Mexico specimen was slightly more constricted
anteriorly. In the summer of 2005, a larger, more robust, left frontal, referable
to the genus Saurornitholestes, was recovered from the upper part of
the Kirtland Formation (De-na-zin Member) in Alamo Wash. This new
frontal forces a reconsideration of the identity of the previously reported
specimen (SMP VP-1270). Here, I report on this new specimen (SMP VP-
1955), and designate a new species of Saurornitholestes, S. robustus, that
adds to our knowledge of Late Cretaceous North American velociraptorines
and specifically the Kirtlandian vertebrate fauna. In this paper, AMNH =
American Museum of Natural History (New York); GIN (IGM) = Mongolian
Institute of Geology (Ulan Bataar); SMP = The State Museum of Pennsylvania,
Harrisburg; and TMP = Royal Tyrrell Museum of Palaeontology,
Drumheller.
SYSTEMATIC PALEONTOLOGY
DINOSAURIA
SAURISCHIA Seeley, 1888
THEROPODA Marsh, 1881
DROMAEOSAURIDAE Matthew and Brown, 1922
Velociraptorinae Barsbold, 1983
SAURORNITHOLESTES Sues, 1978
Type species— Saurornitholestes langstoni Sues, 1978.
Comments— Previously, Saurornitholestes has been considered
to a monotypic taxon known from skeletal material from the Dinosaur
Park Formation of Alberta, Two Medicine Formation of Montana and the
Kirtland Formation of New Mexico (Currie, 2005; Sullivan and Lucas,
2000). Other reports, based solely on teeth, have also been made by Baszio
(1997a,b), Currie et al. (1990), Peng et al. (2001), Rowe et al. (1992),
Sankey (1998, 2003) and Sankey et al. (2002, 2005). The referral of specimens
to cf. Saurornitholestes langstoni from the Horseshoe Canyon and
Scollard formations by Baszio (1997b) should be viewed with skepticism.
Baszio (1997b), himself, noted that isolated teeth from the Scollard, Lance
and Frenchman formations may actually belong to another theropod. In
fact, the newly named Atrociraptor from the Horseshoe Canyon Formation
of Alberta, has a skull that is very different from Saurornitholestes
(e.g., a shorter and deeper face), but its teeth are almost indistinguishable
from Saurornitholestes (Currie and Varricchio, 2004). For the present, it
seems that the genus Saurornitholestes is restricted to the Late Campanian
(Judithian and Kirtlandian).
Saurornitholestes robustus, n. sp.
Holotype— SMP VP-1955, nearly complete left frontal.
Horizon and Type Locality— De-na–zin Member of the Kirtland
Formation. The type locality is SMP loc. no. 388 (Alamo Wash [north fork]);
UTM data is on file and available to qualified researchers.
Age— late Kirtlandian (73 Ma; see Sullivan and Lucas, 2006).
Diagnosis— A species of Saurornitholestes distinguished from
Saurornitholestes langstoni by its larger and more robust frontal
(twice as thick).
Description— The holotype (SMP VP-1955, Fig. 1) is a nearly
complete left frontal. There is some damage behind the orbital region toward
the posterior process and there is also some minor damage along the
anterior-most part of the orbital adjacent to the lacrimal facet. The dorsal
and lateral (orbital) surfaces are weathered and splintered, giving the frontal
a rather rough appearance. The ventral and medial surfaces are largely
unweathered. Despite the weathered and less than complete condition, the
frontal is adequate for diagnostic purposes.
The frontal measures (anteroposteriorly along the midline) 62 mm
in length. Maximum width cannot be established with certainty because of
the damage along the lateral portion of the postorbital, but I estimate it at
about 30 mm.
Dorsally, the frontal is relatively flat, rising slightly toward the rim
of the orbital border. There is a slight depression on the dorsal surface above
where the confluence of the posterior margin of the olfactory bulb and the
anterior margin of the cerebral hemisphere impression occur (on the ventral
side). The area of the frontal-nasal contact is damaged, both medially
and laterally. The lacrimal contact surface is intact, and is well-developed
on the anterolateral side of the frontal, but it is slightly damaged along its
lateroposterior-most portion at the front of the orbital rim.
Ventrally, the olfactory bulb and cerebral hemisphere impressions
are present, although the former is weakly developed. Anteriorly, the frontal
is relatively thick and presumably had a strong contact with the nasal.
The olfactory bulb impression is less pronounced than that of SMP VP-
1270 (Sullivan and Lucas, 2000), but that may be due to the fact that SMP
VP-1270 is water-worn. The bridge between the olfactory bulb impression
and the cerebral hemisphere impression is not as constricted as in SMP
VP-1270, again probably for the same reasons. The cerebral hemisphere
impression is well-defined, bordered laterally by the prominent descending
orbital border. The frontoparietal articulation surface is weakly developed
and slightly damaged.

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FIGURE 1. Saurornitholestes robustus, n. sp. SMP VP-1955 (holotype), nearly complete left frontal, from SMP loc. # 388, De-na-zin Member, Kirtland
Formation. A, dorsal view (stereo pair); B, ventral view (stereo pair); C, orbital rim (lateral view) (dorsal direction is to the right); D, medial view of midline
sutural surface (dorsal direction is to the left). Bar scale = 10 mm.

FIGURE 2. Cf. Saurornitholestes robustus, n. sp. SMP VP-1901 (referred
specimen), islolated tooth, from SMP loc. # 350, De-na-zin Member, Kirtland
Formation. Bar Scale = 5 mm.
Median suture thickness is 8 mm, measured at the confluence of
the posterior margin of the olfactory bulb and the anterior margin of the
cerebral hemisphere impression. The maximum thickness of the frontal is
where the posterior border descends ventrally. Here, the frontal measures
10 mm thick, and is more than twice the thickness of the holotype (TMP
74.10.5) in this region. The median suture surface is very rugose.
The lateral surface of the orbital wall is weathered. No foramina are
visible, due to the condition of the lateral surface.
Comments— The massive nature of the holotype frontal (SMP VP-
1955) clearly distinguishes it from the holotype of Saurornithololestes
langstoni (TMP 74.10.5), leaving little doubt as to its specific distinctiveness.
The ratio of the length (measured along the midline) to the thickness
(posterior part of the frontal) is 6:1 for S. robustus and 10:1 for S. langstoni.
Moreover, the holotype of S. robustus differs from referred specimens of
Velociraptor mongoliensis (Barsbold and Osmólska, 1999), although they
share a number of features (i.e., length/width ratio; shallow depression along
mid-length of frontal; strong frontal nasal contact; lacrimal overlapping the
frontal dorsally, etc.), which are herein considered characters of
velociraptorines in general. Thus, there are sufficient reasons to retain both
Saurornitholestes and Velociraptor as distinct genera.
The frontal, once attributed to the species S. langstoni, has all the
morphological attributes of the genus. These include: 1) triangular shape;
2) posteroventral projection of the frontoparietal contact; 3) sigmoidally
curved ridge running anterolaterally onto the frontal margin of the postorbital
process; and 4) posterior concavity around the supratemporal fossae
(Sues, 1978; Currie, 1987; Sullivan and Lucas, 2000). The specific difference
is the massive thickness of the frontal and somewhat larger size.
Cf. Saurornitholestes robustus
Material— SMP VP-1901, tooth (Fig. 2), SMP loc #350; and SMP
VP-1741, left second pedal ungual (Fig. 3), SMP loc. #361. Both specimens
were collected from the De-na-zin Member, Kirtland Formation, San
Juan Basin, New Mexico
Comments— The tooth (SMP VP-1901) is nearly complete and is
characteristically laterally compressed and strongly recurved. It has a FABL
(fore-aft basal length) of 6.5 mm. The height of the tooth is 12 mm, mea-
255
FIGURE 3. Cf. Saurornitholestes robustus, n. sp. SMP VP-1741 (referred
specimen), left pes ungual (digit II), from SMP loc. # 361, De-na-zin Member,
Kirtland Formation. A, medial view; B, lateral view. Bar Scale = 10 mm.
sured from the base to the tip of the crown along the posterior carina; 15
mm along the anterior carina. Denticles are present along both the anterior
and posterior carinae, and they are slightly hooked toward the tip. Some of
the denticles are damaged, especially along the anterior edge towards the
base. The enamel veneer is largely missing, with only remnants adhering
to the tooth in the vicinity of the denticles on both the anterior and posterior
carinae. Although somewhat larger than most Saurornitholetes teeth, this
tooth cannot be distinguished from those of Saurornitholestes (S. langstoni)
as illustrated by Currie et al. (1990), Sankey et al. (2002) and Brinkman
(2005).
The second pedal ungual (SMP VP-1741) is incomplete both proximally
and distally, making it difficult to accurately determine its total size.
However, it measures 69 mm along the preserved outside curvature. It is
laterally compressed and possesses a prominent lateral groove on each side.
This groove is situated slightly higher on the left lateral side compared to
the right side, thus it is from the left pes (Kirtland et al., 1993; Rahut and
Werner, 1995; Norell and Makovicky, 1997). The pedal ungual is deeper
than that of Velociraptor mongoliensis (IGM 100/985) described and illustrated
by Norell and Makovicky (1997, fig. 6).
DISCUSSION
This is the second frontal of Saurornitholestes known from the San
Juan Basin, New Mexico, and arguably it is the most important, as it documents
features that serve to distinguish it from its presumed predecessor
Saurornitholestes langstoni. The previous report of Saurornitholestes from
the Kirtland Formation was also based on a left frontal from the San Juan
Basin (Sullivan and Lucas, 2000). However, this specimen (SMP VP-1270)
was smaller, slightly water-worn, and in some respects, similar to the holotype
of S. langstoni. At the time of its report, some minor differences were
noted, but these were relegated to individual variation (Sullivan and Lucas,
2000). A re-evaluation of SMP VP-1270, compared to the holotype of S.
langstoni (TMP 74.10.5), which are the same size, shows it to be more
like the holotype of S. robustus in having a thicker frontal. A single tooth
(SMP VP-1901) and a left second pedal ungual (SMP VP-1714) are both
from the De-na-zin Member of the Kirtland Formation and clearly pertain
to Saurornitholestes (Figs. 2, 3). They are provisionally referred to S.

256
robustus based on geographic and stratigraphic parsimony.
Two species of Saurornitholestes are now known: S. langstoni and
S. robustus. Currie (2005) cited three undescribed partial skeletons, two
from the Dinosaur Park Formation of Alberta and one from the Two Medicine
Formation of Montana.
Although Saurornitholestes is considered to be similar to the Asian
dromaeosaurid Velociraptor, recognition of two separate genera has been
the consensus of most workers (Currie, 2005). Differences in the postcranial
skeleton have already been documented by Norell and Makovicky
(1997), and the deeper pedal ungual in Saurornitholestes robustus, if properly
referred, further reinforces the argument for generic and specific distinction.
Although the frontals are not preserved in the holotype of
Velociraptor mongoliensis (AMNH 6515) (Osborn, 1924; Sues, 1977),
frontals are known from referred material (GIN 1000/24, 100/25) described
in detail by Barsbold and Osmólska (1999). Comparison of the frontal material
to that seen in a cast of skull of Velociraptor mongoliensis from a
private collection, also verify the taxonomic distinction between the genera
Velociraptor and Sauronitholestes.
All specimens of Saurornitholestes robustus come from the De-nazin
Member of the Kirtland Formation, which is Kirtlandian age. The duration
of the Kirtlandian is approximately 2 million years (74.8 to 72.8 Ma)
based on recent correlations (see Sullivan and Lucas, 2006). The fossils
from the De-na-zin Member can be more precisely dated at 73 Ma based
on the 40 Ar/ 39 Ar dates of 73.04 Ma (Ash J) and 73.37 Ma (Ash H), pub-
Barsbold, R. and Osmólska, H., 1999, The skull of Velociraptor (Theropoda) from
the Late Cretaceous of Mongolia: Acta Palaeontologica Polonica, v. 44, p.189-
219.
Baszio, S., 1997a, Palaeoecology of dinosaur assemblages throughout the Late Cretaceous
of South Alberta, Canada: Courier Forschungsinstitut Senckenberg, v.
196, p. 1-31.
Baszio, S., 1997b, Systematic Palaeontology isolated dinosaur teeth from the latest
Cretaceous of south Alberta, Canada: Courier Forschungsinstitut Senckenberg,
v. 196, p. 33-77.
Brinkman, D., 2005, An illustrated guide to the vertebrate microfossils from the
Dinosaur Park Formation: Unpublished guide, prepared for the Alberta Paleontological
Society, Workshop on vertebrate microfossils (Jan. 26, 2002); revised
April 3, 2005, 141 p.
Currie, P.J., 1987, Theropods of the Judith River Formation of Dinosaur Provincial
Park, Alberta, Canada; in Currie, P.J. and Koster, E.H., eds., Fourth Symposium
on Mesozoic Terrestrial Ecosystems, Short Papers, v. 3: Drumheller, Tyrrell
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Currie, P.J., 2005, Theropoda, including birds; in Currie, P.J. and Koppelhus, E.B.,
eds., Dinosaur Provincial Park: Bloomington and Indianapolis, Indiana University
Press, p. 367-397.
Currie, P.J., Rigby, J.K., Jr., and Sloan, R.E., 1990, Theropod teeth from the Judith
River Formation of southern Alberta, Canada; in Carpenter, K. and Currie, P.J.,
eds., Dinosaur systematics: Perspectives and approaches: Cambridge, Cambridge
University Press, p.107-125.
Currie, P.J., and Varricchio, D.J., 2004, A new dromaeosaurid from the Horseshoe
Canyon Formation (Upper Cretaceous) of Alberta, Canada; in Currie, P.J.,
Koppelhus, E.B., Shugar, M.A., and Wright, J.L., eds., 2004, Feathered Dragons.
Studies on theTransition from Dinosaurs to Birds. Bloomington, Indiana
University Press, p. 112-132.
Eberth, D.A., 2005, The geology; in Currie, P.J. and Koppelhus, E.B., eds., Dinosaur
Provincial Park: Bloomington and Indianapolis, Indiana University Press,
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Fassett, J.E. and Steiner, M.B., 1997, Precise age of C33N-C32R magnetic-polarity
reversal, San Juan Basin, New Mexico and Colorado: New Mexico Geological
Society, Guidebook 48: 239-247.
Kirkland, J.I., Gaston, R., and Burge, D., 1993, A large dromaeosaur (Theropoda)
from the Lower Cretaceous of Eastern Utah: Hunteria, v. 2, 1-16.
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skeleton II: Information from newly collected specimens of Velociraptor
REFERENCES
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portions of the De-na-zin Member. By contrast, specimens of
Saurornitholestes (S. langstoni) from the Dinosaur Park and Two Medicine
formations are considerably older, 75 Ma at the very least (Eberth,
2005). It is not unreasonable to conclude that given the age difference, that
the morphological differences between the species of Saurornitholestes
are significant. Despite ongoing claims of endemic faunas and provincialism
of Western Interior dinosaurs during the Late Cretaceous (see Sullivan
and Lucas, 2006, for citations), these differences are mostly due to temporal
differences, not biogeographic ones. The cosmopolitan nature of the
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similarities of faunas from the north to the south. The coastal plain along
the western margin of the Western Interior seaway provided a barrier free
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ACKNOWLEDGMENTS
I thank James Gardner (Royal Tyrrell Museum of Palaeontology,
Drumheller) and Phil J. Currie (University of Alberta, Edmonton) for access
to comparative material; and Phil J. Currie and Don Brinkman (Royal
Tyrrell Museum of Palaeontology, Drumheller) for discussions concerning
Saurornitholestes. Don Brinkman and Spencer G. Lucas reviewed this
contribution and I thank them for the comments and suggestions.
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