Kirtland Formation (De-na-zin Member), San Juan Basin, New Mexico. The frontal is larger, .... New Mexico Museum of Natural History and Science Bulletin 35.
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.
253
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 VP1955), 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 VP1270 (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.
254
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.
255
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.
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.
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 VP1955) 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.
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).
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-
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 40Ar/39Ar dates of 73.04 Ma (Ash J) and 73.37 Ma (Ash H), pub-
lished by Fassett and Steiner (1999), which bracket the upper and lower 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 dinosaur faunas during the Late Cretaceous is borne out by the generic 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 corridor for dinosaur dispersal. 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.
REFERENCES Barsbold, R. and Osmólska, H., 1999, The skull of Velociraptor (Theropoda) from the Late Cretaceous of Mongolia: Acta Palaeontologica Polonica, v. 44, p.189219. 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 Museum of Palaeontology, p. 52-60. 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, p. 367-397. 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. Norell, M.A. and Makovicky, P.J., 1999, Important features of the dromaeosaurid skeleton II: Information from newly collected specimens of Velociraptor
mongoliensis: American Museum Novitates, no. 3282, 28 p. Osborn, H.F., 1924, Three new Theropoda, Protoceratops zone, central Monogolia: American Museum Novitates, no. 144, 12 p. Peng, J., Russell, A.P., and Brinkman, D.B., 2001, Vertebrate microsite assemblages (exclusive of mammals) from the Foremost and Oldman formations of the Judith River Group (Campanian) of southeastern Alberta: An illustrated guide: The Provincial Museum of Alberta, Natural History Occasional Paper, no. 25, 54 p. Rauhut, O.W.M. and Werner, C., 1995, First record of the family Dromaeosauridae (Dinosauria: Theropoda) in the Cretaceous of Gondwana (Wadi Milk Formation, northern Sudan). Paläontologische Zeitschrift, v. 69, p. 475-489. Rowe, T., Cifelli, R.L., Lehman, T.M. and Weil, A., 1992, The Campanian Terlingua local fauna, with a summary of other vertebrates from the Aguja Formation, Trans-Pecos Texas: Journal of Vertebrate Paleontology, v. 12, p. 472-493. Sankey, J.T., 2003, New theropod and bird teeth from the Late Cretaceous (Maastrichtian) Hell Creek and Lance Formations (abs): Journal of Vertebrate Paleontology, v. 23, p. 93A. Sankey, J.T., 2005, Late Cretaceous vertebrate paleoecology of the Big Bend National Park, Texas; in Braman, D.R., Therrien, F., Koppelhaus, E.B. and Taylor, W., Dinosaur Park Symposium, p. 89-106. Sankey, J.T., Brinkman, D.B., Guenther, M., and Currie, P.J., 2002, Small theropod and bird teeth from the Late Cretaceous (Late Campanian) Judith River Group, Alberta: Journal of Paleontology, v. 76, p. 751-763. Sues, H.-D., 1977, The skull of Velociraptor monogoliensis, a small Cretaceous theropod dinosaur from Mongolia: Paläontologische Zeitschrift, v. 51, p.173184. Sues, H.-D., 1978, A new small theropod dinosaur from the Judith River Formation (Campanian) of Alberta: Zoological Journal of the Linnean Society, v. 62. p. 381-400. Sullivan, R.M. and Lucas, S.G., 2000, First occurrence of Saurornitholestes (Theropoda: Dromaeosauridae) from the Upper Cretaceous of New Mexico: New Mexico Museum of Natural History and Science, Bulletin 17, p. 105-108. Sullivan, R.M. and Lucas, S.G., 2003, The Kirtlandian, a new land-vertebrate “age” for the Late Cretaceous of Western North America: New Mexico Geological Society, 54th Field Conference, Guidebook, p. 369-377. Sullivan, R.M. and Lucas, S.G., 2006, The Kirtlandian land-vertebrate “age”— faunal composition, temporal position and biostratigraphic correlation in the nonmarine Upper Cretaceous of western North America: New Mexico Museum of Natural History and Science Bulletin, this volume.
