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Journal of Vertebrate Paleontology 20(1):7±11, March 2000 ᭧ 2000 by the Society of Vertebrate Paleontology

RAPID COMMUNICATION

A NEW TROODONTID THEROPOD FROM UKHAA TOLGOD,

MARK A. NORELL1, PETER J. MAKOVICKY1, and JAMES M. CLARK2 1 Division of Paleontology, American Museum of Natural History, 79th Street at Central Park West, New York, New York 10024-5192; 2Department of Biological Sciences, George Washington University, Washington, D.C. 20052

Troodontid theropods, originally described from isolated DiagnosisÐByronosaurus jaffei can be assigned to the Troo- teeth that were at one time confused with those of pachyceph- dontidae on the basis of the following combination of derived alosaurs (Gilmore, 1924), are represented by only a few taxa characters: (1) numerous tightly packed teeth on the anterior known mostly from sparse and fragmentary material (OsmoÂlska part of the row (Currie, 1987); (2) maxillary participation and Barsbold, 1990; Russell and Dong, 1994; Varrichio, 1997). in the posterior margin of the nares (Barsbold and OsmoÂl- Nevertheless, a number of characters have been used to suggest ska,1990) (also present in therizinosaurs [e.g., ]); that these are very closely related to Aves (Gauthier, (3) otosphenoidal crest large and prominent (Currie and Zhao, 1986; Currie and Zhao, 1994; Forster et al., 1997), either as 1994); (4) dentaries that are roughly triangular in side view with part of a group with (e.g., Gauthier, 1986; mental foramina in a deep groove (Currie, 1987); and (5) a Sereno, 1997) or alone (e.g., Forster et al., 1998). lacrimal with an elongate anterior process dorsal to the antor- Here we describe a new taxon of troodontid found bital fossa. at Ukhaa Tolgod, an extremely rich locality in Mongolia's Gobi jaffei can be differentiated from all other troo- Desert (Dashzeveg et al., 1994). This taxon is known from two dontids reported to date on the basis of the following adult individuals, including one of the best preserved troodontid derived characters: (6) teeth devoid of serrations on the anterior skulls ever collected. These specimens allow us to make pre- and posterior carinae (also found in Spinosauridae, Pelacani- liminary statements on troodontid phylogeny and monophyly mimus and some basal ); (7) the presence of a large and to review the distribution of several characters, such as interfenestral bar that is not inset medially from the lateral sur- unserrated teeth and an extensive secondary bony palate, which face of the maxilla; (8) the presence of a connection between have implications for maniraptoran phylogeny and the origin of the nasal passage and the antorbital fenestra through the inter- avialans. fenestral bar; and (9) a horizontal groove on the maxilla adja- cent, and parallel to the tooth row, containing small foramina. SYSTEMATIC PALEONTOLOGY Referred SpecimenÐA second specimen (IGM 100/984), collected July 15, 1996 at a locality called Bolor's Hill, ap- , Marsh 1881 proximately 5 km away from the main Ukhaa Tolgod exposure, TETANURAE Gauthier 1986 can be referred to Byronosaurus jaffei. The fragmentary ros- Gauthier 1986 trum preserves some important features not preserved on the Gilmore 1924 type, including a secondary bony palate formed by both the BYRONOSAURUS JAFFEI sp. nov. premaxillary and maxillary shelves and the vomer. It is referred (Figs. 1, 2) to Byronosaurus jaffei on the basis of the derived characters 6, MaterialÐHolotype: IGM (Institute of Geology, Mongolia) 7, and 8. 100/983, a fragmentary skull and postcranial bones (Fig. 1). DescriptionÐThis description is a composite based on both The skull of IGM 100/983 is preserved in two parts. The rostral the type and referred specimens. fragment and the braincase were found in place adjacent to one Each premaxilla contains four teeth. Large nasal processes another in life position separated by a zone of weathering. The of the premaxillae divide the nasals anteriorly. The premaxilla anterior section is composed of the muzzle and articulated man- lacks a posterior process, and consequently the maxilla forms dible which have been mediolaterally ¯attened. The skull is the posterior margin of the naris, a derived character for troo- broken at the anterior margin of the orbits and the three di- dontids (OsmoÂlska and Barsbold, 1990). A foramen opening mensionally preserved braincase is separate. The roof of the into the supra-alveolar canal is present in the maxilla, below braincase is missing, exposing an eroded endocast. Postcranial the posteroventral corner of the naris and just posterior to the elements were collected as surface ¯oat below the skull along premaxilla±maxilla suture. Both the premaxillae and the max- with the partial remains of an ornithomimid (see Makovicky illae contribute to an extensive secondary bony palate. Poste- and Norell, 1998 for details). Additional remains were collected riorly, the vomer also participated in this feature. during the 1994 and 1995 ®eld seasons as ¯oat. The maxilla is perforated laterally by two large openings, the EtymologyÐNamed in honor of Byron Jaffe in recognition accessory antorbital fenestra anteriorly and the antorbital fe- of his family's support for the Mongolian Academy of Scienc- nestra posteriorly, which are separated by a wide interfenestral es-American Museum of Natural History Paleontological Ex- bar that is con¯uent with the side of the rostrum (Fig. 1). The peditions. accessory antorbital fenestra has an osseous medial wall, LocalityÐThe type specimen (IGM 100/983) was collected whereas the posterior fenestra is lacking a ¯oor. Posteriorly, two at Ukhaa Tolgod. passages run through the interfenestral bar. A ventral passage

7 8 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 20, NO. 1, 2000

FIGURE 1. The rostrum (A) and braincase (B,C) of IGM 100/983. A), left lateral view (scale bar equals 2 cm.), B) posterior view, and C), left lateral view (scale bar equals 1 cm). Abbreviations: aof, antorbital fenestra; axf, accesory antorbital fenestra; bo, basioccipital; cif, crista interfenestralis; d, dentary; hf, hypophyseal fossa; ifb, interfenestral bar; l, lacrimal; n, nasal; pmx, premaxilla; pp, paroccipital process; sr, sub- otic recess; V, trigeminal, VII, facial, X, vagus.

allows communication between the accessory and antorbital fe- low groove ®lled with small foramina extends the length of the nestrae. Dorsally, a passage travels through the bar between the tooth row. antorbital fenestra and the nasal passage, the ¯oor of which Like other troodontids, Byronosaurus jaffei displays numer- bulges into the dorsal part of the accesory antorbital fenestra. ous and heterodont teeth. The most anterior teeth are small and The accessory antorbital sinus is enclosed completely within tightly packed. More posterior teeth are larger and more widely the maxilla, whereas the antorbital fenestra is bordered poste- spaced, but the posteriormost teeth are tiny. The teeth of By- riorly and dorsally by the ``T''-shaped lacrimal that has an ex- ronosaurus jaffei are similar to other troodontids in having a tensive anterior process between the nasal and the maxilla. Pos- constriction between the root and the tooth crown, but are atyp- terior to the postorbital bar on the skull roof, the lacrimal ex- ical of theropods, and similar to avialans and Pelecanimimus, tensively overhangs the orbit. On both the maxilla and dentary, in lacking anterior and posterior serrations (Fig. 2). just dorsal and ventral to the corresponding tooth rows, a shal- The braincase is typical of troodontids in having extensive NORELL ET AL.ÐNEW MONGOLIAN TROODONTID 9

mongoliensis (Norell and Makovicky, 1999) and basal Avialae (Chiappe et al., 1996). Four fragments of distal caudals are preserved. The centra of the distal caudals are long and laterally constricted. The neural canal is diminutive and a distinct sulcus extends above it along the roof of the neural arch. Such a sulcus is also present in other troodontids (Russell, 1969; Russell and Dong, 1994; Varrichio, 1997). Preserved portions of the hindlimb include part of the fem- oral shaft, the right knee, the distal end of metetarsal III, and three phalanges, among them the penultimate phalanx of digit II. This phalanx bears an elongate heel proximoventrally. As in other troodontids, but in contrast to dromaeosaurids, the shaft between the articular ends is not foreshortened (Currie and Peng, 1994). The medial face is vertical whereas the lateral one is convex, a feature that identi®es the phalanx as being from the right foot. Erosion has destroyed much of the distal gingly- mus. In pro®le, the phalanx resembles the corresponding bone FIGURE 2. Teeth of IGM 100/983. Scale bar equals 2 mm. in formosus (Currie and Peng, 1994) and an indeter- minate troodontid from Mongolia (Barsbold et al., 1987).

Discussion pneumatic spaces that lie both ventral and anteroventral to the braincase. Like in other troodontids these are delimited by an The discovery of Byronosaurus jaffei underscores the exten- expanded otosphenoidal crest that creates the ``lateral depres- sive Asian diversity of the Troodontidae. To this point, eight sion'' (see Currie and Zhao, 1994). The trigeminal and facial species of troodontids have been described, and seven of these foramina lie on a continuous sulcus dorsal to the otosphenoidal are from Central Asia. Byronosaurus jaffei also displays a num- crest, rather than being separated by a raised mound as in Troo- ber of character states that are interesting for non-avialan the- don formosus (pers. obs.: TMP [Royal Tyrrell Museum of Pa- ropods. Among these the teeth require special note. laeontology] 86.36.457). The middle ear region is extremely As described above, the unserrated teeth of Byronosaurus well preserved (Fig. 1C), and conclusively shows that Byron- jaffei (Fig. 2) are very similar to those found in basal avialans osaurus jaffei had a fully ossi®ed crista interfenestralis formed like lithographica. This is particularly notewor- exclusively of the prootic. The base of the crista interfenestralis thy in light of claims that serrated teeth are ``characteristic of is level with the braincase wall, unlike in Troodon formosus theropod dinosaurs'' (Feduccia, 1996:77). A second feature of (Currie and Zhao, 1994) and junior (pers. Byronosaurus jaffei is the presence of a secondary palate obs.), where the crista interfenestralis is inset in a deep middle formed by extensive palatal shelves that meet the vomer on the ear cavity. midline. Such a condition has also been reported in the dro- The paroccipital processes are short and their ends are round maeosaur mongoliensis (Norell and Makovicky, in posterior view (Fig. 1B). They are nearly vertical and are 1998) and the ornithomimosaur Garudimimus brevipes (Bars- solid as in other troodontids. A group of small depressions at bold, 1983). the posterodorsal corner of the middle ear cavity above the To evaluate the phylogenetic position of Byronosaurus jaffei crista interfenestralis may represent vestiges of the posterior within the Troodontidae, we conducted a cladistic analysis of tympanic recess. A depression is also apparent in this region in 38 characters (Appendices 1, 2). The ingroup includes Byron- the London specimen of Archaeopteryx lithographica (pers. osaurus jaffei, Troodon formosus, Saurornithoides mongolien- obs.), where it is separate from a secondary opening of the sis, Saurornithoides junior, youngi, and an un- posterior tympanic recess, posterior to the fenestra pseudoro- named Early troodontid (IGM [Institute of Geology, tundum (Witmer, 1990). A well developed subotic recess is pre- Mongolia] 100/44) (Barsbold et al., 1987) from Mongolia. To- sent and extends below the braincase ¯oor. A concave depres- chisaurus nemegtensis, gracilicrus, and Koparion sion ®lled with small pockets lies dorsal and medial to the mid- douglassi were excluded from the analysis because their cod- dle ear cavity on the dorsolateral surface of the braincase. This ings are redundant with a number of other taxa. Additionally, feature has been called the dorsal tympanic recess in other non- nemegtensis and Koparion douglassi cannot be di- avian theropods (Currie and Zhao, 1994; Witmer, 1997; Ma- agnosed by autapomorphies. Other taxa erroneously referred to kovicky and Norell, 1998). However, the homology of this de- the Troodontidae including Ornithodesmus cluniculus (Howse pression with the avian dorsal tympanic recess has not been and Milner, 1993) and Bradycnemae draculai have been shown established. In posterior view, the occiput is higher than wide to have af®nities with other theropod groups (Norell and Ma- (as in Troodon formosus and both species of Saurornithoides) kovicky, 1997; G. Dyke, pers. comm.), and were omitted from and the basal tubera are separated by a small ``V''-shaped the analysis. notch. In developing a phylogenetic analysis of these taxa, consid- Vertebrae include the axis, two other anterior cervicals, dor- eration was given to the lack of consensus in current hypotheses sals, a sacral and four caudals. None of the preserved vertebrae of maniraptoriform relationships when choosing a root. Dro- is complete. The cervicals, which represent the axis and prob- maeosauridae, , Avialae and Ornithomimosauria ably the third and fourth cervicals, have elongate centra with were included as outgroup taxa. Analyses were conducted with signi®cantly wider than tall anterior intercentral faces as in most constrained outgroup con®gurations that re¯ect current, con- coelurosaurs (Gauthier, 1986). The postzygapophyses of the ¯icting hypotheses of maniraptoran relationships (Gauthier, axis and following vertebra are capped by robust epipophyses. 1986; Holtz, 1994; Forster et al., 1998), as well as an uncon- The best preserved of the dorsal centra bears a deep rounded strained analysis rooted on ornithomimosaurs. In no case did hypapophysis, similar to that of olecranus, on the any recombination of the outgroup taxon con®gurations have anterior end of the centrum. The diameter of the neural canal an effect on the topology within Troodontidae, and the outgroup is more then half the diameter of the centrum as in Velociraptor taxa did not fall into the ingroup in the unconstrained analysis. 10 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 20, NO. 1, 2000

by NSF DEB-9407999 and the Frick Laboratory Endowment. Peter Makovicky's work is supported by the Danish Research Academy (Grant 1996-0020-154).

LITERATURE CITED Barsbold, R. 1983. Carnivorous dinosaurs from the Cretaceous of Mon- golia. Joint Soviet-Mongolian Paleontological Expedition, Trans- actions 19:5±120 [Russian]. , H. OsmoÂlska, and S. M. Kurzanov. 1987. On a new troodontid (Dinosauria, Theropoda) from the Early Cretaceous of Mongolia. Acta Palaeontologica Polonica 32:121±132. Chiappe, L. M., M. A. Norell, and J. M. Clark. 1996. Phylogenetic position of Mononykus (Aves: ) from the Late Cre- taceous of the Gobi Desert. Memoirs of the Queensland Museum 39(3):557±582. Chure, D. J. 1994. Koparion douglassi, a new dinosaur from the Mor- rison Formation (Upper ) of Dinosaur National Monument: the oldest troodontid (Theropoda: Maniraptora). Brigham Young University Geological Studies 40:11±15. Currie, P. J. 1987. -like characteristics of the jaws and teeth of troodontid theropods (Dinosauria: ). Journal of Verte- brate Paleontology 7:72±81. , and J. H. Peng. 1994. A juvenile specimen of Saurornithoides mongoliensis from the Upper Cretaceous of Northern China. Ca- nadian Journal of Earth Sciences 30: 2224±2230. , and Zhao, X.-J. 1994. A new troodontid (Dinosauria, Thero- poda) braincase from the Dinosaur Park Formation () of FIGURE 3. Strict consensus tree for phylogenetic relationships among Alberta. Canadian Journal of Earth Sciences 30:2231±2247. troodontids. Unambiguous diagnoses for nodes under all outgroup to- Dashzeveg D., M. J. Novacek, M. A. Norell, J. M. Clark, L. M. Chiappe, pologies: 1 (7, 36), 2 (15), 3 (26, 30), 4 (24). Numbers refer to character A. Davidson, M.C. McKenna, L. Dingus, C. Swisher, and A. Perle. list; all numbered apomorphies represent state 1 of characters. 1995. Unusual preservation in a new vertebrate assemblage from the of Mongolia. Nature 374:446±449. Feduccia, A. 1996. The Origin and Evoluion of . Yale University Press, New Haven. 420 pp. All analyses yield three identical trees with lengths varying Forster, C. A., S. D. Sampson, L. M. Chiappe, and D. W. Krause. 1998. The theropod ancestry of birds: new evidence from the Late Cre- from 44 to 50 steps depending on outgroup con®guration. A taceous of Madagascar. Science 279:1915±1919. strict consensus of troodontid relationships is shown in Figure 3. Gauthier, J. A. 1986. Saurischian monophyly and the origin of birds; As can be determined from this analysis, Byronosaurus jaffei pp. 1±55 in K. Padian (ed.), The Origin of Birds and the Evolution lies in a clade including both described species of Saurorni- of Flight. Memoirs of the California Academy of Sciences 8. thoides and the North American Troodon formosus. The sister- Gilmore, C. W. 1924. On Troodon validus, an ornithopodus dinosaur taxon of this clade is the Central Asian Early Cretaceous Si- from the Belly River Cretaceous of Alberta, Canada. Bulletin of nornithoides youngi. The unnamed Mongolian taxon (IGM 100/ the Department of Geology, University of Alberta.1:1±143. 44) occupies the base of the troodontid tree. Due to large Holtz, T. R., Jr. 1994. The phylogenetic position of the Tyrannosauridae: amounts of missing data, branch support is low, ranging from implications for theropod systematics. Journal of Paleontology 68(5):1100±1117. three steps for Troodontidae (four with dromaeosaurs as nearest Howse, S. C. B., and A. R. Milner. 1993. OrnithodesmusÐa manirap- outgroup) to one step for two of the three internal nodes. toran theropod dinosaur from the Lower Cretaceous of the Isle of In summary, Troodontidae can be diagnosed by two unam- Wight, England. Palaeontology 36:425±437. biguous synapomorphies under all applied outgroup topologies: Kurzanov, S. M., and H. OsmoÂlska. 1991. Tochisaurus nemegtensis gen. presence of a groove along the dentary for the mental foramina, et sp. n., a new troodontid (Dinosauria, Theropoda) from Mongolia. and a slender Mt II that is distinctly shorter than Mt IV. In each Acta Palaeontologica Polonica 36:69±76. analysis a third and occasionally a fourth, separate character Makovicky, P. J. and M. A. Norell. 1998. A partial ornithomimid brain- supports troodontid monophyly (characters 1, 17, 21, 35, and/ case from Ukhaa Tolgod (Upper Cretaceous, Mongolia). Novitates or 38), but these are dependent on outgroup topology. The dis- 3247:1±16. Norell, M. A., and P. Makovicky. 1997. Important features of the dro- tribution of other characters such as a long anterior ramus of maeosaur skeleton: Information from a new specimen. American the lacrimal (character 11) await discovery of additional cranial Museum Novitates 3215:1±28. material. , and . 1999. Important features of the dromaeosaur skel- eton II: information from newly collected specimens of Velocirap- ACKNOWLEDGMENTS tor mongoliensis. American Museum Novitates 3282:1-45. OsmoÂlska, H. and Barsbold, R. 1990. Troodontidae; pp. 259±268 in D. We thank members of the Mongolian Academy of Sciences- B. Weishampel, P. Dodson, and H. OsmoÂlska (eds.), The Dinosau- American Museum of Natural History ®eld crews for excavat- ria. University of California Press, Berkeley. ing the specimens discussed herein. Amy Davidson prepared Russell, D. A. 1969. A new specimen of from the IGM 100/984 and provided partial preparation on IGM 100/ Oldman Formation (Cretaceous) of Alberta. Canadian Journal of 983. William Amaral also provided initial preparation of IGM Earth Sciences 6:595±612. 100/983. Michael Ellison developed Figure 1, Lorraine Meeker , and Dong, Z.-M. 1994. A nearly complete skeleton of a new troodontid dinosaur from the Early Cretaceous of the Ordos Basin, and Chester Tarka are thanked for their work on Figure 2. Field Inner Mongolia, People's Republic of China. Canadian Journal of work for this project supported by NSF DEB-9300700, the Earth Sciences 30:2163±2173. IREX and Eppley Foundations, National Geographic and the Sereno, P. 1997. The origin and evolution of dinosaurs. Annual Review Frick Laboratory Endowment. Special thanks is also extended of Earth and Planetary Sciences 25:435±489. to Lynette, Richard and Byron Jaffe. Other work was supported Varrichio, D. J. 1997. Troodontidae; pp 749±754 in P. J. Currie, and K. NORELL ET AL.ÐNEW MONGOLIAN TROODONTID 11

Padian (eds.), The Encyclopedia of Dinosaurs, Academic Press San Diego, CA. Witmer, L. M. 1990. The craniofacial air sac system of Mesozoic birds (Aves). Zoological Journal of the Linnean Society 100:327±378.

Received 23 June 1999; accepted 29 September 1999.

APPENDIX 1 List of Characters Used to Create Phylogeny in Figure 3 1. Teeth few (Ͻ100) (0), or numerous (1). 2. Absence (0) or presence (1) of enlarged, distally oriented denticles. 3. Teeth serrated (0) or unserrated (1). 4. Osseous canal for maxillary branch of CN V absent (0) or present (1). 5. Maxilla excluded from narial border (0) or participating in narial border (1). 6. Accessory antorbital fenestra small (0) or enlarged (1). 7. Dentary labial foramina situated outside of groove (0) or in a hori- zontal groove (1). 8. In¯ated para-basisphenoid absent (0) or present (1). 9. Lacrimal L-shaped (0) or T-shaped (1). 10. Prefrontal absent (0) or present (1). 11. Anterior ramus of lacrimal short (0) or long (1). 12. Trigeminal nerve exiting through one opening (0) or two or more openings (1). 13. Metotic ®ssure open (0) or constricted (1). 14. Foramen magnum subcircular or wider than tall (0) or taller than wide (1). 15. Basisphenoidal recess absent (0) or present (1). 16. Basal tubera shallow (0) or deep (1). 17. Notch between basal tubera large and U-shaped (0) or small, V- shaped incisure (1). 18. Paroccipital processes elongate and squared-off (0) or abbreviated (1). 19. Posterior tympanic recess invades body of paroccipital process (0) or paroccipital closed off (1). 20. Enlarged subotic recess absent (0) or present (1). 21. Quadrate non-pneumatic (0) or pneumatic (1). 22. Anterior tympanic recess consists of separate prootic recess and basisphenoid recess (0) or prootic recess con¯uent with subotic and delimited by enlarged otosphenoidal crest (lateral depression) (1). 23. Dorsal and ventral dentary margins subparallel (0) or distally con- vergent (1). 24. Dentaries straight in ventral view (0) or medially incurved at sym- physeal extremity (1). 25. Splenial concealed in lateral view (0) or with large lateral exposure (1). 26. Large axial epipophyses absent (0) or present (1). 27. Carotid processes absent (0) or present on posterior cervical ver- tebrae (1). 28. Large hypapophyses on anterior dorsals absent (0) or present (1). 29. Tip of dorsal neural spines unexpanded (0) or expanded to form spine table (1). TABLE 1. Data matrix for characters in Appendix 1. Ð represents inapplicable states. 30. Distal caudals with low neural spine or ridge (0) or with sagittal sulcus above neural canal (1). 31. Proximal gastralia shorter than distal gastralia (0) or proximal seg- ments longer (1). 32. Obturator process positioned proximally (0) or distally (1).

33. Greater and lesser trochanters widely separated by deep notch (0) 1234567891011121314151617181920212223242526272829303132333435363738 0 1Ð1000011011?11000111010000 0 000000 0ÐÐÐ?00000100?00?00100?Ð1000 1 000&1 1 00 00?11?0&100 0 00000000110000100010000011? 1 1111110Ð1100000???1?????100?000?10&100111? 0 01 1 110?11111111110111011111111 1 11??111101111??Ð????0?11?11?111?? 1 ? 11 ???11? 1 11011111111??1001111??111?? 1 ? 11 ?1???? 1 100?111??11???0???????10101 1 1 1? ?0111? ? 1?0???1?????0?101??01????0? 1 ? 11 ?????? 1 1Ð111111111?11001111?11011? 1 1 11 ?1??1? 1 ? ?1 ? or appressed or fused together (1). 34. Rugose lateral ridge on proximal part of femoral shaft absent (0) or present (1). 35. Third metatarsal only slightly constricted (0) or strongly constricted and concealed from anterior view (1). 36. Metapodium symmetrical with mt II equal to mt IV in length (0) or mt II slender and shorter than mt IV (1). 37. Pedal phalanx II-2 elongate and without proximal heel (0) or ab- breviated and tall with proximal heel (1). 38. Enlarged raptorial claw on digit II absent (0) or present (1). Ornithomimidae Oviraptorosauridae Dromaeosauridae Avialae Troodon formosus Saurornithoides mongoliensis Saurornithoides junior Sinornithoides youngi IGM 100/144 Byronosaurus jaffei