PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 3402, 32 pp., 21 ®gures, 1 table March 27, 2003

Osteology and Relationships of Byronosaurus jaffei (: )

PETER J. MAKOVICKY,1 MARK A. NORELL,2 JAMES M. CLARK,3 AND TIMOTHY ROWE 4

ABSTRACT The troodontid Byronosaurus jaffei is known from two specimens from adjacent localities in the Nemegt basin, OÈ mnoÈgov Aimag, Mongolia. These specimens are composed of well- preserved cranial material and fragmentary postcrania. All of these elements are described here. Byronosaurus jaffei is included in a comprehensive phylogenetic analysis of Coeluro- sauria to ascertain its relationships. Several interesting characters of Byronosaurus jaffei have implications both for theropod relationships and for understanding patterns of variation within coelurosaurian theropods. These include the position of a foramen that marks the exit of the supra-alveolar canal (which we suggest is homologous with the subnarial foramen), the ¯at- tened internarial bar, the unusual interfenestral bar, and the unserrated teeth. Additionally, the well-preserved braincase allows detailed comparison with other troodontid taxa.

INTRODUCTION described, most from Central Asia (OsmoÂl- ska and Barsbold, 1990; Russell and Dong, The remains of troodontid are 1993). Here we provide a detailed descrip- extremely rare, and only nine taxa have been tion of Byronosaurus jaffei, a troodontid

1 Assistant Curator, Department of Geology, The Field Museum, Roosevelt Rd. at Lake Shore Drive, Chicago IL 60605. Research Associate, Division of Paleontology, American Museum of Natural History. e-mail: pmakovicky@ ®eldmuseum.org 2 Chairman, Division of Paleontology, American Museum of Natural History. e-mail: [email protected] 3 Ronald S. Weintraub Associate Professor Department of Biological Sciences, George Washington University, Washington, DC 20052. Research Associate, Division of Paleontology, American Museum of Natural History. e-mail: [email protected] 4 J. Nalle Gregory Regents Professor of Geology, and Director, Vertebrate Paleontology Laboratory, Department of Geological Sciences, The University of Texas at Austin, TX 78712. e-mail: [email protected]

Copyright ᭧ American Museum of Natural History 2003 ISSN 0003-0082 2 AMERICAN MUSEUM NOVITATES NO. 3402

Fig. 1. The holotype locality looking north. The arrow signi®es the point of discovery of IGM 100/ 983. from the Djadokhta Formation at Ukhaa Tol- groups will be considered in more detail else- god that was named and brie¯y described by where. Norell et al. (2000). Only one other - tid, mongoliensis (Osborn, MATERIAL 1924), has been reported from Djadokhta, or Djadoktha-type, beds in Asia (Currie and LOCALITY: The holotype specimen (IGM Dong, 2001a). Although troodontids are rare, 100/983) was discovered at the sublocality well-preserved braincases are known for sev- called ``Ankylosaur Flats'' in 1993 by Mi- eral taxa (Osborn, 1924; Barsbold, 1974; chael Novacek (®g. 1). Additional remains of Currie, 1985; Currie and Zhao, 1993; Russell this specimen were collected during the 1994 and Dong, 1993, Xu et al., 2002), and have and 1995 ®eld seasons. Unlike many of the provided important phylogenetic insights fossils at Ukhaa Tolgod, the specimen was into this clade. Few phylogenetic studies within Troodon- not articulated and was found as a loose as- tidae have been undertaken, and broader sociation of bones. Nevertheless, the rostral studies that include troodontids and their pur- fragment and the braincase were found in ported closest relatives (dromaeosaurids place adjacent to one another in life position, [Gauthier, 1986; Sereno, 1999], [Holtz, separated by a zone of weathering. The re- 2001]) have not taken into account the vari- maining parts of the skeleton were collected ation among troodontid taxa and have been as eroded pieces below the skull. Later ex- hindered by fragmentary material and inad- amination of the specimen revealed that re- equate descriptions. Here we expand our spe- mains of a second individual were inter- cies-level phylogenetic analysis of the group mixed with IGM 100/983. This individual (Norell et al., 2000) and examine the evi- (IGM 100/987) is signi®cantly larger than dence for monophyly of this group and more IGM 100/983, and consequently its bones exclusive clades within it. The evidence for can easily be separated. This specimen lacks the relationships of troodontids to other troodontid apomorphies, and is referred to 2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 3 the Ornithomimidae (Makovicky and Norell, DESCRIPTION 1998). The skull of IGM 100/983 is preserved in REFERRED SPECIMEN: A second specimen (IGM 100/984) was collected on July 15, two parts. The anterior section comprises the rostrum and articulated dentaries that have 1996 at the ``Bolor's Hill'' locality approxi- been transversely compressed although most mately 6 km west of the main Ukhaa Tolgod bones appear uncrushed (®gs. 2, 3). It was exposure. While extremely fragmentary, the found in two pieces that have been glued to- specimen preserves some important features gether, and the glue ®lls some large gaps not present on the type. The specimen con- along the contact. The anterior section ends sists of a rostrum in six pieces. It is referred at the anterior margin of the orbits. The to Byronosaurus jaffei on the basis of super- braincase was preserved separately and has numary teeth all lacking serrations, distinc- not been crushed. The top of the braincase is tive lacrimals with a lateral shelf that over- missing, exposing an eroded endocast, and hangs the anterodorsal corner of the orbit, some delicate outer parts of the basicranium and a pair of passages through the interfe- are incomplete. nestral bar, which is not recessed from the IGM 100/984 is very fragmentary. It was plane of the rostrum. preserved as eight separate nodules, only three of which contain recognizable ele- DIAGNOSIS ments. One contains the premaxillae and the anterior ends of the maxillae (®g. 4), a sec- Byronosaurus jaffei exhibits the following ond contains a section of the rostrum at the troodontid synapomorphies (Xu et al., 2002): level of the interfenestral bar, and a third numerous teeth, close packing of dentary contains a section of skull roof at the preor- dentition near the rostral tip of the lower jaw, bital bar preserving the lacrimals and the presence of a distinct groove for the neuro- posterior part of the nasals. vascular foramina on the dentary, and a dor- Our description is primarily based on IGM soventrally ¯attened internarial bar. 100/983, supplemented by IGM 100/984. Byronosaurus jaffei is part of a clade com- Measurements of the holotype IGM 100/983 prising all troodontids except the basal taxon are tabulated in appendix 2. Comparisons changi. This clade is ambigu- with Saurornithoides junior are based on ously diagnosed by: braincase displaying a Barsbold (1974, 1983) and are supplemented large pneumatic sinus (the ``lateral depres- by our own observations. Comparisons with Troodon formosus are based on Currie (1985) sion'' ventral and anteroventral to the brain- and Currie and Zhao (1993), and those with case; presence of a subotic recess; extensive youngi are based on Russell anterior process of the lacrimal forming the and Dong (1993) and our examination of the dorsal border of the antorbital fenestra; and specimen. Comparisons with Saurornithoides maxillary participation in the posterior mar- mongoliensis are based on our own observa- gin of the nares. Ambiguity in the optimi- tions of the type (AMNH 6174). Other com- zation of these characters is due to missing parisons are referenced herein. data and multiple placements of several troo- The face and braincase of the type speci- dontid taxa, including Byronosaurus jaffei, men (IGM 100/983) were scanned separately Sinornithoides youngi, and an unnamed in May 1997 at The University of Texas Mongolian taxon represented by IGM 100/ High-resolution X-ray Computed Tomogra- 44. phy Facility. Both were scanned on the high- Byronosaurus jaffei can be distinguished resolution subsystem, based on a Feinfocus from all other troodontids by the possession microfocal X-ray tube and an image-inten- of several derived characters, including teeth si®er detector system. The face was scanned lacking serrations, an interfenestral bar that with X-ray energy set at 120 kV, 0.200 mA, is not recessed from the plane of the maxilla, at 190% offset in an air wedge, with a and a shallow groove along the buccal mar- source-object distance of 85 mm. Slice thick- gin of the maxilla. ness is 0.25 mm, and 562 consecutive slices 4 AMERICAN MUSEUM NOVITATES NO. 3402

Fig. 2. The rostrum of the holotype specimen IGM 100/983 in right lateral and dorsal views. Ab- breviations are listed in appendix 1. 2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 5

Fig. 3. The rostrum of the holotype specimen IGM 100/983 in left lateral and ventral view. Abbre- viations are listed in appendix 1. 6 AMERICAN MUSEUM NOVITATES NO. 3402

Fig. 4. Stereopair of the ventral surface of the premaxillary area in the referred specimen IGM 100/984. were generated. For each slice, 1200 views bears four teeth (®g. 4), as is primitive for were taken with two samples per view. Re- theropods. The nasal process is gently arched constructed 512 ϫ 512 pixel imagery was and dorsally ¯attened and triangular in cross exported as 8-bit TIF ®les. The braincase section (®g. 2), and it forms the anterodorsal was scanned with X-ray energy set at 120 margin of the external narial opening. It is kV, 0.400 mA, at 190% offset in an air thin anteriorly and increases in breadth in the wedge, with a source-object distance of 45 area where it divides the nasals along the mm. Slice thickness is 0.20 mm, and 150 midline, as in Saurornithoides junior. The consectutive slices were generated. For each maxillary process is short and heavily pitted slice, 1200 views were taken with two sam- by small foramina on its small facial surface. ples per view, and a ®eld of view of approx- Medially, the surface of the maxillary pro- imately 36.3 mm. Reconstructed 1024 ϫ cess arches to form the ¯oor of the naris (®g. 1024 pixel imagery was exported as 8 bit 2). There is no posterior process as in dro- TIF ®les. Original CT datasets were archived maeosaurids that excludes the maxilla from on CD-ROM at the University of Texas and participation in the margin of the narial open- the American Museum of Natural History. ing (®gs. 2, 3). Although obscured, the suture Animated serial section movies and three di- with the maxilla is apparently very simple. mensional volumetric reconstructions are MAXILLA: The maxilla forms most of the also viewable on the Internet at www. lateral surface of the skull. Anteriorly, it DigiMorph.org. forms much of the ¯oor of the large naris. The maxilla contacts the premaxilla along a THE ROSTRUM straight suture anteriorly in this chamber PREMAXILLA: Paired premaxillae form the (®gs. 4, 5). Just posterior to this suture, and anterior margin of the skull. Each premaxilla before the maxilla rises from the narial ¯oor 2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 7

Fig. 5. Detail of the narial region of the ho- lotype specimen IGM 100/983 in left lateral view. Abbreviations are listed in appendix 1.

In Troodon formosus the maxillary fenestra is backed by an unperforated osseous wall. (Brochu, personal commun.). Anteriorly, the rostral end of the maxilla overhangs the max- Fig. 4. Continued. illary fenestra to form a large pocket. The maxillary fenestra is separated from the ant- orbital fenestra by a large, very ¯at plate of to form the posterodorsal margin of the naris, maxilla, here termed the interfenestral bar lies a large, anteriorly directed foramen that (®gs. 2, 7). Unlike Saurornithoides junior, is completely enclosed by the maxilla (®gs. Saurornithoides mongoliensis,orSinorni- 5, 6). A similar foramen is observed in the thoides youngi (Currie and Dong, 2001a), the alvarezsaurid deserti (IGM 100/ interfenestral bar is not inset from the plane 977). This foramen enters into a long supra- of the maxilla. alveolar canal that parallels the row The main antorbital fenestra lacks an os- (®g. 6). The maxilla rises from the narial seous ¯oor, is larger than the anterior fenes- ¯oor in a gentle posterior slope. The lateral tra, is subtrapezoidal in shape, and narrows surface below the narial chamber is covered dorsally. Anteriorly, the margin of the fenes- with small pits and foramina (®g. 5). These tra is formed by the interfenestral bar, which are most apparent as a row of large pits that is scalloped in two places (®g. 7). These lie in a groove just dorsal and parallel to the emarginations lead to dorsal and ventral pas- dentigerous margin. This row of pits runs sages through the interfenestral bar, which posteriorly below the maxillary fenestra, de- are separated by an osseous wall (®g. 7). An creasing in density posteriorly until they dis- erosional cross section of the snout of IGM appear below the antorbital fenestra. 100/984 exposes the course of the two chan- The lateral surface of the maxilla is per- nels through the interfenestral bar (®g. 8). forated by two large, elongated antorbital The dorsal emargination marks the caudal openings (®gs. 2, 3). The maxillary fenestra end of the narial passage, which is visible is elliptical and extends from below the ex- laterally as a swollen ridge on the dorsal part ternal nares to the midway point between the of the osseous wall of the maxillary fenestra maxillary-premaxillary suture and the pre- (®gs. 9, 10). The ventral canal connects the orbital bar. This fenestra has an osseous me- antorbital and maxillary fenestrae. The cau- dial wall, which is perforated by a small hole dal entrance into the supra-alveolar canal, in slightly different positions on each side, which transmits the innervation and blood but these holes may be weathering artifacts. supply of the maxillary dentition, lies on the 8 AMERICAN MUSEUM NOVITATES NO. 3402

Fig. 6. Stereopair of the right dorso-oblique dorsal surface of the rostrum in the referred specimen IGM 100/984. Notice the entrance to the supra-alveolar canal (fsac) which we interpret as the subnarial foramen. Abbreviations are listed in appendix 1. medial face of the interfenestral bar (®g. 9C). of tiny foramina within a shallow depression. Below the antorbital fenestra, the maxilla is The posterior contact with the frontal is not extremely thin, dorsoventrally concave, and preserved on the type specimen, but is visi- devoid of the small foramina that are so con- ble on IGM 100/984. The nasals end just spicuous anteriorly. posterior to the level of the preorbital bar. At On the palate of IGM 100/984, longitudi- the level of the preorbital bar the nasals di- nally extensive palatal shelves of the maxilla verge along the midline (®g. 13) to accom- form the roof of the mouth (®gs. 4, 9, 10). modate a missing anterior wedge of the fron- These large palatal ¯anges extend posteriorly tal, as in Saurornithoides junior. Posterior to from the contact with the premaxilla, and are the preorbital bar on the top of the skull the joined by a median vomer at the border of nasals are ¯at, and together with the lacri- the choanae (®gs. 11, 12. Together these mals they form a ¯at surface above the an- form a solid secondary palate on the roof of terior part of the orbits (®g. 13). the mouth extending to the choanae, which LACRIMAL: The lacrimal forms the anterior lie between the antorbital fenestrae. and anterodorsal border of the orbit. In lat- NASAL: The nasal is a long bone that cov- eral view the lacrimal is T-shaped, as in dro- ers the top of the rostrum for most of its maeosaurids; however, the anterior or supra- length (®g. 2). Anteriorly, the nasal forms the antorbital fenestral process is much longer posterolateral boundary of the large external than the supraorbital process (®gs. 2, 3). Al- naris. The nasal is wider anteriorly than pos- though poorly preserved, several important teriorly. As preserved, anteriorly the nasal is features are apparent. The preorbital bar is a laterally convex, forming a continuous tran- straplike process extending from the skull sition from a vertical plane anteriorly to a roof to contact the jugal. The preorbital bar horizontal plane posteriorly. This transition is is oriented nearly vertically; however, this marked by a sharp angle, but the angle may orientation may be in¯uenced by distortion. be in¯uenced by postmortem distortion. A Contact with the jugal is preserved only on maxillary process extends posteroventrally, the right side, where a posterolaterally open- while a long, attenuate premaxillary process ing foramen appears to be present, but this extends anteriorly to form the posterodorsal area is broken (®g. 2). The posterior surface border of the naris for half its length and con- is slightly convex in cross section. CT scans tact the nasal process of the premaxilla me- reveal that the lacrimal duct exits the orbit dially (®g. 5). This process gives the anterior just ventral to the contact between the ante- surface of each nasal a concave appearance rior and posterior processes. Currie and Dong in lateral view. The lateral edge of the nasal (2001a) suggested that a lacrimal duct is ab- contacts the maxilla anteriorly and the lacri- sent in troodontids, but in Byronosaurus the mal posteriorly along a straight suture. Par- lacrimal duct passes through the lacrimal and allel and just medial to this suture lies a row opens into the posterodorsal corner of the 2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 9

Fig. 6. Continued.

Fig. 8. Antero-ventro oblique view of the re- ferred specimen (IGM 100/984) showing the os- seous wall of the interantorbital canal (iac) sepa- rating it from the nasal chamber just anterior to the interfenestral bar. Abbreviations are listed in appendix 1.

antorbital fossa, as observed in IGM 100/ 984. The posterior, or supraorbital, process is preserved only at its base on the type. On IGM 100/984 the posterior process is appar- ently completely preserved on the right lac- rimal (®g. 13). In this specimen it extends to the same level posteriorly as the nasals. The posterior process forms a laterally expanded shelf above the orbit, as in Saurornithoides junior, and it apparently meets the frontal along a posteromedial-anterolateral diagonal suture (®g. 13). Just posterior to the level of the preorbital bar, the dorsal surface of the lacrimal is perforated by a small anteriorly directed foramen. The anterior process is extensive and forms most, if not all, of the dorsal border of the posterior antorbital fenestra. At the pos- Fig. 7. Postero-oblique view of the left side of the rostrum of IGM 100/983 showing the two terodorsal corner of the antorbital fenestra a passages through the interfenestral bar. The upper large pocket is formed by a buttress between passage (en) connects with the nasal chamber. The the preorbital and anterior processes of the lower passage (iac) connects with the maxillary lacrimal. The lateral surface of this buttress sinus. is extensive and ¯at, forming a wall to the 10 AMERICAN MUSEUM NOVITATES NO. 3402

Fig. 9. CT scans of the rostrum of IGM 100/983. Notice the osseous ¯oor to the nasal cavity (fnc) as it extends through the interfenestral bar in C, D, E, and F. Abbreviations are listed in appendix 1. posterodorsal part of the antorbital fossa. A JUGAL: The jugal is extremely poorly rep- small ridge on the anterior process overhangs resented by only a small strap of bone from the pocket. The pocket that forms at the apex the anterior end, present on the right side of of the buttress is smooth-walled and is IGM 100/983 (®g. 2). The preserved frag- pierced at its apex by the lacrimal duct. ment of the jugal is a very thin bone that 2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 11

Fig. 10. Rendered view of the holotype rostrum of IGM 100/983 showing the narial chamber in relation to the maxillary sinus and the palate. Abbreviations are listed in appendix 1. contacts the jugal process of the maxilla izontal in the holotype skull, they would along an overlapping suture where the jugal probably have met dorsally to form a narrow is exposed on top of and lateral to the un- trough above the choanae, as in derlying maxilla. The element is dorsoven- mongoliensis (Barsbold and OsmoÂlska, trally concave laterally. Only a short section 1999). The anterior tips of the palatines may of the suborbital process is preserved on the have reached the caudal end of the median right side. chamber beneath the nasal passage, although PALATINES: Only the anterior interchoanal there is a lot of breakage in the parts near bar of each palatine is preserved. Although the interefenestral bar. these ¯at bars are displaced and almost hor- VOMER: The base of the paired posterior

Fig. 11. Section of the rostrum in IGM 100/984 in dorsal, right lateral, and ventral views. Abbre- viations are listed in appendix 1. 12 AMERICAN MUSEUM NOVITATES NO. 3402

Fig. 12. Stereopair of the palatal region of IGM 100/984 in ventral view. Abbreviations are listed in appendix 1. section of the vomer is preserved between behind the nares back toward the choanae. the caudal ends of the maxillary palatal On either side of the snout, the accessory shelves in IGM 100/984 (®g. 11). At the lev- antorbital fenestra connects to the antorbital el of the interfenestral bar, the posterior pro- fenestra through the lower, pneumatic pas- cesses conjoin to form a single median ele- sage through the interfenestral bar (®gs. 8, ment (®g. 10B, D). In the serial CT sections 10). A blind, median chamber termed the of the holotype skull, the vomer appears to maxillary antrum by Witmer (1997a) inter- end posterior to the external naris, unlike the cedes between these paired pneumatic sys- condition in Velociraptor mongoliensis (IGM tems (®g. 9C±F). It extends from below the 100/986). caudal end of the naris to the interfenestral bar and is roofed by the nasal passage and PNEUMATIC SYSTEMS OF THE SNOUT ¯oored by the palate. Its lateral walls are The snout comprises a complex of three formed by the vertical laminae of the maxilla longitudinal passageways that pass from just that form the wall of each maxillary fenestra

Fig. 13. Stereopair of the lacrimal-nasal area of IGM 100/984 in dorsal view. Notice the large extensions of the lacrimal over the antorbital bar. Abbreviations are listed in appendix 1. 2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 13

braincase elements are not apparent, indicat- ing that the was an adult. Conse- quently, this description relates to morpho- logical features rather than speci®c cranial el- ements. OCCIPITAL SURFACE: The occipital surface is heavily weathered and the occipital con- dyle is missing (®g. 14). The short paroccip- ital process is oriented nearly parallel to the plane formed by the medial part of the oc- ciput surrounding the foramen magnum (®gs. 14, 15). Although the opisthotic may have contributed to the paroccipital process (Cur- rie, 1985), no suture to distinguish this is ap- parent. The paroccipital process is divided Fig. 12. Continued. into medial and lateral parts by a low ridge that runs ventrolaterally from the top of the occiput. At the ventral surface of the diago- (waof in ®gs. 2, 3, 9B-F). The chamber nal ridge is a small pit that may represent the opens caudally between the interfenestral foramen for the occipital artery. Ventral to bars and just anterior to the level of the cau- the paroccipital process, the margin of the dal end of the nasal passage. The palatines occipital surface is formed by the metotic contribute to the ventromedial ¯oors of the strut, which is separated from the paroccipi- lateral passageways, and possibly to the ¯oor tal process by a small notch (®g. 14). of the median passage. Ventrolateral to the ventral limit of the fo- ramen magnum are three foramina (®g. 14). THE BRAINCASE A large dorsomedial and a smaller ventro- The braincase was collected in a single medial foramen transmitted branches of the weathered nodule (®gs. 14, 15). Although hypoglossal nerve (CN XII) through the several of the elements have been lost to ero- braincase wall. A large, more ventrolateral sion, many important features are well pre- foramen vagi is hidden in posterior view by served and allow additional insight into cra- a scalloped shelf that is a continuation of the nial anatomy and its variation in troodontid ventral border of the paroccipital shelf onto dinosaurs. The front and top of the braincase the occiput. It presumably transmitted the va- are missing, as is the occipital condyle and gus (CN X) and spinal accessory (XI) nerves the right paroccipital process. Sutures among from the metotic foramen through the occi- put. Just ventral to this cluster of foramina near the presumed location of the exoccipi- tal-basioccipital suture lies a small rugose bump. A pair of small basioccipital tubera lie di- rectly ventral to the foramen magnum (®gs. 14, 15). They lack distinct subcondylar fo- ramina opening into the anterior pneumatic spaces, although they are pneumatized lat- erally by the subotic recesses. The tubera are separated by a medial wedge-shaped depres- sion at the apex of which is a small pit that apparently does not lead to a foramen.

LATERAL WALL OF THE BRAINCASE Like other troodontids the lateral surface Fig. 13. Continued. of the braincase is marked by a large lateral 14 AMERICAN MUSEUM NOVITATES NO. 3402

Fig. 14. Stereopairs of the braincase of the holotype specimen IGM 100/983. Top, posterior; middle, anterior; lower, right lateral views. Abbreviations are listed in appendix 1. 2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 15

depression bounded by an otosphenoidal crest anteroventral to the middle ear (®gs. 14, 15). LATERAL DEPRESSION AND SIDE WALL OF THE BRAINCASE: A sharp otosphenoidal crest originates just ventral to the foramen for the facial nerve (CN VII) and curves anteroven- trally, bounding the lateral depression ante- riorly and dorsally (®g. 15). The lateral de- pression is bordered posteriorly by a subotic recess that lies ventral to the metotic strut and invades the basal tuber. Although prom- inent, the lateral depression is not as deep as in Troodon formosus (Currie and Zhao, 1993) and Saurornithoides junior (Barsbold, 1974). Ventrally, the left and right lateral depres- sions appear to be separated only by a thin lamina that extends posterior to the hypo- physeal fossa. Although poor preservation makes it dif®cult to determine the extent of this lamina, connections between the left and right lateral depression may have been pre- sent, as in Troodon formosus (Currie and Zhao, 1993). The wall of the braincase surrounding the hypophyseal fossa de®nes the anteromedial corner of each lateral depression (®gs. 14, 15). Each internal carotid passage descends anteroventrally through an osseous canal and meets its counterpart immediately before en- tering the hypophyseal fossa itself in a com- mon carotid opening (®g. 14). The middle ear is proportionately large in this taxon. An osseous crista interfenestralis subdivides the middle ear into a smaller, tri- angular anterior aperture (the fenestra ovalis) and a larger fenestra pseudorotunda posteri- orly (®gs. 14, 15). The crista interfenestralis is thinner dorsally than ventrally, and a su- ture at the base of the interfenestral crest sug- gests that it is formed entirely from the bone that forms the dorsal rim of the middle ear cavity (the opisthotic in Troodon, according to Currie and Zhao, 1993). Anteromedial to the interfenestral crest, the inner ear forms a deep cochlear recess that is partly divided Fig. 14. Continued. into wider proximal and more constricted distal parts by a low ridge extending oblique- ly along the medial wall. A foramen located in the anterior chamber of the cochlear recess opens anteromedially into the braincase be- low the vestibular pyramid, and probably 16 AMERICAN MUSEUM NOVITATES NO. 3402

Fig. 15. The braincase of the holotype specimen IGM 100/983. Top, left lateral; middle, ventral; lower, dorsal views. Abbreviations are listed in Appendix 1. 2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 17

corresponds to the cochlear branch of CN VIII. A minute foramen exits the anterior wall of the metotic foramen at mid-height. Its small size and proximity to the posterior part of the vestibular pyramid suggest that it is the opening of the perilymphatic duct (Ma- kovicky and Norell, 1998). A similarly lo- cated foramen is present in ornithomimids (AMNH 5355) and Velociraptor mongolien- sis. The posteromedial part of the inner ear is occupied by the metotic foramen. As in Troodon formosus, it is tall and narrow and displays an hourglass shape. The constriction of the metotic foramen is located slightly be- low mid-height. Asymmetry is present be- tween the left and right ears in the region just posterolateral to the metotic foramen. On the right side, the single opening for the vagus and spinal accessory nerves and the dual fo- ramina for the hypoglossal nerve open pos- terolateral to the posterior border of the me- totic ®ssure (®g. 14). An additional foramen is present in this region on the left side of the skull (®g. 15). It opens laterally from the dorsal apex of the metotic foramen into the middle ear recess, thus opening anterior to the metotic strut and leading away from the occipital region of the skull. Another small foramen is present below the one just de- scribed, connecting the posteroventral corner of the middle ear to the subotic recess. Again, this feature is absent on the right side of the skull, although several small depres- sions in the roof of the subotic recess suggest that some resorption of bone has occurred in this region. Because it is only present on one side, this foramen is unlikely to represent the course of the stapedial artery. A pair of foramina open into the base of the paroccipital processes from the roof of the lateral depression (®g. 15) and suggest that a posterior tympanic recess was present, albeit dimintive, unlike the condition de- scribed for Troodon formosus (Currie and Zhao, 1993). Given the abbreviated length of the paroccipital processes, this recess must Fig. 15. Continued. have been relatively small in volume. A pit anterodorsal to the base of each paroccipital process is here interpreted as part of the dor- sal tympanic recess. Two additional pits on the right side suggest that the dorsal tympan- ic recess may have occupied much of the 18 AMERICAN MUSEUM NOVITATES NO. 3402 dorsolateral surface of the posterior region of and medulla. Anterodorsal to the exit for CN the braincase, although its trace is smaller V the laterosphenoids widen laterally to ac- than in other coleurosaurs. The left side is commodate the tectal and optic lobes. The damaged in the region anterodorsal to the anterior border of the tectal lobes is demar- paroccipital process, so the presence of these cated by a deep, oblique groove, which is pits cannot be veri®ed. dorsal to the level of the hypophyseal fossa CN VII exits the braincase laterally and (®gs. 14, 15). Rostral to this groove the right slightly anteriorly through a deep, funnel-like side of the endocast preserves the base of the channel situated at the posterodorsal termi- cerebral hemisphere, but this structure is in- nus of the otosphenoidal crest (®gs. 14, 15). suf®ciently preserved to deduce relative size Only the posterior rim of the opening of CN compared with other cerebral structures. V is preserved on either side. In relative di- The inner surface of the posterior region mensions, Byronosaurus jaffei has a propor- of the braincase has been prepared up to the tionately larger CN V opening than either level of the opening of CN VII. The vestib- Troodon formosus (Currie and Zhao, 1993) ular pyramid bulges into the brain cavity op- or Saurornithoides junior, although this is posite the anterodorsal corner of the base of probably allometric. Both of these foramina are situated in a wide, shallow trough bor- the paroccipital process. The apex of the ves- dered by the otosphenoidal crest below and tibular pyramid is directed anteromedially. a horizontal ridge that borders a lateral ex- Anteroventral to the apex of the pyramid, the pansion of the braincase above. The latter auricular fossa containing the foramina for structure markedly overhangs the exit of CN CN VII and the cochlear and vestibular V, and a small pocket opens dorsally into this branches of CN VIII extends laterally from overhang. Other surfaces of the trough are the base of the cerebellar portion of the en- also scalloped, a feature often associated docast. On both sides of the braincase, the with the presence of pneumatic diverticula. bony wall of the braincase is shattered in the The braincase is wide just dorsal to the region of the ¯occular recess. Damage to the exits of CN V (®g. 15). Dorsal to this lateral surface of both pyramids precludes identi®- expansion, each side of the braincase curves cation of the opening of the endolymphatic medially, and the lateral surface is concave duct. The vertical vestibular canal is not ex- both transversely and parasagittally. The dor- cessively swollen. The metotic foramen is sal sagittal region is not preserved, but this situated posteroventral to the vestibular pyr- area would have been roofed over by the amid, while the exit of CN VII constrains the posterior recesses of the parietals anteriorly base of the vestibular pyramid anteriorly. and supraoccipital posteriorly. The exit of CN VII is visible as a larger fo- ramen located anterior to the vestibular pyr- INTERIOR OF BRAINCASE AND ENDOCAST amid, at approximately the same level as the opening for CN VIII. Most of the roof and lateral walls of the SCLERAL OSSICLES: Articulated anterior braincase are missing, but a well-consolidat- segments of both scleral rings are preserved ed natural endocast of the braincase with a smooth surface is preserved (®g. 15). The re- in the anterior part of the orbit on IGM 100/ gion of the endocast representing the cere- 983, two ossicles on the right and three on bellar region lies beneath the supraoccipital the left (®gs. 2, 3). Some of the remaining and above the foramen magnum. It is prom- ossicles may lie in a mass of fragmentary inently domed both sagittally and transverse- bones just ventral to the orbit, but most of ly, and it indicates that the cerebellar region these fragments seem to be from the jugal of the brain was proportionately as large as and mandible. The ®ve ossicles are incom- in Troodon formosus (Currie and Zhao, pletely exposed, but they are generally sim- 1993). Beneath the cerebellar area, the ¯oor ilar to those of troodontids (e.g., Saurorni- of the braincase is deeply concave. The com- thoides mongoliensis) and other basal coe- bination of this deeply concave ¯oor and the lurosaurs in being rectangular with only a large foramen magnum suggest a large pons gentle lateral convexity. 2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 19

THE MANDIBLE and little morphology can be ascertained (®g. 2). It is transversely convex and apparently DENTARY: The dentaries are partly eroded, narrow dorsal to the mandibular fenestra, but from the preserved sections it can be de- suggesting that the mandibular fenestra was termined that they are thin with slightly con- large, unlike the condition in dromaeosaur- cave dorsal and convex ventral margins (®gs. ids. Just ventral to the lacrimal buttress, a 2, 3). Like other troodontids they are tall pos- large, elliptical surangular foramen perfo- teriorly and taper anteriorly toward the sym- rates the surangular; anterior to this foramen physis. The lateral surface is scoured by a the bone is slightly concave, but lacks a dis- deep median groove ®lled with numerous tinct groove as in some other theropods (e.g., small foramina, just below the tooth row. Ornitholestes hermanni and Posteriorly, this groove disappears as the lat- albertensis). eral surface of the dentary becomes ¯at; a ANGULAR: The right angular is also ex- small ridge overhangs the lateral surface dor- tremely fragmentary (®g. 2). The element is sally. CT scans show that the groove is level crescentic, with an expanded lateral surface lying with the implanted bases of the tooth anteriorly. More anteriorly the bone is frag- roots. Small perforating canals that pass be- mented. tween the roots of consecutive teeth connect the lateral canal and the Meckelian groove, DENTITION transmitting branches of the mandibular nerve and artery. CT scans also show that The dentition of Byronosaurus jaffei is un- the Meckelian groove parallels the lateral usual for theropod dinosaurs in that no ser- groove along the inner surface of the man- rations are found on the anterior or posterior dible. The groove terminates dorsal to a fo- carinae (®g. 16), a feature known only in al- ramen varezsaurids, the ornithomimosaur Pelecani- Similar to Sinornithoides youngi (Rusell mimus polyodon, and toothed avialans. Four and Dong, 1993) the dentaries are parallel in premaxillary teeth are present, but it is dif- ventral view, and there is no medial curva- ®cult to determine the number of maxillary ture of the rami at the mandibular symphysis, or dentary teeth because these areas are poor- as there is in Saurornithoides mongoliensis, ly preserved. Nevertheless, the preserved Saurornithoides junior, and Troodon formo- teeth are densely packed, and a conservative sus (®g. 3). estimate using the spacing of the preserved SPLENIAL: Only fragments of the splenial teeth to estimate the number of missing teeth are exposed, in ventral view. The anterior ex- is that at least 30 maxillary teeth and 30 den- tent of the splenial is hard to determine; how- tary teeth were present. ever, it reaches at least as far anteriorly as All of the teeth are recurved with sym- beneath the accessory antorbital fenestra (®g. metrical carinae and somewhat bulbous 3). It is apparently longer than in Deinony- crown bases (®g. 16). The tooth bases are chus antirrhopus and more similar to the marked by mediolateral, longitudinal condition in the dromaeosaurids Velociraptor grooves. The premaxillary teeth are small mongoliensis and Dromaeosaurus alberten- and closely packed, and they are round in sis where the splenial is at least half the cross section. The maxillary teeth are small length of the dentary. Posteriorly, the splen- compared with similar-sized theropods. They ial wraps around the ventral surface of the are also heterodont, with larger teeth occu- dentary to become exposed laterally, beneath pying the 11th and 12th tooth positions. The the anterior end of the antorbital fenestra. more anterior teeth are round in cross sec- tion, grading into elliptical and then subrect- Whether the splenial is signi®cantly exposed angular teeth posteriorly. laterally as a triangular surface as in Sauror- nithoides junior, Saurornithoides mongolien- POSTCRANIAL SKELETON sis (personal obs.), and dromaeosaurids (Cur- rie, 1995) cannot be determined. Limited postcranial remains were pre- SURANGULAR: The surangular is only pre- served with the holotype, including several served as pieces on the right side of the skull, vertebrae from the presacral part of the ver- 20 AMERICAN MUSEUM NOVITATES NO. 3402

Fig. 16. The teeth of the right maxilla and dentary of the holotype (IGM 100/983). Notice both the lack of serrations and the heterodont condition. tebral column and tail. Hindlimb elements in- borders of the neural arch arc ventrally and clude a heavily weathered element that rep- overhang the centrum. The broken posterior resents the proximal end of the right femur, end of the centrum reveals that the interior the right knee, a number of broken and was hollow, and a small foramen on the side weathered sections of hollow limb bones, of the centrum probably marks the pneumatic and a number of pedal phalanges. None of connection to the interior. Nearly complete the preserved vertebrae are complete. cervical ribs are preserved in articulation The preserved cervical elements are iden- with both the centrum and arch of the second ti®ed as parts of the axis and third and fourth cervical in the nodule. The capitulum and tu- cervicals. Two nodules each contain parts of berculum are axially elongate. A short, an- two articulated anterior cervicals. Only the terior process extends a short distance ante- posterior half of the neural arch and the pos- rior to the juncture between the tuberculum teriormost end of the centrum are preserved and capitulum, and a small pneumatic fora- on the more anterior cervical. The postzy- men is visible on the medial face of the left gapophyses are set far from the midline and rib in this region. Posterior to this juncture are surmounted by small epipophyses. The the main shaft of the cervical rib has a long, base of the neural spine is preserved and it tapering, transversely ¯attened shaft. appears to have been anteroposteriorly short A second, smaller nodule contains the an- along the midline. The following cervical is terior portion of the anterior vertebra from better preserved and comprises the majority the nodule described above as well as the of the centrum and the anterior half of the posterior part of the preceding element. This, neural arch (®g. 17). The prezygapophyses the anteriormost preserved cervical, has a diverge widely from the midline and face an- small but proportionately massive neural terodorsally and slightly medially. Small, an- arch. The arch narrows rostrally, suggesting terior pedicular fossae are present just lateral that it was triangular in dorsal view, as is to the ¯oor of the neural canal. Laterally, the characteristic of the theropod axis. The single 2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 21

Fig. 18. Dorsal vertebrae of IGM 100/983 in right and left lateral views. Abbreviations are list- ed in appendix 1. Fig. 17. Cervical and caudal vertebrae of IGM 100/984 in (A) posterodorsal and (B) dorsal views. Abbreviations are listed in appendix 1. toward their medial borders, forming hypo- spheneal articular facets. A single, apneumatic centrum, which is preserved postzygapophysis is surmounted wider than tall, may derive from the sacrum, by a massive, rugose epipophysis, suggesting but this cannot be determined for certain. that it derives from the axis (Gauthier, 1986). Four fragments of distal caudals are pre- The following element is heavily abraded but served, two of these are articulated and ap- displays a steeply angled anterior intercentral pressed to the larger piece containing ante- articulation typical of the anteriormost post- rior cervicals. The centra of the distal caudals axial cervicals. are large and laterally constricted. The pre- Four weathered sections of the dorsal se- zygapophyses are broken but do not appear ries are preserved. One of these sections to have been elongated as in basal tetanuri- comprises two centra with a tall narrow cross nes, because their preserved parts taper rap- section and large hypapophyses (®g. 18). The idly. The neural canal is diminutive, and a more anterior vertebra has the hypapophysis distinct sulcus extends above it along the situated at the front end of the centrum, roof of the neural arch. A similar sulcus is whereas this structure occupied a greater also present in other troodontids (Russell, length on the second vertebra. These verte- 1969; Russell and Dong, 1993). brae clearly derive from the very front of the A right knee composed of the proximal dorsal series. More posterior sections have ends of the tibia and ®bula and the distal ex- more rounded centra and diminutive zyga- tremity of the femur is poorly preserved, and pophyseal facets that are situated close to the little morphology is apparent (®g. 19). The midline. The postzygapophyses arc ventrally distal femur possesses a marked popliteal 22 AMERICAN MUSEUM NOVITATES NO. 3402

Fig. 20. Pedal phalanges of the holotype spec- imen IGM 100/983. Abbreviations are listed in appendix 1. Fig. 19. Right knee of the holotype specimen IGM 100/983 in (A) medial and (B) lateral views. Abbreviations are listed in appendix 1. only part of the metatarsus preserved. The articulation is narrow and simple, with a more pronounced medial than lateral collat- fossa. The distal condyles are narrow and ex- eral ligament fossa. A pronounced, narrow tend far posterior to the femoral shaft. A ridge forms the posteromedial edge of the ar- long, narrow supracondylar crest extends ticulation but is broken a short distance from the lateral condyle toward the proximal above its distal terminus. end of the femur. Fragments of three pedal phalanges were The proximal end of the tibia is subtrian- found with the holotype of Byronosaurus jaf- gular in proximal view. Both the cnemial fei (®g. 20). One of these can be identi®ed crest and the lateral condyle of the tibia are as the second phalanx of the second digit be- well developed and project far from the cause of the presence of an elongated heel shaft. The cnemial crest is subtrapezoidal in proximoventrally. As in other troodontids, medial view and curves laterally at its ante- but in contrast to derived dromaeosaurids rior rim. A large, suboval tuberosity marks like Velociraptor mongoliensis and Deinon- the proximodorsal extremity of the cnemial ychus antirrhopus, the pinched shaft between crest, and the anterior border below it is the articular ends is not foreshortened (Currie thickened for muscular insertion. No details and Peng, 1993). The medial face is vertical, are evident on the short preserved section of whereas the lateral one is convex, a feature the ®bula. A number of hollow limb bone that allows determination that the phalanx fragments pertain to the hindlimbs, but they derives from the right foot. Erosion has de- do not warrant description. stroyed much of the distal ginglymus. In pro- The distal end of right metatarsal II is the ®le, the phalanx resembles the corresponding 2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 23 bone in Troodon formosus (Currie and Peng, interfenestral bar; however, they apparently 1993) and an indeterminate troodontid from did not extend along the roof of the palate Mongolia (Barsbold et al., 1987). anteriorly beneath the nares as in Velocirap- The other phalangeal fragments consist of tor mongoliensis. articulated distal and proximal ends of two An unusual feature of Byronosaurus jaffei, phalanges. Dorsodistally, the articulation of which is also present in the alvarezsaurid the proximal element bears a lappetlike ex- Shuvuuia deserti, is the presence of a small tension of the articular surface. The articu- tubular foramen on the maxilla just dorsal to lated proximal end of one phalanx is trian- the tooth row on the ventral border of the gular in cross section and the rest of the el- narial fossa (®g. 6). We interpret this feature ement appears to have been relatively slen- as homologous to the subnarial foramen, der. By comparison with which typically lies in the premaxillary-max- gracilicrus (OsmoÂlska, 1987), these frag- illary suture and level with the supraalveolar ments may represent the phalanges III-2 and canal in theropods and presumably transmit- III-3. ted parts of the maxillary branch of CN V that innervated the tip of the snout. This DISCUSSION opening extends into a tubular sinus adjacent to the tooth row (the supra-alveolar canal Byronosaurus jaffei is one of the best pre- [Norell et al., 2000]). A very similar con®g- served troodontid dinosaurs yet discovered uration is seen in Shuvuuia deserti (IGM and extends our knowledge on the diversity 100/977). If our interpretation of this feature of Late troodontids. A number of is correct, then Sereno's (2001) assertion that important characters such as an extensive alvarezsaurids lack a subnarial foramen secondary palate, the opening here interpret- needs to be modi®ed. ed as a subnarial foramen, and the connec- The internarial bar in most theropod di- tion between the antorbital and accessory fe- nosaurs is an oval or cylindrical rodlike nestrae though the interfenestral bar were structure composed of the premaxillae that previously unrecognized in troodontids. Un- meet the nasals posteriorly. Sereno (2001: usually among troodontids, the teeth are un- 93) asserted, in reference to his placement of serrated. Prior to the discovery of Byrono- alvarezsaurids as the sister group to orni- saurus jaffei and Sinovenator changii (Xu et thomimosaurs, that ``the unusual dorsoven- al., 2002), most known troodontid teeth were tral ¯attening of the internarial bar . . . is characterized by large, apically curved den- unique to these two groups.'' However, this ticles with ``blood pits'' between them (Cur- con®guration is also present in the troodontid rie et al., 1990). The only exception is an taxa Saurornithoides junior and Byronosau- unnamed troodontid from Mongolia, which rus jaffei. apparently has reduced denticles that extend The interfenestral bar in Byronosaurus jaf- perpendicular to the axis of the tooth (Bars- fei shows some unique features (®gs. 7±10). bold et al., 1987). Unlike other troodontids, the interfenestral Byronosaurus jaffei provides us with the bar is not inset from the plane of the maxilla, ®rst data on the anatomy of the anterior part but is ¯ush with the lateral surface of the of the troodontid palate. The palate is formed snout. As in both species of Saurornithoides, anteriorly by the premaxilla and more pos- a connection between the antorbital and ac- teriorly by the palatal shelves, which are ¯at cessory antorbital fenestrae passes through anteriorly and become more vaulted posteri- the interfenestral bar (®gs. 9, 10). The nasal orly (®gs. 4, 12). Premaxillary contribution passage terminates at the dorsal end of the to the roof of the palate is limited compared interfenestral bars in Byronosaurus jaffei, to other coelurosaurs. The vomers appear and it opens into the choanae at the level of fused anteriorly and are divided posteriorly the antorbital fenestra (®g. 9). Our previous at the level of the interfenestral bar (®g. 12). description (Norell et al., 2000) stated un- The vomers apparently lay between the pal- clearly that the nasal passages are connected atal shelves posteriorly and lie dorsal to them to the antorbital fenestra by a connection more anteriorly. They extend anterior to the through the interfenestral bar, but an exposed 24 AMERICAN MUSEUM NOVITATES NO. 3402 cross section of this region in the referred via gracilicrus and nemegtensis specimen clearly shows that it is the nasal were excluded, because apart from the auta- passage proper that transits the interfenestral pomorphic phalangeal characters of the for- bar. mer, the single known specimen of each spe- Although Byronosaurus jaffei shares de- cies only shows characters that are common rived dental characters related to tooth spac- to all troodontids more derived than Sinov- ing and shape with other troodontids, it dif- enator, and thus provides no information to- fers in having unserrated teeth (®g. 16). This ward elucidating troodontid relationships. has important implications toward under- Our comprehensive approach of analyzing standing the phylogenetic position of troo- troodontid relationships in the broader con- dontids. Some previous analyses have text of coelurosaurian phylogeny is justi®ed grouped the Troodontidae with therizinosau- by the lack of consensus regarding the rela- roids (and the edentulous oviraptorosaurs) in tionship of Troodontidae to outgroup taxa. part because of the enlarged and apically Furthermore, the high amounts of homoplasy hooked serrations seen in members of both and missing data that pervade coelurosaurian groups. The presence of small serrations in relationships complicate the correct estab- the recently described basal troodontid Si- lishment of character polarities. In our view, novenator (Xu et al., 2002) and the absence the least assumptive approach to this prob- of serrations in Byronosaurus jaffei clearly lem is to analyze a comprehensive range of show that large denticles can no longer be outgroup taxa (i.e., Coelurosauria) to achieve regarded as plesiomorphic within Troodon- an accurate optimization of ancestral states at tidae, and are diagnostic of a more exclusive the root node of Troodontidae. clade within Troodontidae. All characters were treated as unordered. Analysis of this dataset was conducted using PHYLOGENY NONA version 1.9 (Goloboff, 1999) run through the Winclada interface (Nixon, In our preliminary description of Byrono- 1999). One thousand repetitions of the tree saurus jaffei (Norell et al., 2000), we exam- bisection regrafting algorithm were used to ined the relationship of Byronosaurus jaffei ®nd islands of shortest trees, followed by to other troodontids and found it to be the branch swapping to ®nd all shortest trees. sister group to a clade of derived large-bod- The analysis found 432 trees (TL ϭ 586, CI ied troodontids comprising Troodon formo- ϭ 0.45, RI ϭ 0.75), and a strict consensus sus and both species of Saurornithoides. tree is presented in ®g. 21. Troodontid taxa More recent studies of coelurosaurian rela- are boldfaced. tionships (Norell et al., 2001; Xu et al., 2002) Troodontids are found to be monophyletic have included almost the same sampling of and are the sister taxon of the Dromaeosaur- troodontid taxa as our preliminary study and idae within a monophyletic Deinonychosau- have yielded results that are largely congru- ria, which in turn is the sister taxon to Avi- ent with it. alae. Troodontid monophyly is wellsupported Here we have expanded the Xu et al. by the nine characters discussed below. (2002) study, which included the data of No- Dentary nutrient foramina situated in a rell et al. (2000), by adding an unnamed tax- horizontal groove on labial face of den- on from the Early Cretaceous of Mongolia tary: The labial margin of the dentary is (Barsbold, 1987) and several characters, pierced by a disparate number of nutrient and some of which pertain to troodontid relation- nervous foramina in theropods. In Byrono- ships. Seven troodontid species are included saurus jaffei (®gs. 2, 3), Troodon formosus along with 42 other coelurosaurian taxa and (Russell, 1969), Saurornithoides mongolien- the tetanuran Allosaurus, which was used to sis, Saurornithoides junior (Barsbold, 1974), root the most parsimonious trees. The com- and Sinovenator changii (Xu et al., 2002) plete matrix has 220 characters and 50 taxa, these foramina line the ¯oor of a distinct sul- and the character list and data matrix are cus on the labial side of the dentary (Currie, available at http://research.amnh.org/users/ 1987). In Sinovenator changii this groove is norell/index.html. The troodontids Borogo- restricted to the posterior part of the dentary 2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 25

Fig. 21. Strict consensus of coelurosaurian interrelationships found in our phylogenetic analysis of 220 characters in 50 coelurosaurian taxa. Troodontids are indicated in bold. 26 AMERICAN MUSEUM NOVITATES NO. 3402 ventral to the caudal end of the tooth row, of troodontids appears to be dorsoventrally but in more derived taxa it extends further compressed (®gs. 2, 5). Each premaxillary toward, but does not reach, the symphysis. In internarial process is wider than tall and has dromaeosaurids, oviraptorids, and other man- a dorsally ¯at surface, and the short nasal iraptorans the neurovascular foramina are contribution to the narial bar forms a lateral linearly arranged but do not lie in a groove. extension of the base of the bar. This con- Pneumatic quadrate with pneumato- trasts with the round internarial bar of dro- pore on posterior face: The quadrate bears maeosaurids such as Velociraptor mongo- a pneumatic foramen posteriorly in the troo- liensis (AMNH 6215) and uncrested ovirap- dontid taxa where it is known, including torosaurs. A ¯attened internarial bar is de- Troodon formosus (Varrichio, 1997), an un- rived independently in ornithomimosaurs and named Early Cretaceous troodontid (Bars- alvarezsaurids (Sereno, 1999). According to bold et al., 1987), and Sinovenator changii our analysis, this feature does not unambig- (Xu et al., 2002). Quadrate pneumaticity is uously optimize as a synapomorphy of any variably expressed among coelurosaurian combination of these clades. theropods. It is observed in tyrannosaurids, Closely packed anterior dentition in advanced oviraptorids (Maryanska and Os- symphyseal region of dentary: The lower moÂlska, 1997), the compsognathid Sinosau- dentition of troodontids is moderately heter- ropteryx prima (Currie and Dong, 2001b), odont. In the dentary, the anteriormost teeth derived avialans, and possibly some, but not are closely packed, and toothless jaws show all, ornithomimosaurs (Makovicky and No- that the alveoli are con¯uent within a groove rell, 1998). Quadrate pneumaticity is absent in the anterior part of the jaw (Currie, 1987). in many maniraptorans, including basal ovi- Posteriorly, the alveoli are separated by walls raptorosaurs such as , alvarezsaur- of porous interdental bone. In contrast, the ids, therizinosauroids (Clark et al., 1994), teeth of most theropods are more equidis- and Ornitholestes hermanni. It also appears tantly separated and set in separate alveoli to be absent in lithographica throughout the whole jaw, as in dromaeo- (Witmer, 1990; Elzanowski and Wellnhofer, saurids (IGM 100/986, AMNH 5356) and 1996) and sanctus (Chiappe Archaeopteryx lithographica (Elzanowski et al., 1999). and Wellnhofer, 1996). The teeth of alvar- Basisphenoid recess absent: A blind ezsaurids and the basal ornithomimosaur Pe- pocket, derived from the median pharyngeal lecanimimus are set in a groove throughout system, invades the basisphenoid ventrally in the dentary, but are subequal in size. In ther- most theropods and forms a deep pneumatic izinosauroids, all teeth are set in individual recess on the basicranium (Witmer, 1997). alveoli separated by septa, and the teeth This basisphenoid recess is widespread in grade toward larger size rostrally (Clark et coelurosaurs and is seen in ornithomimids al., 1994; Xu et al., 2001). (OsmoÂlska et al., 1972), oviraptorids (Clark Depression on the ventral surface of the et al., 2002), and dromaeosaurids (e.g., Dro- postorbital process of the laterosphenoid: maeosaurus albertensis AMNH 5356). This A ventral pit is visible on the postorbital pro- recess is secondarily lost in troodontids (Cur- cess of the laterosphenoid in Troodon for- rie, 1985), although the basisphenoid is hol- mosus (Currie, 1985), Saurornithoides junior low and pneumatic in at least Troodon for- (IGM 100/1), and Sinovenator changii (Xu mosus, Byronosaurus jaffei (®g. 15), and et al., 2002). The pit has smooth walls, and Saurornithoides junior. Secondary loss of may have been pneumatic. The postorbital the basisphenoid recess is also observed in process of dromaeosaurids (AMNH 5356, the therizinosauroid andrewsi PIN 100/25), Archaeopteryx (Walker, 1985; (Clark et al., 1994) and in advanced aviali- London specimen), oviraptorosaurs, therizi- ans. The condition is unknown for Archae- nosauroids (Clark et al., 1994), alvarezsaur- opteryx lithographica and Confuciusornis ids, and ornithomimosaurs does not bear sanctus. such a pit. Dorsoventrally ¯attened internarial Reduced basal tubera that lie directly bar: As discussed above, the internarial bar ventral to the occipital condyle: The basal 2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 27 tubera are unusual among coelurosaurs in be- are relatively short in dromaeosaurids such ing reduced and situated close to the midline, as millenii (Xu et al., 1999) so that they are directly ventral to the occip- and Velociraptor mongoliensis (IGM 100/ ital condyle in Asian troodontid taxa (®g. 986) and in the basal avialans Archaeopteryx 14). The tubera are separated by a narrow V- lithographica (Berlin specimen) and Rahon- shaped notch in all troodontids. The tubera avis ostromi (Forster et al., 1998). Ovirap- are larger in Troodon formosus, but they are torosaurs have short, wide transverse pro- still positioned close to the midline (Russell, cesses on the trunk vertebrae (Makovicky 1969) and ventral to the condyle. In many and Sues, 1998). Proportionately long trans- outgroup taxa, including dromaeosaurids, verse processes are seen in ornithomimids oviraptorosaurs, and ornithomimosaurs, the and in alvarezsaurids. tubera are more pronounced and lie in a para- In all troodontids for which the tail is sagittal plane lateral to the occipital condyle. known, the distal caudals have a sulcus on Alvarezsaurids have reduced tubera, but the dorsal midline in place of a neural spine, these are widely separated from each other but the condition is unknown for the most by a notch that is wider than the diameter of basal troodontid Sinovenator and is thus an the occipital condyle. The basal tubera of the ambiguous synapomorphy of the clade. Sev- therizinosauroid Erlikosaurus andrewsi are eral characters used to diagnose Troodonti- virtually obliterated by the ventral expansion dae in the past no longer optimize at the base of the pneumatic basisphenoid (Clark et al., of Troodontidae, following the discovery of 1994). a number of new deinonychosaurs in recent Large number of teeth: Among advanced years. The troodontid metatarsus is asym- theropods, troodontids are characterized by metrical with a slender metatarsal II that is having a very large number of teeth (®g. 2). markedly shorter than metatarsals III and IV, Although the precise number is unknown in and a robust metatarsal IV, unlike the more Byronosaurus jaffei, a minimum of 4 pre- symmetrical metapodium of most other the- maxillary and 30 maxillary teeth were pre- ropods, and this asymmetry has been used to sent in the upper jaws, while the dentary held diagnose the clade in the past (Wilson and 30 or more teeth. In dromaeosaurids the Currie, 1985). Somewhat asymmetric meta- tooth count is substantially smallerÐ 4 pre- podia are also observed in the basal dro- maxillary, and 9 (Dromaeosaurus alberten- maeosaurids zhaoianus (Xu et sis) to15 ( antirrhopus) maxil- al., 2000) and, to a lesser degree, in Sinor- lary teeth and 11 (Dromaeosaurus alberten- nithosaurus millenii (Xu et al., 1999), and sis)to16(Deinonychus antirrhopus) dentary this character may be diagnostic of Deinon- teeth (Ostrom, 1969, Currie, 1995). Archae- ychosauria rather than of Troodontidae. opteryx lithographica has a dental formula of Possession of large, apically hooked den- 4 premaxillary plus 8 maxillary and 11 den- ticles has also been considered a diagnostic tary teeth (Wellnhoffer, 1974). Oviraptoro- character of troodontids (Barsbold, 1974; saurs and advanced ornithomimids are eden- Currie, 1987). The recently discovered Si- tulous, but the basal ornithomimid Pelecan- novenator changii has small denticles direct- imimus polyodon (Perez-Moreno et al., 1994) ed perpendicular to the long axis of the tooth, has more than 220 teeth in its jaws, and a however, and this character now diagnoses a large number of teeth are also present in the more exclusive clade within Troodontidae. alvarezsaurid Shuuvuia and in therizinosau- An in¯ated bulla at the base of the cultriform roids. According to our analysis, these taxa process has been observed in Saurornithoi- derive the large number of teeth independent des junior and Troodon formosus, and it was of troodontids. previously viewed as a troodontid synapo- Long, slender transverse processes: The morphy. It is, however, absent in the basal transverse processes of the trunk vertebrae troodontid Sinovenator changii and its pres- are relatively long and slender and curve ence cannot be ascertained in several other slightly in troodontid taxa that preserve taxa including Byronosaurus jaffei. them, including Sinoventor changii and Within troodontids, Sinovenator changii is Troodon formosus. The transverse processes the sister taxon to a clade comprising all oth- 28 AMERICAN MUSEUM NOVITATES NO. 3402 er troodontids. Two unambiguous synapo- thoides junior, but is dorsal to it in Byrono- morphies are present at this node. saurus jaffei (®gs. 14, 15). Presence of a subotic recess on the side Presence of enlarged, distally oriented of the braincase ventral to the middle ear: denticles: Theropod teeth are primitively ser- A pneumatic pocket invades the lateral wall rated. In velociraptorine dromaeosaurids of the basicranium ventral to the middle ear there are 5±7 denticles per millimeter (Currie in Troodon formosus, Saurornithoides ju- et al., 1990), while this ratio is 3±3.2 denti- nior, Byronosaurus jaffei (®gs. 14, 15), and cles per millimeter in Dromaeosaurus alber- the Early Cretaceous troodontid from Kha- tensis (Currie, 1996). By contrast the number mareen Us (Barsbold et al., 1987). Witmer of denticles is 2.3±3 in Saurornithoides mon- (1997b) termed this feature the subotic re- goliensis (Russell, 1969) and less than 2 per cess. A subotic recess is absent in dromaeo- mm in Troodon formosus (Currie et al., saurids, basal avialans, alvarezsaurids, and 1990) and Saurornithoides junior (Barsbold, oviraptorosaurs. It is convergently present in 1974). In addition, the denticles of these at least some derived ornithomimosaurian troodontid taxa are long and separated by taxa (Makovicky and Norell, 1998). deep blood grooves with small proximal Metatarsal IV oval in cross section, blood pits. The teeth of Sinornithoides youn- deeper than wide: Basal paravians have a gi have porportionately smaller serrations mediolaterally expanded metatarsal IV with than do those of the three larger taxa listed a lateral ¯ange on the side. This condition is above, but they are still proportionately large seen in Archaeopteryx lithographica and Ra- relative to tooth size (Currie and Dong, honavis ostromi and in dromaeosaurids, in- 2001a). cluding Microraptor zhaoianus and Veloci- Relationships between Byronosaurus jaf- raptor mongoliensis. In higher troodontids, fei, Sinornithoides youngi, and a clade com- the fourth metatarsal reverses to a more co- prising the three larger bodied taxa Troodon lumnar shape with an oval cross section that formosus and both species of Saurornithoi- is longer along the palmar-plantar axis. des cannot be parsed out, partly because the Byronosaurus jaffei and Sinornithoides braincase is undescribed for Sinornithoides youngi form a clade together with the larger and the dentition is autapomorphic in Byron- bodied, Troodon formosus, osaurus jaffei. The Troodon ϩ Saurornithoi- Saurornithoides mongoliensis, and Sauror- des clade is derived in its possession of a nithoides junior. Two cranial characters un- medially de¯ected symphysis on the dentary ambiguously diagnose this group. and by the secondary loss of a dorsal tym- Large otosphenoidal crest de®ning lat- panic recess. eral depression on side of braincase: Troo- dontids, with the exceptions of an unnamed ACKNOWLEDGMENTS Early Cretaceous form (Barsbold et al., We thank the ®eld crews of the 1993, 1987) and Sinovenator changii, bear a well- 1994, and 1996 ®eld seasons. For help in developed ``lateral depression'' (Barsbold, Mongolia, we would especially thank D. 1974) on the side of the braincase covering Baatar, T. Boldsukh, D. Dashzeveg, and R. parts of the laterosphenoid, basisphenoid, Barsbold. Mike Novacek found the type prootic, and possibly part of the opisthotic. specimen (IGM 100/983). Martin KundraÂt This lateral depression is delimited anteriorly carefully went over the manuscript. Amy Da- and dorsally by the expanded otosphenoidal vidson and Bill Amaral prepared the speci- crest, which is formed by the laterosphenoid mens and Mick Ellison developed the ®g- anteriorly and by the prootic and possibly ures. This work was supported by the Divi- also the opisthotic posteriorly. A number of sion of Paleontology at the American Mu- small laminae subdivide the lateral depres- seum, the Field Museum Department of sion into subcompartments, such as the pro- Geology, and the Jaffe Family Foundation. otic recess rostrally. The internal carotids en- ter the hypophyseal fossa near the rostral part REFERENCES of the lateral depression. The middle ear lies Barsbold, R. 1974. Saurornithoididae, a new fam- within the lateral depression in Saurorni- ily of small theropod dinosaurs from Central 2003 MAKOVICKY ET AL.: BYRONOSAURUS JAFFEI 29

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APPENDIX 1

ANATOMICAL ABBREVIATIONS II-2 pedal phalanx II-2 lpmx left premaxilla aof antorbital fenestra ls left splenial atr anterior tympanic recess mbg groove separating forebrain and mid- boc base of occipital condyle brain of endocast bot basioccipital tuber mf metotic foramen cc common carotid foramen mg mandibular groove cf carotid foramen ms metotic strut cif interfenestral crest mxf maxillary fossa cnc cnemial crest nar naris cnV cranial nerve v nc nasal chamber cnVI foramen for cranial nerve VI ns neural spine cnVII foramen for cranial nerve VII otc otosphenoidal crest cn X,XI foramen for cranial nerves X and XI pn pneumatic space cnXII foramen for cranial nerve XII pop paroccipital process csu caudal vertebral sulcus prz prezygapophysis ctr caudal tympanic recess ps palatal shelf dg dentary groove pvh posteroventral heel dr dorsal rib pz postzygapophysis dtr dorsal tympanic recess r reconstructed en entrance to nasal chamber ra right angular ep epipophysis rd right dentary ®c ®bular crest fm foramen magnum rf right ®bula fnc ¯oor of nasal chamber rj right jugal fo foramen ovale rl right lacrimal fp foramen pseudorotunda rmx right maxilla fsac foramen for supraalveolar canal rn right nasal (ϭ subnarial foramen) rpmx right premaxilla hf hypophysial fossa rs right splenial hy hypapophysis rsa right surangular iac interantorbital canal rt right tibia in internarial bar sac supraalveolar canal ifb interfenestral bar sf surangular foramen ld left dentary sor subotic recess ll left lacrimal t tooth lmx left maxilla v vomer ln left nasal waof wall of antorbital fossa 32 AMERICAN MUSEUM NOVITATES NO. 3402

APPENDIX 2 MEASUREMENTS OF THE HOLOTYPE SPECIMEN IGM 100/983 (in mm)

a This paper meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper).