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Byronosaurus Jaffei (Theropoda: Troodontidae)

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Byronosaurus Jaffei (Theropoda: Troodontidae) 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 (Theropoda: Troodontidae) 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 dinosaurs 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 q 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 troodon- tid, Saurornithoides 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], birds [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 Sinovenator 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 Sinornithoides 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 3 512 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.
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