New Information on the Braincase of the North American Therizinosaurian (Theropoda, Maniraptora) Falcarius utahensis

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1 New Information on the Braincase of the North American Therizinosaurian (Theropoda, Maniraptora) Falcarius utahensis Author(s): David K. Smith, Lindsay E. Zanno, R. Kent Sanders, Donald D. Deblieux, and James I. Kirkland Source: Journal of Vertebrate Paleontology, 31(2): Published By: The Society of Vertebrate Paleontology URL: BioOne ( is an electronic aggregator of bioscience research content, and the online home to over 160 journals and books published by not-for-profit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne s Terms of Use, available at Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder. BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research.

2 Journal of Vertebrate Paleontology 31(2): , March by the Society of Vertebrate Paleontology ARTICLE NEW INFORMATION ON THE BRAINCASE OF THE NORTH AMERICAN THERIZINOSAURIAN (THEROPODA, MANIRAPTORA) FALCARIUS UTAHENSIS DAVID K. SMITH, *,1 LINDSAY E. ZANNO, 2 R. KENT SANDERS, 3 DONALD D. DEBLIEUX, 4 and JAMES I. KIRKLAND 4 1 Biology Department, Northland Pioneer College, Show Low, Arizona 85901, U.S.A., dsmith@npc.edu; 2 The Field Museum of Natural History, Chicago, Illinois 60605, U.S.A., lzanno@fieldmuseum.org; 3 University of Utah Health Sciences Center, Salt Lake City, Utah 84112, U.S.A., kent.sanders@hsc.utah.edu; 4 Utah Geological Survey, Salt Lake City, Utah 84114, U.S.A., dondeblieux@utah.gov; jameskirkland@utah.gov ABSTRACT Many disarticulated bones from multiple individuals of a primitive therizinosaurian, referred to Falcarius utahensis, were found in the paucispecific Crystal Geyser bonebed in the Lower Cretaceous Cedar Mountain Formation of eastern Utah. To date, more than 2000 specimens from this species have been excavated. Included in this collection are two partial braincases, one of which is designated the holotype. Here we describe the braincase morphology of Falcarius utahensis. These specimens help establish the primitive cranial condition for the Therizinosauria and further substantiate intraspecific and contralateral braincase pneumatic variation in theropods. When combined with new observations on the cranial remains of the therizinosaurid Nothronychus mckinleyi derived from computed tomographic (CT) scans, the braincase morphology of Falcarius clarifies several evolutionary trends within the Therizinosauria and establishes a suite of synapomorphies for the Therizinosauridae. Trends within the clade include increased basicranial pneumatization (the development of a basisphenoid bulla and loss of external subcondylar recesses), anterior deflection of the supraoccipital, and the reduction of points of origin of the craniocervical musculature, associated with the loss of discrete basipterygoid processes, probably due to incorporation of these structures into the expanded hyperpneumatic bone. Finally, CT scans reveal a complete, nearly avian, inner ear with bird-like semicircular canals and a long cochlea indicating broad frequency discrimination. INTRODUCTION Therizinosaurians, such as Erlikosaurus andrewsi (Perle, 1981), Segnosaurus galbinensis (Perle, 1979), and Therizinosaurus cheloniformis (Maleev, 1954), comprised a rare, unusual group of Cretaceous maniraptoran theropods historically known exclusively from Asia (Barsbold, 1976; Perle, 1979; Russell and Dong, 1993; Clark et al., 2004). However, recent discoveries from North America (Gillette and Albright, 2001; Kirkland and Wolfe, 2001; Kirkland, Zanno, et al., 2005) indicate that the clade was more widespread and actually had a pan-laurasian distribution, at least during the Early and early Late Cretaceous. In general, they are distinguished from other theropods by having small heads bearing leaf-shaped teeth, long necks, functionally four-toed feet, and stout bodies (Zanno, 2004a, 2004b). At least 13 species are known, but skull remains are rare. Asian therizinosaurians with skull material include a partial braincase noted, but not yet figured, for the therizinosauroid Neimongosaurus yangi (Zhang et al., 2001) from China and a nearly complete skull, including the braincase, of the therizinosaurid Erlikosaurus andrewsi (Clark et al., 1994). No skull material is described for the intermediate therizinosaurians Erliansaurus bellamanus (Xu, Zhang, et al., 2002), or Suzhousaurus megatherioides (Liet al., 2007). The North American therizinosaurians include Nothronychus mckinleyi from the Upper Cretaceous (middle Turonian) Moreno Hill Formation of western New Mexico (Kirkland and Wolfe, 2001), Nothronychus graffami, which lacks braincase material, from the early Turonian Tropic Shale of southern Utah (Gillette and Albright, 2001; Zanno et al., 2009), and Falcarius utahensis, from the Lower Cretaceous (early Barremian) of Utah. * Corresponding author. Excavations of the paucispecific bonebed at the Crystal Geyser Quarry, southeast of Green River, in the Yellow Cat Member of the lower Cretaceous Cedar Mountain Formation, east-central Utah, were undertaken by representatives of the Utah Geological Survey and the University of Utah. Thousands of well-preserved, disarticulated specimens of Falcarius were recovered from the quarry representing individuals of various sizes and, presumably, ontogenetic stages (Zanno and Erickson, 2006). Among the specimens collected are two partial braincases preserved in carbonate nodules collected from near the top of the bone-bearing horizon, plus a juvenile basioccipital from the base of the bone layer. One of the braincases was designated as the holotype for Falcarius utahensis (Kirkland, Zanno, et al., 2005) and the other was referred to the same species. Numerous studies have recovered therizinosaurians close to oviraptorosaurians (Sues, 1997; Makovicky and Sues, 1998; Rauhut, 2003; Zanno, 2004a, 2006; but see Sereno, 1999, and Zanno et al., 2009 for different results). Collectively, therizinosaurians and oviraptorosaurians appear to represent a radiation of predominantly herbivorous theropods (Smith, 1992; Russell, 1997; Xu, Cheng, et al., 2002; Zanno, 2004a, 2004b; Kirkland, Zanno, et al., 2005). Falcarius (Kirkland, Zanno, et al., 2005) is the most basal therizinosaurian known to date and its braincase exhibits a mosaic of primitive and derived characters observed in other theropod and therizinosaurian taxa (Figs. 1 4). Therefore, in the following description, we take a comprehensive approach, comparing the braincase of Falcarius with specimens and descriptions in the literature of a suite of coelurosaurian and non-coelurosaurian braincases. These theropods include the therizinosaurians Nothronychus (Figs. 5 6 and Kirkland, Smith, et al., 2005) and Erlikosaurus (Clark et al., 1994); the oviraptorids Chirostenotes (Sues, 1997) and Citipati (Clark et al., 2002); 387

3 388 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 31, NO. 2, 2011 FIGURE 1. Stereophotographs of Falcarius utahensis, partial braincase, UMNH VP 15000, Lower Cretaceous Cedar Mountain Formation, Crystal Geyser Site, Utah, in A, posterior; B, dorsal; and C, left lateral views. Scale bar equals 1 cm. thedromaeosauridsdeinonychus (Brinkman et al., 1998), Velociraptor (Barsbold and Osmólska, 1999; Norell et al., 2004), and Dromaeosaurus (Currie, 1995); the ornithomimid Gallimimus (Osmólska et al., 1992); the alvarezsaurid Shuvuuia (Chiappe et al., 2002); the basal avian Archaeopteryx (Whetstone, 1983); and the non-coelurosaurians Allosaurus (Madsen, 1976; Rogers, 1999), Ceratosaurus (Madsen and Welles, 2000; Sanders and Smith, 2005), and Majungasaurus (Sampson and Witmer, 2007), among others. We follow the taxonomic definitions of Zanno et al. (2009) and Zanno (2010) for the taxa Therizinosauria, Therizinosauroidea, and Therizinosauridae. For the purposes of this discussion, Therizinosauridae refer to Erlikosaurus and Nothronychus, and excludes Falcarius. The endocranial morphology of Falcarius is compared with new observations and computed tomographic (CT) scans of the therizinosaurid Nothronychus, which is included to present trends in therizinosaurian evolution. Descriptions of other theropods (e.g., Madsen, 1976; Currie and Zhao, 1993; Norell et al., 2006; Sampson and Witmer, 2007) are used to illustrate the non-therizinosaurian condition. Finally, the inner ear of Falcarius was digitally extracted using CT data and is compared with results for Ceratosaurus (Sanders and Smith, 2005), Allosaurus (Rogers, 1999), Majungasaurus (Sampson and Witmer, 2007), Archaeopteryx (Dominguez-Alonso et al.,2004), and other theropods (Witmer and Ridgely, 2009). Institutional Abbreviations AZMNH, Arizona Museum of Natural History, Mesa, Arizona, U.S.A.; MWC, Museum of Western Colorado, Grand Junction, Colorado, U.S.A.; RTMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta, Canada; UMNH, Utah Museum of Natural History, Salt Lake City, Utah, U.S.A. Anatomical Abbreviations AA, anterior ampulla; ASC, anterior semicircular canal; BC/SCA, m. biventer cervicis/m. splenius capitis attachment; BO, basioccipital; BPP, basipterygoid

4 SMITH ET AL. THE BRAINCASE OF FALCARIUS 389 process; BPR, basipterygoid recess; BS, basisphenoid; BSR, basisphenoid recess; BT, basal tubera; BTS, basituberal sinus; C, cochlear duct; CE, cerebellar eminence; CC, common crus; CF, cerebellar fossa; CIF, crista interfenestralis; EX, exoccipitalopisthotic; FM, foramen magnum; FO, fenestra ovalis; FPR,foramen pseudorotunda; FR, floccular recess; H, hypophyseal fossa; HSC, horizontal semicircular canal; IAC, internal auditorycanal; J, jugular foramen; MF, metotic foramen; MS, metotic strut; OC, occipital condyle; OCC, ostium canalium caroticorum (= carotid canal); OR, otic recess; OTI, optic tract impression; PF, pituitary fossa; PA, posterior ampulla; PI, pontine impression; PMI, pontomedullary impression; POPR, paroccipital process; POPRS, paroccipital process sinus; PO, prootic; POR, prootic recess; PSC, posterior semicircular canal; RCA, rectus capitis anterior origin; SCR, subcondylar recess; SG, stapedial groove; SO, supraoccipital; SOR, suboticrecess; TG, trigeminal ganglion; U, utricle; VCMP, middle cerebral vein, posterior branch; VE, vestibular eminence. III XII, cranial nerve foramina. METHODS AND MATERIALS CT scans of both braincases of Falcarius (holotype UMNH VP 15000, referred UMNH VP 15001) and that of Nothronychus (AZMNH 2117) were performed at the University of Utah Health Sciences Center and then rescanned at Ohio University. A soft tissue algorithm was used to reduce the effects of beam hardening artifact (Curry et al., 1990). Coronal slices were taken at 1.5-mm intervals at the University of Utah using a medical scanner. Good CT results were obtained for both braincases of Falcarius and Nothronychus. The inner ear was reconstructed using SurfDriver 8 and is compared with CT results for the non-maniraptoran theropod Ceratosaurus (Sanders and Smith, 2005) and other theropods in the literature (e.g., Witmer and Ridgely, 2009). Because it is currently undergoing research elsewhere, a complete pneumatic and soft-tissue reconstruction of Falcarius (Dufeau and Witmer, 2008) and related large-scale evolutionary trends in these structures for coelurosaurians will not be presented here, only their immediate relevance to the observed osteology. DESCRIPTION FIGURE 2. Interpretive illustrations of Falcarius utahensis, partial braincase, UMNH VP 15000, Lower Cretaceous Cedar Mountain Formation, Crystal Geyser Site, Utah, in A, posterior; B, dorsal; and C, left lateral views. Scale bar equals 1 cm. Scale bar equals 5 cm. Drawings by David K. Smith. The posterior region of the braincase of Falcarius utahensis is well preserved in the two specimens, UMNH VP and UMNH VP (Figs. 1 4). The two braincases exhibit slight differences in the development of pneumatic and internal features that can be attributed to intraspecific variability, as is seen in other theropods such as Allosaurus fragilis (Chure and Madsen, 1996; Smith and Lisak, 2000). The supraoccipital, exoccipital-opisthotic, basioccipital, basisphenoid-parasphenoid, and prootic are present in both specimens, but the occipital condyle is absent in UMNH VP Although most sutures are fused, the parietal is interpreted as unpreserved in both specimens, based on the locations of the observed vascular foramina. Therefore, the contribution of the parietal to the occiput was reduced in therizinosauroids relative to other coelurosaurs, such as Velociraptor (Barsbold and Osmólska, 1999). In general, Falcarius exhibits the cranial and endocranial foreshortening typical of most coelurosaurs (Witmer and Ridgely, 2009), though not as exaggerated as that seen in the therizinosaurids Nothronychus and Erlikosaurus (Clark et al., 1994), as well as derived oviraptorids (Clark et al., 2002), and ornithomimids (Makovicky and Norell, 1998). Both oviraptorosaurs and known therizinosaurians are striking in their degree of skull pneumatization (Clark et al., 1994; Witmer, 1997a; Kundrát and Janáček, 2007).

5 390 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 31, NO. 2, 2011 FIGURE 3. Stereophotographs of Falcarius utahensis, partial braincase, UMNH VP 15000, Lower Cretaceous Cedar Mountain Formation, Crystal Geyser Site, Utah, in A, ventral; B, anterior; and C, right lateral views. Scale bar equals 1 cm. Supraoccipital The typical theropod supraoccipital forms a vertical descending wedge over the foramen magnum, separating the adjacent exoccipital-opisthotics and thereby contributing to the occipital plate. A variably developed supraoccipital knob is present in other theropods, including ceratosaurians (Madsen and Welles, 2000), Majungasaurus (Sampson and Witmer, 2007), and basal tetanurans (Madsen, 1976), as well as the more derived troodontids (Currie and Zhao, 1993; Makovicky et al., 2003; Norell et al., 2006). The supraoccipital is only partially preserved in the holotype specimen of Falcarius, but is more complete in UMNH VP (Figs. 1 4). Falcarius and Nothronychus (Figs. 5 6) are unusual among theropods in possessing a more horizontal orientation of the supraoccipital than is typical, thereby contacting the parietal anteriorly and forming the posterior portion of the skull roof. In neither taxon is there any evidence of a distinct nuchal crest. The supraoccipital of Falcarius, but not Nothronychus, contains a convex cerebellar prominence at the midline over the cerebellar fossa, like the hesperornithian bird Enaliornis (Galton and Martin, 2002). The prominence extends above the level of the dorsal margin of the paroccipital process, but unlikeenaliornis, it does not extend beyond the occipital condyle, as the condyle of Falcarius isoriented posteriorly rather than posteroventrally (Galton and Martin, 2002). Passing anteriorly, the cerebellar prominence becomes increasingly pronounced and probably continues onto the parietals. Lateral to the prominence, the braincase is concave, but these regions lack distinct margins. In Enaliornis, this area serves as the attachment points for the m. biventer cervicis and m.

6 SMITH ET AL. THE BRAINCASE OF FALCARIUS 391 FIGURE 4. Interpretive illustrations of Falcarius utahensis, partial braincase, UMNH VP 15000, Lower Cretaceous Cedar Mountain Formation, Crystal Geyser Site, Utah, in A, ventral; B, anterior; and C, right lateral views. Scale bar equals 5 cm. Drawings by David K. Smith. splenius capitis (Galton and Martin, 2002). As in Garudimimus (Kobayashi and Barsbold, 2005), the cerebellar prominence in Falcarius is faintly bifurcate posteriorly, dividing above the foramen magnum with a shallow, posteriorly directed pit at the base. It terminates at a slightly rugose margin surrounding the foramen magnum that reflects the points of origin for the craniocervical musculature. The pit probably reflects the attachment point for a weak nuchal ligament. In Erlikosaurus (Clark et al., 1994), the supraoccipital retains a more typical configuration, with an anterodorsal orientation combined with a pronounced nuchal crest, whereas in Nothronychus, the supraoccipital lacks any crest or cerebellar prominence. The reduced crests in Nothronychus and Falcarius may be associated with weakening of the craniocervical musculature relative to other theropods (Sampson and Witmer, 2007). As in many theropods (Currie and Zhao, 1993; Brochu, 2003; Sampson and Witmer, 2007), the supraoccipital of Falcarius is hollow and has a large, oval, laterally oriented venous foramen, originating within the endocranial cavity and exiting the top of the skull. This foramen is described as accommodating the posterior branch of the middle cerebral vein in other theropods (Currie and Zhao, 1993; Brusatte and Sereno, 2007; Sampson and Witmer, 2007). In UMNH VP 15001, this foramen is located on the medial side of an enlarged, dorsally oriented fossa (Figs. 1 4) that may correspond with the depression noted in the description of the braincase of Velociraptor (Norell et al., 2004) associated with the boundary between the supraoccipital and a separate epiotic. The foramen may be homologous with the one that accommodates the external occipital vein seen in birds (Baumel and Witmer, 1993). Although venous structures characteristically exhibit considerable individual variation, in both specimens of Falcarius, the foramen for the middle cerebral vein is far more anteriorly positioned than in Velociraptor (Norell et al., 2004). The development of the associated fossa apparently also reflects individual variation, because it is more poorly developed in UMNH VP than in UMNH VP The supraoccipital is fused with the exoccipital-opisthotic so its lateral extent and exact contribution to the dorsal margin of the foramen magnum cannot be determined. The foramen magnum is circular in Falcarius, Nothronychus, and Erlikosaurus (Clark et al., 1994), a character that is shared with Troodon (Currie and Zhao, 1993), but distinct from Byronosaurus (Makovicky et al., 2003), Zanabazar (Norell et al., 2009), and probably Tsaagan (Norell et al., 2006), in which it is oval with a vertical major axis. The expansion of the foramen magnum margin at this point is not as distinct as in Erlikosaurus (Clark et al., 1994). Within the cranial cavity, there is a pronounced dorsal excavation in Falcarius that corresponds with a cerebellar fossa expanding within this region of the endocranium as it passes anteriorly. This fossa is enlarged relative to the condition in Nothronychus. The medial surface is completely smooth, with no evidence of transverse ridges. In a description of a specimen of Ingenia, the endocranium exhibited vascular markings (Osmólska, 2004). She combined this characteristic with the general convex shape of the endocranial cavity below the roofing bones to support the contention that nervous tissue completely filled the endocranium in oviraptorids. Osmólska (2004) further proposed this trait as a maniraptoran synapomorphy. Witmer and Ridgely (2009) were more cautious about the conclusion that nervous tissue necessarily filled the posterior endocranium. No such vascular traces and no visible demarcation for the occipital sinus were observed in Falcarius, but the domed shape of the supraoccipital suggests that the cerebellum may have filled the endocranial cavity in posterior endocranial cavity. Exoccipital-Opisthotic Although the left exoccipitalopisthotic complex is almost complete in UMNH VP (Figs. 1 4), the right complex is broken above the base of the paroccipital process. The paroccipital process of Falcarius projects posterolaterally with a minor ventral component, reaching the level of the occipital condyle, as in dromaeosaurids (Brinkman et al., 1998; Barsbold and Osmólska, 1999). However, the paroccipital processes are not as strongly posteriorly angled

7 392 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 31, NO. 2, 2011 FIGURE 5. Stereophotographs of Nothronychus mckinleyi, partial braincase, AZMNH-2117, Upper Cretaceous Moreno Hill Formation, Zuni Basin, New Mexico, in A, posterior; B, dorsal; and C, left lateral views. Some structural identifications in Kirkland, Smith, and Wolfe, 2005, have been revised. Scale bar equals 1 cm. as in more basal tetanurans (Madsen, 1976; Allain, 2002) and ceratosaurians (Madsen and Welles, 2000; Sampson and Witmer, 2007). This orientation is also unlike the condition observed in oviraptorosaurians in which the paroccipital processes are more pendant (Clark et al., 2002), and those of dromaeosaurs, other than Tsaagan (Norell et al., 2006), in which it projects laterally from the foramen magnum. The paroccipital process of Erlikosaurus, however, converges on the condition seen in ornithomimids (Makovicky and Norell, 1998; Clark et al., 2004) and extant ostriches (Struthio) (Sanders, pers. observ.) in that it projects laterally from the level of the foramen magnum. This orientation is probably related to a transverse expansion of

8 SMITH ET AL. THE BRAINCASE OF FALCARIUS 393 FIGURE 6. Stereophotographs of Nothronychus mckinleyi, partial braincase, AZMNH-2117, Upper Cretaceous Moreno Hill Formation, Zuni Basin, New Mexico, in A, ventral; B, anterior; and C, right lateral views. Some structural identifications in Kirkland, Smith, and Wolfe, 2005, have been revised. Scale bar equals 1 cm. the basicranium and the associated skull morphology. The end of the paroccipital process is better preserved in UMNH VP than UMNH VP It possesses an anterior bend at its distal-most extent, similar to the condition in Tyrannosaurus (Brochu, 2003). However, it is not twisted to face posterodorsally in either specimen, as it is in Troodon (Currie and Zhao, 1993), Archaeopteryx (Currie, 1995), Velociraptor (Norell et al., 2004), abelisaurs, and dromaeosaurs (Sampson and Witmer, 2007). The dorsal edge of the process is weakly convex, whereas the ventral edge is weakly concave in UMNH VP These margins

9 394 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 31, NO. 2, 2011 are straight in UMNH VP A long groove runs along the dorsal margin of the paroccipital process that marks the contact with the parietal and squamosal in both specimens. As in Velociraptor (Norell et al., 2004), the base is nearly vertical. There is a roughly rhomboid distal expansion of the paroccipital process in UMNH VP 15001, as in Shuvuuia (Chiappe et al., 2002), but the process is considerably longer in Falcarius. The distal end of the paroccipital process is not expanded in UMNH VP 15000, but this morphology results from post-depositional erosion. The above combination of characters in the paroccipital process is similar to that of the ornithomimids Gallimimus (Osmólska et al., 1972), Garudimimus, andstruthiomimus (Kobayashi and Barsbold, 2005), but Falcarius lacks the distal pneumatopore on the posterior surface of the paroccipital process that appears in ornithomimids. Posterior to the paroccipital process of Falcarius, there is a shallow depression lateral to the foramen magnum, as in Velociraptor (Norell et al., 2004) and Dromaeosaurus (Currie, 1995). A poorly defined ridge separates this depression from the dorsal plate of the supraoccipital. The ridge in Velociraptor is similar, but better defined. On the anterior face of the paroccipital process is a long, concave, anteriorly directed facet that demarcates the contact with the quadrate. This facet is shallow in UMNH VP 15000, but deep in UMNH VP This variation suggests that this structure is not phylogenetically useful. It is more anteriorly oriented than that described for Zanabazar (Norell et al., 2009), in which there is a dorsal component. This facet is not present in most theropods (Norell et al., 2009). In comparison with the condition described for Troodon (Currie and Zhao, 1993), the nutrient foramina cannot confidently be identified on the quadrate facet, so the presence of a cartilaginous cap inferred this region in Troodon remains unsupported for Falcarius. In both Falcarius specimens, as in many theropods, there is a broad and shallow distal excavation on the posterior face of the paroccipital process that marks part of the origin for the m. depressor mandibulae, as in birds (Bock 1964; Baumel, 1993) and alligators (Chiasson, 1962). Proximally on the paroccipital process, a basal ridge extends from the ventral margin of the process on the posterior side, to merge with the elevated, sloping margin around the foramen magnum at a point adjacent to the base of the foramen magnum. A similar ridge is reported in Tsaagan (Norell et al., 2006) and Zanabazar (Norell et al., 2009). In Tsaagan, but not Falcarius, an accessory oblique ridge extends from the dorsal margin of the foramen magnum to the base of the paroccipital process. It may represent the base of the origin of the m. depressor mandibulae. The loss of this ridge in Falcarius may reflect some reduction in the associated craniocervical musculature. The lower basal strut, the pedicle of the exoccipital-opisthotic (Brusatte and Sereno, 2007), extends ventromedially to the dorsolateral corner of the occipital condyle, giving the condyle a dorsally concave and transversely expanded outline like that of most coelurosaurs (Clark et al., 1994; Sues, 1997; Makovicky and Norell, 1998; Barsbold and Osmólska, 1999; Kirkland and Wolfe, 2001; Xu, Norell, et al., 2002; Norell et al., 2004). The pedicle is absent in the therizinosaurid Nothronychus in which the occipital condyle is almost rectangular, with no dorsal concavity in posterior view. In the occipital condyles of the two braincase specimens of Falcarius, the exoccipital-opisthotics are fused with the basioccipital. The paroccipital process in Falcarius is invaded by at least one pneumatic diverticulum derived from the otic region and extending almost the length of the process, a condition widespread in coelurosaurian theropods (Clark et al., 1994; Makovicky and Norell, 1998; Brochu, 2003; Makovicky et al., 2003; Norell et al., 2009). The complete paroccipital process of Nothronychus is not preserved, but the base is even more extensively pneumatic and transversely inflated than in Falcarius. Below the pedicle, the exoccipital-opisthotic is deeply excavated, possessing a pronounced and highly pneumatic subcondylar recess similar to, but more extremely developed than, that in ornithomimids (Makovicky and Norell, 1998) and Tsaagan (Norell et al., 2006). The metotic strut forms the lateral wall of this excavation. On the ventral side of the paroccipital process is a flange that connects the base of the process with the crista tuberalis. It then becomes a vertically directed ridge connecting the process with the basal tuber. This architecture is typical for theropods, but the crista is deeply notched in carcharodontosaurids (Coria and Currie, 2002; Brusatte and Sereno, 2007). Unlike in Nothronychus, the crista of Falcarius is also typical for theropods in that it is neither inflated nor pneumatic. It meets the dorsal surface of the basal tuber, extending to the end, but does not contribute to it as in Majungasaurus (Sampson and Witmer, 2007). In Falcarius, the stapedial groove is on the ventral margin of the paroccipital process anterior to the metotic strut rather than on the anterior face, as in Nothronychus. The groove leads directly to the otic recess in both taxa, but is transversely broader in Nothronychus than in Falcarius. This structure is better defined in UMNH VP than UMNH VP In both specimens of Falcarius, there is an enlarged pneumatic pit contained within the opisthotic, about halfway down the length of the stapedial groove that corresponds to the posterior tympanic recess of most coelurosaurs and birds (Chiappe et al., 2002), exceptions being Troodon (Witmer, 1997b) and Saurornithoides (Norell et al., 2009). The exoccipital-opisthotic makes up the posterior margin of the middle ear of Falcarius, but due to fusion with the prootic, its anterior extent cannot be determined. Typically, however, it extends to the fenestra ovalis in theropods (Sampson and Witmer, 2007). UMNH VP possesses a moderately developed fossa on the dorsolateral face, just anterior to the paroccipital process, which corresponds to the dorsal tympanic recess that is widespread in theropods (Rauhut, 2004), including coelurosaurs (Witmer, 1997b; Brinkman et al., 1998; Norell et al., 2004; Norell et al., 2006). However, this feature is isolated and does not possess a groove connecting the dorsal tympanic recess to the otic region as noted for ornithomimids (Makovicky and Norell, 1998). The recess is very faint and shallow in UMNH VP 15000, very similar to that seen in Dromaeosaurus (Currie, 1995). The otic recess of UMNH VP is well preserved on the left side (Figs. 1 4). It is moderately depressed, generally more similar to more basal theropods (Madsen, 1976; Sampson and Witmer, 2007), rather than the deep, well-defined pit observed in derived theropods such as Sinovenator (Xu, Norell, et al., 2002), Tsaagan (Norell et al., 2006), and Troodon (Currie and Zhao, 1993). Three foramina are apparent within the otic recess of Falcarius. The two anterior foramina are identified as the fenestra ovalis (= fenestra vestibuli: Sampson and Witmer, 2007) and the fenestra pseudorotunda. They are separated by a welldefined, but very thin, crista interfenestralis and are typical in size for theropods, not exaggerated as in Byronosaurus (Makovicky et al., 2003). In UMNH VP 15000, the fenestra ovalis is nearly triangular and smaller than the more oval fenestra pseudorotunda, as in Byronosaurus and Tsaagan. In contrast, the fenestra ovalis is larger than the fenestra pseudorotunda in Sinovenator (Xu, Norell, et al., 2002). Posterior to the crista interfenestralis is an anteriorly directed fenestra pseudorotunda and behind that, a posteriorly directed metotic foramen as in Dromaeosaurus but in contrast to the condition observed in Troodon (Currie, 1995). In Troodon, there is some question as to whether the crista interfenestralis is ossified, because it is only marked by incipient ridges in the described RTMP (Currie and Zhao, 1993), whereas it is complete in Byronosaurus (Makovicky et al., 2003). In Falcarius, the crista interfenestralis, although pronounced, is located deep within the otic recess, and is markedly variable between braincase specimens, trending vertically in UMNH VP and horizontally in UMNH VP This variation indicates that

10 SMITH ET AL. THE BRAINCASE OF FALCARIUS 395 the orientation of the crista may not present a well-supported phylogenetic signal (Currie and Zhao, 1993). The jugular vein and the glossopharyngeal (IX), vagus (X), and spinal accessory (XI) nerves enter the metotic foramen at the posterior side of the otic recess primitively in theropods (Currie and Zhao, 1993; Sampson and Witmer, 2007). In many theropods, a vagal canal transmits diverted vagus and spinal accessory nerves to the occiput from the back of the metotic foramen. The glossopharyngeal nerve (IX) and jugular vein are regarded as exiting laterally out of the braincase through the metotic foramen (Currie and Zhao, 1993). This configuration has frequently been interpreted as derived for theropods, having been described in Troodon and ornithomimids (Currie and Zhao, 1993), but it has also been observed in ceratosaurs (Sampson and Witmer, 2007) and basal tetanurans (Currie and Zhao, 1993), indicating that its evolution is complex. Falcarius possesses the described derived configuration. On the endocranial wall of Falcarius is an oval, horizontally directed floccular recess (= subarcuate fossa: Clark et al., 1994) on the internal side that accommodated the flocculus. It is pronounced, taller than wide, and very similar to that of Erlikosaurus (Clark et al., 1994). In both genera, it and, presumably, the associated flocculus are much larger relative to the rest of the preserved endocranial cavity than in Majungasaurus (Sampson and Witmer, 2007). In contrast to the condition observed in most coelurosaurs (Witmer and Ridgely, 2009), there is no evidence of a distal expansion of the flocculus in Falcarius. The flocculus is involved in coordinating eye and head movements in animals that exhibit rapid bipedal running, as in birds and presumably extinct carnivorous theropods (Sampson and Witmer, 2007), such that an increase in size is associated with an increased reliance on quick head movement and eye stabilization. This reliance can, therefore, be inferred for all three of the therizinosaurians considered herein. The presence of a well-developed flocculus in carnivorous dinosaurs but not in many herbivorous dinosaurs including sauropods (Witmer et al., 2008) and ceratopsians (e.g., Zuniceratops: Smith et al., 2007; Pachyrhinosaurus: Witmer and Ridgely, 2008) may be correlated with visual reflex and tracking in carnivorous dinosaurs. Its distribution and development has been extensively studied by Witmer et al. (2009). Considering their hypothesized ecological niche, this structure is probably a retained primitive trait in therizinosaurians. The rim of the fossa extends into the endocranial cavity and occupies much of the base of the paroccipital process. It is medially bordered by a swollen vestibular eminence. This condition is very similar to the morphologies contained in the endocrania of Velociraptor, Shuvuuia (Chiappe et al., 2002), and Troodon (Currie and Zhao, 1993). Norell et al. (2004) describe this eminence as surrounding the vertical vestibular canal. The swelling of the eminence in Falcarius is more exaggerated than in many theropods, except Velociraptor (Norell et al., 2004) and birds (Chatterjee, 1991), but in Falcarius, the ventromedial expansion of the eminence is enlarged compared to the lateral border of the fossa. In Enaliornis (Galton and Martin, 2002), the expanded vestibular eminence forms a complete ring around the fossa. In Falcarius, the top of the vestibular eminence forms the base of the dorsal cerebral vein. Unlike in Velociraptor (Norell et al., 2004), there is no indication of an endolymphatic duct contained within the eminence in Falcarius or Nothronychus. A single, small foramen is apparent ventral and slightly posterior to the floccular recess on the left endocranial wall. This foramen probably corresponds to that ascribed to the vestibulocochlear nerve (VIII) by Norell et al. (2004) in Velociraptor. In the Ukhaa Tolgod ornithomimid (Makovicky and Norell, 1998), there are two distinct foramina in this region that are ascribed to separate branches of the vestibulocochlear nerve. Ventral to the base of the floccular recess lies a strut, the ventromedial extension of the otic capsule into the endocranium (Currie, 1997), that extends from the enlarged vestibular eminence bordering the fossa vertically to meet the floor of the endocranium. This architecture is similar to, but not as expanded as, the condition in Velociraptor. Basioccipital The basioccipital is fused with the surrounding bones (Figs. 1 4). The occipital condyle is broken off posterior to the pneumatic system in UMNH VP 15001, but is complete in UMNH VP The occipital condyle is narrower in Nothronychus (Figs. 4 5) than in Falcarius or Erlikosaurus (Clark et al., 1994). The basioccipital makes up most of the occipital condyle, but the exact proportion shared with the exoccipitals is difficult to determine due to fusion. However, the basioccipital is separated from the exoccipital-opisthotic in a smaller, juvenile specimen (UMNH VP 16670), in which it can be observed comprising roughly 60% of the condyle, as in Troodon (Currie and Zhao, 1993). Like most theropods (Makovicky and Norell, 1998), there is a longitudinal trench dorsal to the occipital condyle between the adjacent exoccipital-opisthotic that becomes deeper anteriorly. Generally, in small-headed theropods, the occipital condyle is smaller than the round foramen magnum (Currie and Zhao, 1993) and this condition holds in Falcarius. As noted above, it is dorsally concave and transversely broad as a result of enlarged pedicles, like in Erlikosaurus (Clark et al., 1994), and in contrast to Nothronychus. The occipital condyle possesses a distinct neck and is posteriorly oriented, with no ventral deflection. As in ornithomimids (Makovicky and Norell 1998), the condylar surface of Falcarius laps onto the ventral surface of the neck, but does not extend as far anteriorly. A ventral lip is present on the condyle in Falcarius, producing a concave ventral margin in lateral view. This lip is absent, however, in both Erlikosaurus (Clark et al., 1994) and Nothronychus. Falcarius has lateral invaginations into the neck as in Majungasaurus (Sampson and Witmer, 2007), separate from another shallow concavity on the ventrolateral corner lateral to the condylotuberal crest. This lower fossa is confluent with the subcondylar recess but apparently can be identified as the posterior condylar recess. This feature was described in Conchoraptor and regarded as a possible oviraptorid synapomorphy (Kundrát and Janáček, 2007). Falcarius possesses the dorsal elaboration of the subcondylar recess described in many theropods. It is referred to as the paracondylar recess in Majungasaurus (Sampson and Witmer, 2007), anditisalsopresentinpiatnitzkysaurus (Rauhut, 2004) and Carcharodontosaurus (Brusatte and Sereno, 2007). Such a pneumatic structure appears to have been reduced in more derived theropods, such as Troodon (Currie and Zhao, 1993) and Tsaagan (Norell et al., 2006). Within the dorsal margin of the paracondylar recess (Sampson and Witmer, 2007), a groove leads to a small, well-developed foramen that penetrates the neck of the occipital condyle. This foramen accommodated the second branch of the hypoglossal nerve (XII) (Allain, 2002). Lateral to this accessory foramen, the larger, subequal hypoglossal (XII) and vagal (X and XI) canals open into the paracondylar recess (Sampson and Witmer, 2007). The hypoglossal canal accommodated cranial nerve XII and exits ventral and slightly medial to the vagal foramen. Both foramina are directed posteriorly, as in Chirostenotes (Sues, 1997). There is a well-developed basicranial fontanelle in the basioccipital of Falcarius. It is verticalized under the occipital condyle (Sampson and Witmer, 2007), forming a basicranial fontanelle, as in Majungasaurus and many other theropods. Unlike the condition in many theropods (Madsen, 1976; Currie and Zhao, 1993, Norell et al., 2006; Sampson and Witmer, 2007), in Falcarius the posterior face does not form a smooth to gently concave anterior wall for a simple subcondylar recess. Rather, the subcondylar recess is subdivided by a descending bar, the condylotuberal crest, into right and left halves. The posterior face is more extensively invaded by the subotic recess than is typical for theropods. The basioccipital probably meets the exoccipital-opisthotic within the

11 396 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 31, NO. 2, 2011 excavation associated with the subcondylar recess as in other theropods (e.g., Madsen, 1976), but the suture is fused in Falcarius. Two deep accessory pneumatic fossae are housed within the ventral region of the subcondylar recess on either side. They are enlarged relative to the jugular foramen and vagal canal and vary from side to side and between specimens. On the left side in UMNH VP 15000, the lower one is larger than and slightly medial to the upper one. The ridges dividing the pits on the right side were apparently resorbed. Similar enlarged, asymmetric pneumatic features are known in the ornithomimids Gallimimus (Osmólska et al., 1972) and Struthiomimus (Makovicky and Norell, 1998). The condylotuberal crest extends vertically from the neck of the occipital condyle and is marked by a midline groove, which is better expressed on UMNH VP than UMNH VP This groove probably corresponds to the pit described by Currie and Zhao (1993) that they ascribed to a pneumatic diverticulum in Troodon and tyrannosaurs. However, Witmer (pers. comm., 2010), using CT data, found this pit to be non-pneumatic, at least in tyrannosaurs. There is a similar excavated crest in Piatnitzkysaurus (Rauhut, 2004), but it is much narrower in Falcarius. The condylotuberal crest is absent in Nothronychus and reduced in Erlikosaurus (Clark et al., 1994). Both therizinosaurids lack externalsubcondylar recesses. InFalcarius, the crest bifurcates ventrally to support the basal tubera. The tubera are divergent, as in Velociraptor, whereas in Dromaeosaurus,they arenearlyparallel (Norell et al., 2004). Unlike the condition in troodontids, the apex is more U - than V -shaped (Makovicky et al., 2003; Norell et al., 2009). In Chirostenotes, the condylotuberal crest does not fork at this point (Sues, 1997). Rather, the basal tubera are reduced to knobs on very short processes that are bridged by an extensive bony web that is absent in Falcarius.The crest of Falcarius divides the subcondylar recesses and is more elongate and narrower than in the majority of other coelurosaurs, with the possible exception of ornithomimids (Makovicky and Norell, 1998). This overall configuration is plesiomorphic for Falcarius (Witmer, pers. comm., 2010). The distance spanned by the basal tubera of Falcarius is about the same as the width of the occipital condyle, as in Allosaurus, but not in most other theropods, in which the distance is greater (Sampson and Witmer, 2007). No foramen between the basal tubera, such as was reported in Troodon and Saurornithoides (Norell, 2009), was observed in Falcarius, but its function may have been accommodated by the notch between the tubera, so these structures may be functionally homologous. There is a shallow pit on the posterior surface of each basal tuber similarto Velociraptor and Itemirus (Currie and Zhao, 1993) that was interpreted as the origin for the m. rectus capitus anterior in Itemirus (Kurzanov, 1976). In Nothronychus, the basal tubera, basipterygoid processes, and subcondylar recesses are apparently almost completely incorporated into the walls of the enlarged pneumatic chamber and are, therefore, highly modified. Additionally, the points of origin for the craniocervical musculature associated with the occipital plate and basioccipital are reduced in Falcarius and almost unidentifiable in Nothronychus and Erlikosaurus (Clark et al., 1994). In Nothronychus, the origins for the craniocervical muscels are reduced to roughened tuberosities located ventral to the paroccipital processes and contained within the basicranial fontanelle (Witmer, pers. comm., 2010). The remaining attachment points for the craniocervical musculature are not clear. Internally, the basioccipital of Falcarius makes up the posteromedial floor of the endocranium. The floor is smooth, lacking the longitudinal ridge, the medullar eminence, seen in some specimens of Carcharodontosaurus (Brusatte and Sereno, 2007), Bambiraptor (Burnham, 2004), dromaeosaurs (Currie, 1995), Struthiomimus, and Velociraptor (Norell et al., 2004). However, it is quite deep in Falcarius, with a concave medullary fossa, reflecting a strong pontine flexure that is similar to that in the bird Enaliornis and which has been regarded as one characteristic of some avian braincases (Galton and Martin, 2002). Basisphenoid-Parasphenoid The basisphenoid and parasphenoid are considered separately in this discussion, despite the extensive fusion between the two elements and resulting difficulty in precisely distinguishing them (Sampson and Witmer, 2007). The basisphenoid is an endochondral bone, whereas the parasphenoid is intramembranous (de Beer, 1937), and this embryological interpretation has implications for the hypothetical derivation of the derived therizinosaurian basicranium. The basisphenoid of Falcarius (Figs. 1 4) is expanded and pneumatic, but not inflated as it is in Nothronychus (Figs. 5; 6) and Erlikosaurus (Clark et al., 1994), thereby contributing to the basisphenoid bulla in the latter taxa. Its relationship with the associated bones in Falcarius is typical for theropods (Sampson and Witmer, 2007). The basipterygoid processes are posteriorly displaced, unlike at the more typical anterolateral corners of the basisphenoid recess of most theropods. The deep, ventrally open basisphenoid recess is well developed and almost typical for theropods, unlike Nothronychus (Figs. 1 6) and Erlikosaurus (Clark et al., 1994) in which it is extensively modified. In Falcarius, the recess is roughly oval, with a length about 1.2 times the width. The shape is similar to that in Shuvuuia (Chiappe et al., 2002), but Falcarius lacks the posterior deflection accommodating the expanded endocranium observed in that genus. The recess is an elongate triangle in Majungasaurus (Sanpson and Witmer, 2007). In most other theropods, including allosauroids (Madsen, 1976), ceratosaurians (Madsen and Welles, 2000), and ornithomimids, but not tyrannosaurids (Brochu, 2003), it is anteroposteriorly elongate and often oval to rectangular, with a length at least 1.5 times the width (Rauhut, 2003). However, in Tsaagan, the recess is again triangular (Norell et al., 2006). In some theropods, including Struthiomimus (Mackovicky and Norell, 1998) and Velociraptor (Norell et al., 2004), there is a dorsal longitudinal septum partially subdividing the recess that is absent in Falcarius. The ventrally directed basipterygoid processes of Falcarius are displaced posteriorly, such that the basisphenoid recess extends anteriorly some distance beyond the base of the processes, although they are short compared to most theropods (Madsen, 1976; Madsen and Welles, 2000; Norell et al., 2006; Sampson and Witmer, 2007). The basipterygoid processes extend some distance from the broadest point of the basisphenoid recess, spanning a transverse distance greater than that of the basal tubera. They lack the distal expansion observed in many theropods, but do not taper. As in troodontids (Norell et al., 2009), CT scan reveals that the basipterygoid processes of Falcarius are hollow, with a single large, camerate space occupying each process. The adjacent chambers are separated by a single thin, asymmetrical median septum. This entire configuration is very similar to that of a camerate vertebra (Wedel et al., 2000). Unlike Velociraptor, the basipterygoid processes of Falcarius are roughly at the same level as the basal tubera and are connected to them by short, thick posteromedially directed ridges (the crista ventrolateralis per Sampson and Witmer, 2007) that form the posterolateral corners of the basisphenoid recess. The ridges exhibit a slight medial excavation. As discussed previously, the basipterygoid processes are structurally absent in Nothronychus and the oviraptorid Citipati (Clark et al., 2002), but their presence in Falcarius indicates that this loss in the latter two genera is likely a convergent character. The walls of the basisphenoid recess extend anteromedially from the basipterygoid processes to meet at the base of the cultriform process. These walls are longer and much thinner than those connecting the basipterygoid processes to the basal tubera. Externally, the lateral wall is extensively pneumatized above the level of the basisphenoid recess, as in Citipati. The recess pattern varies from side to side and between UMNH VP and UMNH VP 15001, reflecting an extensive complex pattern of

12 SMITH ET AL. THE BRAINCASE OF FALCARIUS 397 bone resorption in this region (de Beer, 1937) or growth around a previously existing cavity in the soft tissue (Molnar, pers. comm. 2010). The pneumatic pattern is simpler on the left than the right side of UMNH VP 15000, so the recesses are identified on the left, following Witmer (1997b). The basipterygoid recess is located on the lateral side of the basipterygoid process, as previously discussed. It contains multiple fossae divided by thin, oblique laminae. Dorsal to the basipterygoid recess, and effectively merging with it, is a subotic recess. Unlike the laterally oriented basipterygoid recess, it is oriented posteroventrally, toward the ventral margin of the otic recess. There is a shallow pit ventral to the otic capsule that may represent a further evagination of this pneumatic chamber. On the left side, the two recesses are separated by a thin sheet of bone. On the right, all of the laminae appear to have been resorbed, resulting in a single large, deep recess with an incipient bony sheet extending partway over the posterodorsal segment. On the left side is a further, deep pocket dorsal to the subotic recess, anterior to the facial foramen, that is regarded as the prootic recess/trigeminal fossa (Witmer, 1997b), completely contained within the prootic. It is similar to those found in Velociraptor, Gallimimus, Struthiomimus, the Ukhaa Tolgod ornithomimid (Makovicky and Norell, 1998), and Troodon (Currie and Zhao, 1993). On the right side, the prootic recess is not confidently identified. This recess is often considered part of the anterior tympanic recess (Makovicky and Norell, 1998). The basipterygoid recess of Falcarius is expanded, extending into the anterior surface of the basipterygoid process from the wall of the basisphenoid as in many theropods (Chure and Madsen, 1996; Barsbold and Osmólska, 1999; Rauhut, 2004), but more exaggerated. However, on the left side, it is subdivided into a series of pits by thin laminae. This subdivision is reduced on the right side. A diverticulum probably extended dorsally to meet the prootic recess/trigeminal fossa. All of these excavations together are taken to form the anterior tympanic recess (Witmer, 1997b). The origin and significance of the basipterygoid recess is unclear. It may represent a diverticulum from a pneumatic sinus (Witmer, 1997b). Taking its proximity to the anterior tympanic recess in Allosaurus, the basipterygoid recess may be associated with that system (Chure and Madsen, 1996; Rauhut, 2004). In any case, it was used in combination with the smooth distal surface to confirm the identity of the basipterygoid processes, because they are in an unusual position. Anteriorly, what appeared to be a well-developed, very large carotid canal through the pituitary fossa (Currie and Zhao, 1993) in the holotype specimen of Falcarius was revealed by CT scan to be a blind pneumatic cavity. The basisphenoid makes up most of the floor of the endocranium, which is smooth and deeply excavated. A transverse ridge is present on the lateral wall ventral to the floccular recess, likely marking the division between the pons and the medulla. It is not possible to distinguish the parasphenoid from the basisphenoid, except in general terms (Witmer, pers. comm., 2010). It has been described as an intramembraneous bone with three centers of ossification in Gallus (de Beer, 1937). Typically, there is a median cultriform process with two lateral wings fused to the endochondrally derived basisphenoid. The posterior base of the parasphenoid is partially preserved in UMNH VP It extends anteriorly from what is interpreted as a partial subsellar recess. The preserved portion probably represents the base of an anteriorly directed cultriform process. It is broken anteroventral to the trigeminal foramen and posterior to the hypophyseal fossa. This bone is pneumatized, with two basal chambers and one medial chamber ventral to the base of the endocranium. It is, however, unknown if the anterior parasphenoid is inflated, as in ornithomimids (Holtz, 2000) and troodontids (Currie, 1985), because this region is not preserved in either Falcarius specimen. Dorsolateral to the medial chamber is a small foramen that probably accommodated the internal carotid artery. Prootic The prootic is only partially preserved in both specimens of Falcarius. It forms the lateral side of the braincase and the pontine floor anterior to the exoccipital-opisthotic and dorsal to the basisphenoid, but the sutures are fused so its extent can only be estimated (Figs. 1 4). The otic structures in Nothronychus are resolved. What was identified as the abducens foramen (VI) in Nothronychus (Kirkland, Smith, et al., 2005) is probably actually the trigeminal (maxillomandibular branch) nerve (V 2 3 ) exit, corresponding with Falcarius, an identification that should be resolved with CT data. Thus, a distinct abducens foramen was not identified in either Falcarius or Nothronychus. The lateral wall contains, in part, a complex of pneumatic pits. As in the basisphenoid, the exact pattern varies from side to side. This element forms the anterior margin of the otic recess, typically meeting the exoccipital-opisthotic within the otic capsule (Currie, 1997). The prootic of Falcarius is posteriorly fused with the opisthotic, so its posterior extent is unclear, but there is no reason to believe that it is different from other theropods. The otosphenoidal crest makes up the dorsal and anterior margins of the prootic recess as in Dromaeosaurus (Norell et al., 2004). The lateral side of the prootic anterior to the otosphenoidal crest is characterized by pneumatic pits, constituting the prootic recess/trigeminal fossa, that are probably an extension of the anterior tympanic recess as a dorsally directed diverticulum from the region of the basisiphenoid. This pattern is observed in extant alligators (Witmer et al., 2008, Dufeau and Witmer, 2008). Endocranium and Associated Structures In many respects, thebraincase of Falcarius follows a typical theropod architecture, but the basicranial pneumatic system is expanded (Figs. 1 6), resulting in some degree of internal distortion and alteration of the endocranium and associated structures. The endocrania of most theropods (e.g., Hopson, 1979; Rogers, 1999; Brochu, 2003; Sanders and Smith, 2005; Sampson and Witmer, 2007; Witmer and Ridgely, 2009) are serially arranged, with shallow pontine and cephalic flexures. InFalcarius, however, the endocranium has an unusually flexed aspect in lateral view. The hypophyseal fossa and infundibulum are both visible in the CT data of UMNH VP (Fig. 7). These structures are also visible in the broken specimen of Nothronychus (AZMNH 2117) In Falcarius, the infundibulum is oriented anteriorly. The preserved portion of the proximal end of the optic chiasma (II) is also visible as a short, anteriorly directed canal on either side of the hypophyseal fossa. Lateral to the optic chiasma and tract is a small canal that probably transmitted both the oculomotor (III) and abducens (VI) nerves, referred to as the oculomotoabducens canal (Fig. 7). Posterior to this region, at the level of the inferred posterior pituitary, CT scans reveal a large space, a possible trigeminal ganglion cavity ventrolateral to the hypophyseal fossa. However, Witmer and Ridgely (2009) noted that this structure is more typically contained within the endocranial cavity in tetanuran theropods. In some theropods, including tyrannosaurs, troodontids, and birds, two discrete foramina transmit branches of the trigeminal nerve (Currie, 1995; Norell et al., 2004; Witmer et al., 2008) and CT data reveal that that isthecaseinfalcarius. Passing anteriorly, running parallel to cranial nerves II, III, and VI, the ophthalmic (V 1 ) and maxillomandibular (V 2 3 ) branches extend from the ganglion. The ophthalmic branch is between the hypophysis and the oculomotoabducens canal. The maxillomandibular branch is lateral to the canal. CT data reveal two possible candidates for the facial (VII) nerve canal. The most likely candidate is a small canal exiting the braincase immediately adjacent to the prootic recess and within the prootic. The presence of the facial foramen within the prootic is widespread in coelurosaurs, for example Troodon (Currie, 1995) and Archaeopteryx (Whetstone, 1983). The vagus nerve passes into the metotic canal, with the glossopharyngeal (IX) and spinal accessory (XI) nerves, but all are deflected posteriorly to the occiput, where they exit the braincase lateral to

13 398 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 31, NO. 2, 2011 FIGURE 7. Falcarius utahensis, UMNH VP 15001, Lower Cretaceous Cedar Mountain Formation, Crystal Geyser Site, Utah. A, braincase in left lateral view; B D, selected coronal CT slices located as indicated in A. Data courtesy of Larry Witmer and Ryan Ridgely. Scale bars equal 1 cm. the occipital condyle, as is typical for derived theropods (Currie and Zhao, 1993; Sampson and Witmer, 2007). All of these posterior cranial nerves travel behind the basicranial pneumatic system. Inner Ear In both specimens of Falcarius, the morphology of the inner ear (Fig. 8) was extracted using CT data from the University of Utah data set, resulting in a preliminary reconstruction. UMNH VP has a nearly complete inner ear, whereas it is only partially preserved in UMNH VP 15000, in which only the cochlear duct and horizontal semicircular canal are present. In general, Falcarius possesses a mosaic of primitive and derived inner ear morphologies relative to those of more basal theropods (Rogers, 1999; Sanders and Smith, 2005; Sampson and Witmer, 2007), but the pattern of inner ear evolution is complex, with abundant homoplasy in different characters (Witmer, pers. comm., 2009). Theropods are characterized by having longer anterior semicircular canals than horizontal or posterior (Sampson and Witmer, 2007), and this is true in Falcarius. The anterior semicircular canal of Falcarius is long relative to basal theropods, spanning 15.7 mm, but not as exaggerated as in more derived forms. It does not extend far behind the common crus, so it is only slightly twisted, more similar to tyrannosaurs than other coelurosaurs, such as Struthiomimus (Witmer and Ridgely, 2009). Additionally, in Falcarius the posterior semicircular canal, spanning 10.6 mm, does not extend much below the horizontal semicircular canal, as in Majungasaurus (Sampson and Witmer,