Tyrannosaurus rex Osborn 1905

The primary specimen used in this study was CMNH 7541, a nearly complete skull of a probably juvenile tyrannosaurid, collected from the Hell Creek Formation of Carter County, Montana (see Gilmore, 1946, for collection details and geological setting). A variety of other tyrannosaurid specimens were studied in connection with this project (for a full listing, see Witmer and Ridgely, 2009). In addition to general observation, the major tool used in this study was CT scanning (computed x-ray tomography), followed by 3D analysis and visualization. CMNH 7541 has been CT scanned no fewer than three times. The first scans were done in 1990 on a medical scanner at a Toronto hospital, in the presence of Michael Williams, Andrew Leitch, and Robert T. Bakker (Bakker, 1992). Scanning details have not been published, but results were presented in a popular article by Bakker (1992), who reconstructed the brain cavity and, within the nasal cavity, an elongate turbinate. The same CT data were used later by J. A. Ruben to refute the finding of respiratory turbinates in this skull (Ruben, 1996; Ruben et al., 1996, 1997). The Cleveland skull was scanned for the second time in 1998 at the Boeing Rocketdyne CT Lab in California under the auspices of the Cleveland Museum of Natural History with funding from the Field Museum of Natural History. The skull was scanned at a slice thickness of 1 mm with slices in the horizontal plane, yielding 300 slices; voxel sizes were 0.4042 X 0.4042 X 1.0 mm. As part of the collaboration with M. Williams, the Cleveland Museum of Natural History provided Witmer with the full 16-bit dataset from these scans. In hopes of revealing aspects of anatomy that are unclear in the Boeing dataset, the skull was scanned for the third time in 2005 at O’Bleness Memorial Hospital, Athens, Ohio, on a General Electric LightSpeed Ultra MultiSlice CT scanner equipped with the Extended Hounsfield option, which enhances the ability to resolve details from dense objects such as fossils. Because the Boeing dataset was derived from scans of the skull in the horizontal plane, the x-rays had to pass through the full length of the skull, which can produce artifacts. Thus, we scanned the skull axially, which minimized the amount of material the x-rays had to penetrate. The full skull was scanned helically with a slice thickness of 1.25 mm at 140 kV and 300 mA, yielding 483 slices and voxel sizes of 0.082 X 0.082 X 1.25 mm. The skull was then scanned again, this time focusing on the braincase, tightening the field of view and extending from the caudal end of the skull through the orbits and ending rostrally just in front of the lacrimals’ jugal processes; slice thickness was 625 microns (0.625 mm) at 140 kV and 170 mA, yielding 385 slices and voxel sizes of 0.049 X 0.049 X 0.625 mm. No CT scan is free from artifact, and both the Boeing and Athens datasets have their advantages and disadvantages. Ultimately, much of the work presented here is based on a composite ‘‘superdataset’’ created by registering and combining the Boeing and multiple Athens datasets. Viewing, analysis, and visualization of the scan data were done using Amira 3.1.1 and 4.2 (Mercury-TGS, Chelmsford, MA) on 32- and 64-bit Windows XP PCs equipped with nVidia Quadro FX graphics cards and 2–8 GB of RAM. Structures of interest were highlighted (segmented) using Amira’s segmentation tools and visualized either in isolation or combined with other structures or the whole skull. Both surfaces and volumes were generated, and these served as the basis for most of the illustrations in this article. Additional information, such as interactive movies and 3D PDFs, are available on the authors’ website: www.ohio.edu/witmerlab. Museum abbreviations used in this paper are: AMNH, American Museum of Natural History, New York City, New York; BMR, Burpee Museum of Natural History, Rockford, Illinois; CMNH, Cleveland Museum of Natural History, Cleveland, Ohio; FMNH, Field Museum of Natural History, Chicago, Illinois; MOR, Museum of the Rockies, Bozeman, Montana; OMNH, Oklahoma Museum of Natural History, Norman, Oklahoma; ROM, Royal Ontario Museum, Toronto, Ontario; and TMP, Royal Tyrrell Museum of Palaeontology, Drumheller, Alberta. Results CMNH 7541 has been the subject of at least three major anatomical studies (Gilmore, 1946; Bakker et al., 1988; Carr, 1999), and thus there is no need for in-depth anatomical description here. Instead, our intent is to focus on a few areas that have not been adequately discussed in print and that are revealed by the CT scan data. It is also our intent to capitalize on the new-found visualization capabilities provided by the new imaging technologies and software, and so we emphasize illustration over text (in the spirit of the old adage of a picture being worth a thousand words). General attributes Figures 1–3 provide stereophotographs of the skull in several views, showing not just bony structure but also the amount of matrix remaining in the skull and, to a certain extent, the amount of plaster restoration. Plaster is relatively low density and homogenous and is easily identified and removed in volume renderings of the CT data. Figure 4 presents views of the skull with the plaster (and some regions of thin matrix) dropped out. Examination of Figure 4 reveals that the snout had been fractured along an oblique transverse plane running caudodorsally to rostroventrally (arrows in Figure 4C) such that, when repaired and restored in plaster (quite expertly, it may be said), much of the central parts of the nasal and antorbital cavities wound up being largely plaster. Braincase The braincase of CMNH 7541 is visible in dorsal, caudal, and ventral views in the actual (physical) specimen (Figures 2 D–E, 3A–C), but the other surfaces are obscured by matrix and other bones. To more easily compare the braincase to those of other theropods, the braincase of CMNH 7541 was digitally ‘‘disarticulated’’ along sutures and then extracted so that it could be viewed in isolation (Figures 5–8). The general conformation of the braincase is fairly similar to those of other tyrannosaurs in having a well developed otosphenoidal crest (= crista prootica) running from the otoccipital bone caudally and arching rostrally and then ventrally on the prootic, laterosphenoid, and basisphenoid bones (Figure 6B). Tucked ventrally or caudoventrally below the margin of the otosphenoidal crest, from caudal to rostral, are the caudal tympanic recess aperture, columellar recess (with the columella [= stapes] in place in the fenestra vestibuli), the maxillomandibular and facial foramina (for CN V 2–3 and CN VII, respectively), the rostral tympanic recess, and cerebral carotid foramen. As in other tyrannosaurids, the facial foramen opens so close to the maxillomandibular foramen that the two essentially share an aperture laterally (Witmer et al., 2008; Witmer and Ridgely, 2009; Brusatte et al., 2009). As in other tyrannosaurids, perhaps all coelurosaurs, and many tetanurans (Sampson and Witmer, 2007; Witmer and Ridgely, 2009), the ophthalmic nerve (CN V 1) exits the laterosphenoid rostrally via its own foramen. Another attribute shared with other tyrannosaurids is the almost complete ossification of the front of the braincase, with orbitosphenoid and sphenethmoid ossifications contacting their fellows in the midline and enclosing foramina for the olfactory bulbs and tracts, optic nerves (CN II), oculomotor nerves (CN III), and abducens nerves (CN VI) (Figures 6, 8). The sphenethmoid is particularly significant because it preserves details relating to the olfactory apparatus. The sphenethmoid itself is divided by a midline osseous septum (mesethmoid), which in life would have separated the rostral terminations of the olfactory tracts (i.e., the olfactory bulbs), as described for other theropods (Sampson and Witmer, 2007; Ali et al., 2008; Witmer and Ridgely, 2009). Lateral to the region of the olfactory bulbs are a series of thin bony laminae descending from the roof of the sphenethmoid and possibly also the frontal (Figures 6A, B; 7B, C; 8). These laminae are external to the neural domain and would be within the nasal cavity, and thus these laminae are best interpreted as olfactory turbinates that would have supported the sensory olfactory epithelium, as observed in extant taxa. The olfactory apparatus indeed seems well developed in CMNH 7541, which is consistent with its large olfactory bulbs, as well as with the large bulbs and expansive nasal olfactory regions seen in other tyrannosaurs (Witmer et al., 2008; Witmer and Ridgely, 2009). CMNH 7541 shares with other tyrannosauroids modestlysized but widely-spaced basal tubera (Bakker et al., 1988; Carr, 1999; Currie et al., 2003; Li et al., 2010), yet it retains strong basipterygoid processes. Likewise, all tyrannosaurids, including CMNH 7541, share extensive pneumaticity associated with the middle ear and pharynx (rostral and caudal tympanic recesses, basisphenoid recesses, subcondylar recess, subsellar recess; Witmer, 1997b; Witmer and Ridgely, 2009; see below). Despite the similarities with other tyrannosaurids noted above, CMNH 7541 displays a number of noteworthy differences. For example, the cultriform process (parasphenoid rostrum) is relatively low and straight (Figures 5A, 6A), rather than, as in most other tyrannosaurids (Russell, 1970; Carr, 1999; Brochu, 2003; Currie, 2003b), having a strongly arched ventral margin that sweeps dorsally before leveling off parallel to the frontals. This may relate to CMNH 7541 having a relatively smaller subsellar recess (a ventral pneumatic chamber in the base of the cultriform process; Figures 6B, 9) relative to other tyrannosaurids. Among tyrannosaurids, the conformation of the cultriform process and subsellar recess is most similar to that of Gorgosaurus (Witmer and Ridgely, 2009) and Alioramus (Brusatte et al., 2009). Another difference is that CMNH 7541 has a

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