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Home , Acrocanthosaurus, Carcharodontosaurus, Endocast, Tyrannosaurus

Cranial of the Theropod atokensis Author(s): Jonathan Franzosa and Timothy Rowe Source: Journal of , Vol. 25, No. 4 (Dec. 30, 2005), pp. 859-864 Published by: The Society of Vertebrate Paleontology Stable URL: http://www.jstor.org/stable/4524512 Accessed: 12/04/2010 21:03

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860 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 25, NO. 4, 2005

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FIGURE 1. A, Rostraland B, rightlateral views of the braincaseof the of Acrocanthosaurusatokensis, OMNH 10146.This is the braincase fromwhich the digitalendocast was constructed.Abbreviations: f.o., fenestraovalis; i.c., internalcarotid canal; pit., pituitaryfossa; I, olfactorynerve foramen;II, optic nerve foramen;III, oculomotornerve foramen,IV, trochlearnerve foramen;V, trigeminalnerve foramen;VI, abducensnerve foramen;VIIH, hyomandibularbranch of the facial nerve foramen;VII,, palatinebranch of the facial nerve foramen.

and reconstruction of 900. A ring-free correction filter was endocast are terms meant for the actual , in keeping with later applied to each slice, to reduce ring artifact. tradition (e.g. Jerison, 1973; Hopson, 1979). Although we realize To facilitate segmentation of the endocast as a subset of the that an endocast does not completely and faithfully reproduce total volume, the dataset was digitally re-sliced into 277 coronal the , the use of brain terms to describe the endocast dis- slices extending from the tip of the frontals to the back end of the courages the creation of new terms for endocast features that can (with the same slice thickness and spacing as the original be described more easily by the terms that were created for the dataset) in Scion Image Beta 4.0.2 (Scion Corp., Frederick, feature on an actual brain. ). The slices were processed for viewing in Adobe Pho- The endocast of Acrocanthosaurus atokensis measures 14.88 toshop 6.0 (Adobe Systems, Inc., San Jose, California). Con- cm from the front of the olfactory bulbs to the foramen magnum, struction of the endocast was done manually, by outlining the and has a volume of 190.8 cm3 (Fig. 2). The rostralmost portion endocranial and in cavity nerve passageways each slice, then of the endocast represents the olfactory bulbs and tracts (CN I). all contained a color value assigning pixels unique (Franzosa, The olfactory bulbs are swollen and distinctive, evidently filling The endocranial volume thus labeled in the dataset was 2001). the rostral endocranial fossa (ethmoid fossa), which is enclosed volume-rendered and isosurfaced an in-house using isosurfacing dorsally by the frontals, and ventrally and laterally by the mes- to a endocast whose surfaces and vol- program generate digital ethmoid, which also a median the ume could be measured. supplies septum separating two bulbs. This cavity lacks a rostral wall, and the endocast is therefore constrained rostrally by the rostral extent of the por- DESCRIPTION tion of the mesethmoid that forms the ventrolateral limit of the bulb and tract. As the bulb and For the following description of the endocast, the terminology olfactory delimited, olfactory tract measure 6.6 cm is predominately that of Nomina Anatomica Avium (Baumel et long. The tracts toward the cerebral al., 1993). We acknowledge that the terms used to describe the olfactory project hemispheres as separate bilateral tracts that merge along the midline caudal to the median septum before joining the cerebral hemispheres. A TABLE 1. web resourcesavailable at: Ancillary www.DigiMorph.org/ comparable septum also occurs in and Gi- specimens/acrocanthosaurus_atokensis/ ganotosaurus (Larsson, 2001; Coria and Currie, 2002). In Allo- Web resourcesavailable saurus and , there is no evidence that the olfactory tract was separated by the mesethmoid (Madsen, 1976; Currie and Transverseslice movie Zhao, 1993), but it is unclear whether this is an artifact of pres- Sagittalslice movie Horizontalslice movie ervation. The tract meets the cerebral hemispheres in a transition Transversevolumetric rendering of braincase involving the ventral and lateral expansion of the forebrain. The Sagittalvolumetric rendering of braincase cerebral hemispheres reach their maximum width (4.35 cm) dor- Horizontalvolumetric rendering of braincase sal to the trigeminal (CN V) and facial (CN VII) nerves. 3-D model of endocast A longitudinal ridge on the endocast overlies the cerebral Inspectorapplet and the sinus dorsalis. The Cutawaymovies of braincase hemispheres represents sagittalis right and left optic nerves (CN II) project to the cerebral hemi-

FRANZOSA AND ROWE-A CROCANTHOSA UR US ENDOCAST 863 and hypoglossal (CN XII) nerves. The foramina for each of these ganotosaurus, but not in or Sinraptor, help to nerves are found in a deep pit in the exoccipitals of the braincase. strengthen the postulated relationships. Several other examples On the left side of the braincase is a third foramen, not found on such as this can be found throughout the description above. the right side, that is probably either a separate exit for the Second, the external shape of the endocast can tell us several glossopharyngeal nerve (CN IX), or possibly for the external things. The brain is only slightly sigmoidal, with very little ex- ophthalmic artery and vein. The accessory nerve (CN XI) is not pansion of the cerebral hemispheres, and the optic lobes are still distinct, and probably exited the braincase with the vagus nerve, in a dorsomedial position. This general shape more closely re- as did the glossopharyngeal nerve, if not through the third fora- sembles what is seen in than , and this same men found on the left side of the braincase. Just caudal to these general appearance is seen in most early theropods (pers. obs.). nerves is the foramen magnum, marking the end of the medulla This is important because even though Acrocanthosaurus was a oblongata, and of the endocast. The diameter of the foramen theropod, its brain still retains a condition that is more similar to magnum is 35 mm transversely, and 27 mm vertically (Stovall that of ancestral than that of modern theropods and Langston, 1950). (birds). The endocast, by preserving the semicircular canals, There are several major features of the brain that cannot be shows the proper orientation of the head. Most previous studies discerned in the endocast, the most important of which are the of theropods treat the orientation of the head as if the frontals optic lobes. Based on the shape of the endocast, the optic lobes were habitually held horizontally. Recent analyses of in Acrocanthosaurus probably had not yet obtained the ventro- (Witmer et al., 2002, 2003) have re-emphasized that lateral position seen in modern birds, and due to the venous the horizontal semicircular canal is a more accurate indicator of sinuses in the braincase, the optic lobes did not leave any im- habitual head orientation, and that it doesn't always afford the pressions on the inner walls. They were most likely positioned same perspective as the frontals. In Acrocanthosaurus, the hori- more dorsally and caudal to the cerebral hemispheres, more like zontal semicircular canal is in a horizontal position when the what is seen in modern crocodilians. Also indiscernible in the rostral portion of the skull is rotated ventrally approximately 25 endocast is the main body of the cerebellum. Some of the lateral degrees (Franzosa, 2004). This rotation suggests that rather than margins can be seen, and the general position of the main body habitually looking straight ahead, the correct orientation of the can be inferred based on the position of the floccular lobes, but, skull would have the habitually looking somewhat down just as with the optic lobes, a large portion of it is covered with in front of itself. venous sinuses. While the venous sinuses and the internal carotid Lastly, having endocasts allows quantitative comparisons to be arteries are visible, the rest of the vasculature of the braincase is made between different theropods. As was mentioned previ- indiscernible. ously, along with Acrocanthosaurus, digital endocasts have been created for (Brochu, 2000), Carcharodontosaurus ORIENTATION OF THE HEAD (Larsson et al., 2000; Larsson, 2001), and also and Allosaurus for dissertation 2004). The datasets The scan of this was such that the my (Franzosa, original specimen olfactory can be used as a source of characters, as was mentioned before, bulbs and tracts were in a horizontal While there is position. and these can be used not for total-theropod-tree phyloge- with this it is not in with the only nothing wrong convention, keeping netic but can also be used in smaller analyses, such as actual orientation of the head the lifetime of the animal. analyses, during that Larsson et al. (2001). Such analyses are the With the anterior semicircular canals rather than performed by horizontal, basis of in As more endocasts as extend the other canals are manuscripts currently preparation. sloping dorsally, they posteriorly, become available, these will become more meaningful oriented et Rotat- analyses similarly incorrectly (Witmer al., 2002, 2003). as the taxa increase in number. It will also allow the rostral end of the endocast 25 represented ing ventrally approximately some of the gaps that are currently seen in the of the degrees (Fig. 2) places the semicircular canals in an orientation brain to be filled. similar to that seen in Carcharodontosaurus and Allosaurus. The theropod of the canals is also similar in all three taxa. physical appearance ACKNOWLEDGMENTS The anterior semicircular canal extends posterodorsally, lateral of the semicir- to the floccular lobe. The anterior edge posterior We thank Richard Ketcham, Matthew Colbert, and Jessie cular canal near the of the anterior semi- begins posterior edge Maisano for scanning and processing the original Acrocantho- circular canal, and extends posteroventrally. The horizontal saurus dataset. We would also like to thank Richard Cifelli for semicircular canal ventral to the anterior of the an- begins edge loan of the and Wann Langston for allowing us access terior semicircular and extends ventral to specimen, canal, posteriorly just to the Lastly, we would like to thank the vertebrate the floccular lobe. After the of the specimen. extending past posterior edge students at The University of at floccular the horizontal semicircular canal meets the paleontology graduate lobe, pos- Austin for all their helpful comments, discussions, and sugges- terior semicircular one continuous canal in the canal, forming tions. Scanning was funded by NSF grants IIS-0208675 and IIS- endocast, although the two canals were probably not continuous 9874781. like this in life. This orientation gives the same subtriangular shape of the semicircular canals described for Carcharodonto- CITED saurus and Allosaurus. LITERATURE Baumel,J. J., A. S. King, J. E. Breazile,H. E. Evans, and J. C. Vanden DISCUSSION Berge. 1993. Handbook of Avian Anatomy: Nomina Anatomica Publicationsof the Nuttall an endocast of Acrocanthosaurus allows several ques- Avium, Second Edition. Ornithological Having No. 779 tions to be addressed and answered. First, the braincase and Club, 23, ,MA, pp. A. 2000. A endocast for Tyrannosaurus be used to corrobo- Brochu, C. digitally-rendered endocast can give us characters that can help rex. Journalof VertebratePaleontology 20:1-6. rate or discredit ideas about the of Acrocanthosau- relationship Carlson,W. D., T. Rowe, R. A. Ketcham,and M. W. Colbert. 2003. to other taxa. As was mentioned Acrocanthosaurus rus above, Geological applicationsof high-resolutionX-ray computedtomog- has been placed as the to Carcharodontosaurus sa- raphy in petrology, meteoriticsand palaeontology;pp. 7-22 in F. haricus by Sereno et al. (1996), forming the monophyletic clade Mees, R. Swennen,M. Van Geet, and P. Jacobs(eds.), Applications , with . Characters such of X-rayComputed Tomography in the Geosciences215. Geological as the division of the olfactory bulbs and tracts by a mesethmoid, Society, London. which occurs in Acrocanthosaurus. Carcharodontosaurs, and Gi- Conroy,G. C., and M. W. Vannier.1984. Noninvasive three-dimensional 864 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 25, NO. 4, 2005

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