ON THE ORBIT OF THEROPOD DINOSAURS

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1 GAIA N 15, lisboallisbon, DEZEMBRO/DECEMBER 1998, pp (ISSN: ) ON THE ORBIT OF THEROPOD DINOSAURS Daniel J. CHURE Dinosaur National Monument. Box 128, JENSEN, UT USA dan_chure@nps.gov ABSTRACT: Primitively, theropod orbits are roughly circular in outline and this pattern is retained in most theropods, Large-headed theropods show a much greater diversity in the shape of the orbit, ranging from strongly elliptical to keyhole shaped, to a near complete division of the orbit at mid-height by projections of the postorbital, lacrimal, or both. Orbit shape is not congruent with current theropod phylogenies. The functional and biological significance ofthese diverse orbital shapes in large-headed theropods remains unknown. INTRODUCTION Theropod dinosaurs have long captured the imagination of the public and paleontologists, and there has been much speculation aboutlheir biology (BAKKER, 1986; PAUL, 1988), some even identified as such (FARLOW, 1976). While the visual system has been the subject of relatively little speculation, there have been claims of binocular vision in Tyrannosaurus and Nanotyrannus (PAUL, 1988). However, overlapping visual fields do not necessarily imply stereopsis (MOLNAR & FARLOW, 1990; MOL NAR, 1991). Cranial morphology tells us pitifully little about the visual system of theropods. However, there is a striking range of size and shape in the orbits of theropods, and this diversity presumably has some biological andlor functional significance. DESCRIPTION In primitive theropods, such as Coelophysis bauri (COLBERT, 1989), Eoraptor lunensis (SERENO et al., 1993), Herrerasaurus ischigualastensis (SER ENO & NOVAS, 1993), Syntarsus rhodesiensis (COL BERT, 1989), and S. kayentakayae (ROWE, 1989) the orbit is large and roughly circular (Fig. 1A). This condition is retained in many coelurosaurs, such as Omitholestes (OSBORN, 1903A), Compsognathus (OSTROM, 1978), ornithomimids, oviraptorids, dromaeosaurids, therezinosaurids, troodontids, and most tyrannosaurids (Albertosaurus libratus Rus SELL, 1970, Oaspletosaurus torosus RUSSELL, 1970, and Nannotyrannus lancensis BAKKER, WIL LIAMS & CURRIE, 1988). While sclerotic rings are not well known in theropods, they are known in Herrerasaurus ischigualastensis (SERENO & NOVAS, 1993), Syntarsus kayentakayae (ROWE, 1989), and the ornithomimid Struthiomimus samueli (PARKS, 1928) and the size of these rings strongly suggests that the eye occupied all or nearly all of the circular orbit. This is the primitive and most widespread condition of the orbit and eye in theropods and many other amniotes. However, unusual orbital shapes do occur in theropods with large skulls. In the most extreme shape the orbit is nearly divided into a dorsal and ventral component. This constriction is usually caused by an anterior projection of the postorbital, as in Abelisaurus comahuensis (BONAPARTE & No VAS, 1985), Carcharodontosaurus saharicus (SER ENO et al., 1996), Camotaurus sastrei (BONAPARTE, 1985), and Tyrannosaurus rex (OSBORN, 1912) (Fig. 1 J-N). The condition is ontogenetically variable to some extent in Tyrannosaurus bataar. In the type, PIN (MALEEV, 1974: fig. 48) there is a postorbital projection into the orbit. The smaller, referred skulls (PIN and 553-1) show a smaller postorbital projection (CARPENTER, 1992). In Acrocanthosaurus atokensis (STOVALL & LANGSTON 1950) the constriction is due to both a posterior projection of the lacrimal and an anterior projection ofthe postorbital (ANONYMOUS, 1994) (Fig. 1 L). In theropods where the orbit is constricted the part for the eye is dorsal and the smaller of the two spaces (with the possible exception of Tyrannosaurus bataar), making these theropods beady-eyed killers. Sinraptor dongi (CURRIE & ZHAO, 1993) (Fig. 1 F) has a small projection from both the lacrimal and the postorbital, but the orbit is not constricted anywhere near to the degree seen in Acrocanthosaurus. A number of large-headed theropods show conditions intermediate between the circular and constricted orbital shapes. The simplest of these is a vertically elongated orbit, as in Alioramus remotus (KURZANOV, 1976), Ceratosaurus nasicomis (GI L MORE, 1920), Torvosaurus tanneri (BRITT, 1991), Yangchuanosaurus shangyuensis (DONG, ZHAO & ZHANG, 1983) (Fig. 1 C-E). Where the eye would be 233 artigos/papers

2 D.CHURE N Fig. 1 - Left orbits and circumorbital bones of selected theropods discussed in text. All drawn with orbits to same vertical height to show proportional differences, rostral to left. Circumorbital bones: J = jugal; L = lacrimal; PO = postorbital. A - Eoraptor lunesis (after SERENO et al., 1993, reversed). B - Nanotyrannus lancensis (after BAKKER, WILLIAMS, & CURRIE, 1988). C - Ceratosaurus nasicornis (after GILMORE, 1920, reversed). 0 - Torvosaurus tanneri(after BRITT, 1991). E - Yangchuanosaurus shangyuensis (after DONG, ZHAO & ZHANG, 1983, reversed). F - Sinraptordongi (after CURRIE & ZHAO, 1993). G -Allosaurus n. sp., DINO H - Monolophosaurusjiangi (after ZHAO & CURRIE, 1993).1- Cryolophosaurus ellioti (after HAMMER & HICKERSON, 1994, reversed). J - Carcharodontosaurus saharicus (after SERENO et al., 1996). K - Tyrannosaurus rex (after OSBORN, 1912). L - Acrocanthosaurus atokensis (after ANONYMOUS, 1994). M - Carnotaurus sastrei (after BONAPARTE, NOVAS & CORIA, 1990). N - Abelisaurus comahuensis (after BONAPARTE & NOVAS, 1985). 234

3 ON THE ORBIT OF THEROPOD DINOSAURS and the size olthe eye can not be easily determined in these forms. In Cryolophosaurus ellioti (HAMMER & HICKERSON, 1994) (Fig. 11) the upper third of the orbit is circular and the ventral two-thirds is elongate and tapering and the eye would presumably be in the circular part. Monolophosaurus jiangi (ZHAO & CUR RIE, 1993) (Fig. 1 H) has a large circular orbit with a short tapering ventral part. Presumably the eye in Monolophosaurus was very large. Two new and undescribed specimens of Allosaurus show a condition intermediate between Sinraptor dongi and those forms with elliptical orbits. The first, MOR 693, is a nearly complete skeleton with a superb skull from the Brushy Basin Member of the Morrison Formation near Shell Wyoming. The second of these, DINO 11541, is a new species of Allosaurus (CHURE, in prep.) from the Salt Wash Member of the Morrison Formation in Dinosaur National Monument. The orbital shape in Allosaurus is somewhat variable. It is always elliptical in shape, but in MOR 693 (Fig. 2B) and AMNH 600 (OSBORN, 1903b) the ventral edge is rounded, in DINO (Fig. 1G) it is flat, and in DINO 2560 (the basis forthe skull restoration in MADSEN, 1976) it has a short tapering ventral margin. However, in the latter specimen there is crushing in the orbital region and the shape may be more elliptical than it appears. The postorbital is concave anteriorly and does not project into the orbit in Allosaurus. However in MOR693 and DINO there isa short projection from the posterodorsal margin of the lacrimal into the orbit (Fig. 2). This projection is slightly more pronounced in MOR 693. This projection probably marks the anteroventral margin olthat part of the orbit occupied by the eye. Parts of sclerotic rings were found in the left orbit of both MOR 693 and DINO In MOR 693 the sclerotic ring is collapsed upon itself as a jumble of plates. In DINO the sclerotic ring is only partly visible (eight articulated plates) in the posterodorsal corner of the orbit (Fig. 2A). Preservation is such that it is difficult to determine the pattern of plate overlap. Nevertheless, in both specimens the sclerotic plates are restricted to the dorsal part of the orbit and in DINO the half or one-quarter circle of plates preserved indicates that the eye could fit within the area of the orbit delineated by the lacrimal projection. In birds, the Ligamentum suborbitale is a thin fasciailligamentous band which stretches from the lacrimal to the postorbital process and participates in forming the ventrolateral wall of the orbit (BAUMEL & RAIKOW, 1993: 150, fig. 5.1A). Lacrimal and postorbital processes in theropods are probably manifestations of this ligament in theropods. DISCUSSION As stated above, the primitive, and most common orbit shape in theropods is large and circular. Theropods with large skulls exhibit a much wider range of orbil shapes than small headed-theropods. These large-headed theropods do not form a monophyletic group. SERENoetal. (1994, 1996)dividethe basal tetanurans (i.e. non-coelurosaurian tetanurans) into two major clades, the Spinosauroidea and the Allosauroidea. HOLTZ (1994) has three distinct clades of basal tetanurans, only one of which is named (Allosauridae). CURRIE (1995) unites all basal tetanurans into a single clade, the Carnosauria. _ In addition, CURRIE (1995) incudes Ceratosaurus, Abelisaurus, and Carnotaurus in his Carnosauria, taxa which Sereno and Holtz consider to belong to the primitive theropod clade Ceratosauria. In spite of these differing views, all these authors exclude the Tyrannosauridae from basal tetanurans and place them in the Coelurosauria. Under any of the phylogenetic schemes of CURRIE (1995), HOLTZ (1994), and SERENO et al. (1994, 1996) there is convergence in the extreme shape where the orbit is nearly divided in two. This condition occurs in Abelisaurus, Acrocanthosaurus, Carnotaurus, Tyrannosaurus, and to a lesser extent in Carcharodontosaurus. This is not a function of size, as the smallest of these skulls, Carnotaurus, is 48% the length of the largest, Tyrannosaurus bataar(table I). In addition, some of the taxa with constricted orbits, such as Carnotaurus, have shorter skull lengths than taxa with unconstricted orbits, such as Sinraptor dongi (TABLE I). Taxa with constricted orbits do not constitute a monophyletic group under any of the phylogenetic schemes cited above, and in one of them (HOLTZ, 1994) they occur in widely disparate clades. Even within the monophyletic clade Tyrannosauridae a constricted orbit occurs only in Tyrannosaurus, the other genera being more similar to the primitive theropod pattern. lithe eye occupied only the dorsal part of the orbit in large headed theropods, then what occupied the rest of the orbit? The eye in living birds is large and fills the orbit. There are no living terrestrial vertebrates with the unusual orbital shapes discussed in this paper. In a detailed study of archosaur cranial pneumaticity WITMER (1997) suggested that the ventral part of the orbit in Allosaurus fragilis is occupied by the diverticulum suborbitale of the craniofacial pneumatic system. However, it is not clear that there is any relationship between the presence of this diverticulum in the orbit and the various orbital shapes in large-headed theropods. Smaller theropods were probably similar to birds in that pneumatic diverticula occupied only a small part of the orbit (see WITMER, 1997: fig. 6). 235

4 D.CHURE A B Fig. 2 - Orbital region in Allosaurus. A - DINO 11541, left orbit, rostral to left. Large arrow points to partial sclerotic ring. Small arrow points to projection of lacrimal marking probable anteroventral margin of part of orbit occupied by eye. Scale bar = 5 cm. B - MOR 693, right lateral view, arrow points to projection of lacrimal marking probable anteroventral margin of part of orbit occupied by eye. Scale bar = 10 cm. 236

5 ON THE ORBIT OF THEROPOD DINOSAURS TABLE I Skull length for large-headed theropods mentioned in text. Alioramus remotus, Cryolophosaurus ellioti, and Torvosaurus tanneri are excluded because insufficient cranial material exists. TAXON SKULL LENGTH (mm) SPECIMEN SOURCE Abelisaurus comahuensis 850 Acrocanthosaurus atokensis 1325 Albertosaurus libratus 1050 Allosaurus fragi/is 753 Allosaurus n. sp. 640 Carcharodontosaurus saharicus "1600" Carnotaurus sastrei 596 Ceratosaurus nasicornis 620 Daspletosaurus torosus 1040 Monolophosaurus jiangi 670 Nanotyrannus lancensis 572 Sinraptor dongi 900 Sinraptor hepingensis 1040 Tyrannosaurus bataar 1220 Tyrannosaurus rex 1210 Yangchuanosaurus magnus 1110 Yangchuanosaurus shangyuensis 810 MC BONAPARTE & NOVAS (1985) no cat. no. pers. obs. AMNH 5434 MATTHEW & BROWN (1923) MOR 693 pers. obs. DINO pers. obs. SGM-Din 1 SERENO et al. (1996) MACNCH 894 BONAPARTE et al. (1990) USNM 4735 GILMORE (1920) NMC 8506 BAKKER et al. (1988) IVPP ZHAO & CURRIE 1993 CMNH 7541 BAKKER et al. (1988) IVPP CURRIE & ZHAO (1993) ZDM 0024 GAO (1992) PIN MALEEV (1974) AMNH 5027 OSBORN (1912) ChM V 216 DONG et al. (1983) ChM V 215 DONG etal. (1983) * "approximately 1.6m" in SERENO et al. (1996) Most forms with a strongly constricted orbital vacuity also have bony projections wh ich overhang the orbit dorsally. In Carnotaurus these projections take the form of laterally projecting frontal horns with flat dorsal surfaces. PAUL (1988: 285) suggests that the postorbital projection dividing the orbit may have been to reduce eye-damage during "horn-butting fights". The great width across the frontal horns and their flat dorsal surface suggests that such "butting" would probably be more in the form of pushing with the dorsal surface of the head. Other forms with greatly restricted orbits (Abelisaurus, Acrocanthosaurus, and Carcharodontosaurus) do not have horns, but do have shelf-like projections over the orbit which might also indicate a head pushing behavior like Carnotaurus. The exception to this pattern is Tyrannosaurus rex, which is reported to have a large supraorbital boss orrugosity (OSBORN, 1912). However, as noted by MOLNAR (1991), this rugosity is subject to considerable variation. This may suggest that T rex was not a head-pusher. Conversely, there may be more variation in the supraorbital structures in Abelisaurus, Acrocanthosaurus, Carcharodontosaurus, and Carnotaurus than we know, as each of these are on ly known from only one complete or fairly complete skull. Be that as it may, why headpushing would functionally necessitate the restriction of the orbit is unclear. Furthermore, the cranial architecture is strikingly different between Carnotaurus, Abelisaurus, Acrocanthosaurus, Carcharodontosaurus, and Tyrannosaurus. For example, Carnotaurus is pug-faced with an extremely thin postorbital bar, whereas Carcharodontosaurus has a long and lightly built skull and moderate postorbital bar, and Tyrannosaurus rex has a long, massive skull with a broad postorbital bar (Fig. 1 K-M). What functional reasons there could be for a constricted orbital vacuity among such differently constructed sku lls is unknown. TABLE II shows the size of the orbit as a percentage of skull length for selected theropods. In Coe/ophysis bauri there is a growth series and, not surprisingly, the orbit is a relatively larger in juveniles than adults (COLBERT, 1989, 1990). Theropods which had a small adult body size have an orbit which is relatively larger than theropods with large adult body size, except, surprisingly, for adult Coe/ophysis, wh ich is closer to large theropods than other theropods closer to it in body length, such as Ornitho/estes. TAB LE II shows thatthe orbit, and by infer- 237

6 D.CHURE TABLE II Orbital length as a percentage of skull length in selected theropods discussed in text. Taxa are arranged in order of increasing skull length. SKULL ORBIT ORBIT AS TAXON LENGTH LENGTH % SKULL SOURCE (mm) (mm) LENGTH Coelophysis bauri largest 250 smallest 68 Compsognathus longipes 70 Omilholestes hermanni 138 Ceratosaurus nasicomis 550 Nanotyrannus lancensis 572 Camotaurus sastrei 596 Monofophosaurus jiangi 670 Allosaurus fragilis 753 Tyrannosaurus rex 1210 Acrocanthosaurus atokensis ' 80 85' % 29.4% 27.1% 25.4% 14% 15.4% 13.4% 12.7% 10.4% 8.3% 7.5% COLBERT (1989) COLBERT (1989) COLBERT (1989) COLBERT (1989) pers. obs. (USNM 4735) BAKKER et al. (1988) BONAPARTE et al. (1990) ZHAO & CURRIE (1993) pers. obs. (MOR 693) pers. obs. (AMNH 5027) pers.obs.""* * Estimated from illustration. ** Cast of a privately owned specimen. ence the eye, becomes relatively smaller with increasing skull length, although in absolute terms the eyes are, in fact, larger. The implications of these observations for understanding the paleobiology of theropods is uncertain. Most crepuscular and nocturnal birds have larger eyes than diurnal birds (WELTY, 1982: 92). RUSSELL & SEGUIN (1982) suggested that the small theropod Troodon (= their Stenonychosaurus) was crepuscular or nocturnal based in part of the relatively large size of the orbit. In terms of relative size of the orbit (as a percentage of skull length), one might infer niche segregation in theropods, with large-headed forms being diurnal predators, and smaller forms being crepuscular or nocturnal hunters. However, given what the fossil record has left us this is a very difficult hypothesis to test. Much has been written in popular books about the paleobiology of theropods. Unfortunately, most of this speculation is very difficult to formulate as testable hypotheses. MOLNAR & FARLOW (1990: 210) provide a sobering review of carnosaur biology, in which they write: "These interpretations seem plausible, but it must be emphasized that the plausibility of a hypothesis does not guarantee its correctness, an unfortunate fact of life often overlooked." The wide range of orbit shapes in theropods reflects something in their biology, but what that is can not yet be determined. ACKNOWLEDGMENTS I thank Ray Jones (Radiological Health Dept., University of Utah) who used his gamma scintillator to locate the still buried skull of DINO long afterwe had given up hope and abandoned the quarry. Ann Elder and Scott Madsen (Dinosaur National Monument), and volunteers Rod Joblove and Rod Hopwood excavated the skull of DINO Ann Elder prepared the orbital region of the skull and the sclerotic plates. Marcus Schmidt (Fire Management Officer, Dinosaur National Monument) provided the helicopter needed to lift the skull back tothe preparation lab. I thank Jack Horner and Pat Leiggi (Museum of the Rockies) for allowing me to study MOR 693. Rich Cifelli (University of Oklahoma Museum) and Ken Carpenter (Denver Museum of Natural History) allowed me to study casts of the skull of Acrocanthosaurus atokensis, the original of which is privately owned. This research is part of a larger Ph.D. study currently underway on the systematics of the Allosauridae. Bob Schiller (Grand Teton National Park) and the National Park Service's Natural Resources Preservation Program provided funding for that program under which I was able to study MOR 693. INSTITUTIONAL ABBREVIATIONS AMNH - American Museum of Natural History, New York City, N.Y., USA; ChM - Chongqing Museum, Chongqing, People's Republic of China; 238

7 ON THE ORBIT OF THEROPOD DINOSAURS CMNH - Cleveland Museum of Natural History, Cleveland, OH, USA; DINO - Dinosaur National Monument, Jensen, UT, USA; IVPP -Institute ofvertebrate Palaeontology and Palaeoanthropology, Beijing, People's Republic of China; MACHCH - Museo Argentino de Ciencias Naturales, Chubut, Argentina; MC - Museo de Cipolleti, Cipolleti, Argentina; MOR - Museum of the Rockies, Bozeman, MT. USA; NMC - National Museum of Canada, Ottawa, Canada; PIN - Palaeontological Institute, Moscow, Russia; SGM - Ministere de l'energie et des Mines, Rabat, Morocco; USNM - National Museum of Natural History, Washington, D.C. USA; ZDM - Zigong Dinosaur Museum, Zigong, People's Republic of China REFERENCES ANONYMOUS (1994) - Catalog 46. Geological Enterprises Inc., Ardmore. Oklahoma, USA, cover illustration. BAKKER. R.T. (1986) - The Dinosaur Heresies. William Morrow & Co., New York, 481 pp. BAKKER, R.T.: WILLIAMS, M. & CURRIE, P. J. 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