On the Orbit of Theropod Dinosaurs

GAIA N·15, lISBOAlLISBON, DEZEMBRO/DECEMBER 1998, pp. 233-240 (ISSN: 0871-5424) ON THE ORBIT OF THEROPOD DINOSAURS Daniel J. CHURE Dinosaur National Monument. Box 128, JENSEN, UT 84035. USA E-mail: [email protected] ABSTRACT: Primitively, theropod orbits are roughly circular in outline and this pattern is re tained 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 di vision 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 orbit. This is the primitive and most widespread con dition of the orbit and eye in theropods and many Theropod dinosaurs have long captured the other amniotes. imagination of the public and paleontologists, and there has been much speculation aboutlheir biology However, unusual orbital shapes do occur in (BAKKER, 1986; PAUL, 1988), some even identified theropods with large skulls. In the most extreme as such (FARLOW, 1976). While the visual system shape the orbit is nearly divided into a dorsal and has been the subject of relatively little speculation, ventral component. This constriction is usually there have been claims of binocular vision in Tyran caused by an anterior projection of the postorbital, nosaurus and Nanotyrannus (PAUL, 1988). How as in Abelisaurus comahuensis (BONAPARTE & No ever, overlapping visual fields do not necessarily VAS, 1985), Carcharodontosaurus saharicus (SER imply stereopsis (MOLNAR & FARLOW, 1990; MOL ENO et al., 1996), Camotaurus sastrei (BONAPARTE, NAR, 1991). Cranial morphology tells us pitifully little 1985), and Tyrannosaurus rex (OSBORN, 1912) (Fig. about the visual system of theropods. However, 1 J-N). The condition is ontogenetically variable to there is a striking range of size and shape in the or some extent in Tyrannosaurus bataar. In the type, bits of theropods, and this diversity presumably has PIN 551-1 (MALEEV, 1974: fig. 48) there is a postor some biological andlor functional significance. bital projection into the orbit. The smaller, referred skulls (PIN 551-3 and 553-1) show a smaller postor DESCRIPTION bital projection (CARPENTER, 1992). In Acrocantho saurus atokensis (STOVALL & LANGSTON 1950) the In primitive theropods, such as Coelophysis constriction is due to both a posterior projection of bauri (COLBERT, 1989), Eoraptor lunensis (SERENO the lacrimal and an anterior projection ofthe postor et al., 1993), Herrerasaurus ischigualastensis (SER bital (ANONYMOUS, 1994) (Fig. 1 L). In theropods ENO & NOVAS, 1993), Syntarsus rhodesiensis (COL where the orbit is constricted the part for the eye is BERT, 1989), and S. kayentakayae (ROWE, 1989) dorsal and the smaller of the two spaces (with the the orbit is large and roughly circular (Fig. 1A). This possible exception of Tyrannosaurus bataar), mak condition is retained in many coelurosaurs, such as ing these theropods beady-eyed killers. Sinraptor Omitholestes (OSBORN, 1903A), Compsognathus dongi (CURRIE & ZHAO, 1993) (Fig. 1 F) has a small (OSTROM, 1978), ornithomimids, oviraptorids, dro projection from both the lacrimal and the postorbital, maeosaurids, therezinosaurids, troodontids, and but the orbit is not constricted anywhere near to the most tyrannosaurids (Albertosaurus libratus Rus degree seen in Acrocanthosaurus. SELL, 1970, Oaspletosaurus torosus RUSSELL, 1970, and Nannotyrannus lancensis BAKKER, WIL A number of large-headed theropods show con LIAMS & CURRIE, 1988). While sclerotic rings are not ditions intermediate between the circular and con well known in theropods, they are known in Herre stricted orbital shapes. The simplest of these is a rasaurus ischigualastensis (SERENO & NOVAS, vertically elongated orbit, as in Alioramus remotus 1993), Syntarsus kayentakayae (ROWE, 1989), and (KURZANOV, 1976), Ceratosaurus nasicomis (GI L the ornithomimid Struthiomimus samueli (PARKS, MORE, 1920), Torvosaurus tanneri (BRITT, 1991), 1928) and the size of these rings strongly suggests Yangchuanosaurus shangyuensis (DONG, ZHAO & that the eye occupied all or nearly all of the circular ZHANG, 1983) (Fig. 1 C-E). Where the eye would be 233 artigos/papers D.CHURE N Fig. 1 - Left orbits and circumorbital bones of selected theropods discussed in text. All drawn with orbits to same verti cal 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 11541. H - Monolophosaurusjiangi (after ZHAO & CURRIE, 1993).1- Cryolopho saurus ellioti (after HAMMER & HI CKERSON, 1994, reversed). J - Carcharodontosaurus saharicus (after SERENO et al., 1996). K - Tyrannosaurus rex (after OSBORN, 1912). L - Acrocanthosaurus atokensis (after ANONYMOUS, 1994). M - Car notaurus sastrei (after BONAPARTE, NOVAS & CORIA, 1990). N - Abelisaurus comahuensis (after BONAPARTE & NOVAS, 1985). 234 ON THE ORBIT OF THEROPOD DINOSAURS and the size olthe eye can not be easily determined DISCUSSION in these forms. In Cryolophosaurus ellioti (HAMMER As stated above, the primitive, and most com & HICKERSON , 1994) (Fig. 11) the upper third of the mon orbit shape in theropods is large and circular. orbit is circular and the ventral two-thirds is elongate Theropods with large skulls exhibit a much wider and tapering and the eye would presumably be in the range of orbil shapes than small headed-theropods. circular part. Monolophosaurus jiangi (ZHAO & CUR These large-headed theropods do not form a mono RIE, 1993) (Fig. 1 H) has a large circular orbit with a phyletic group. SERENoetal. (1994, 1996)dividethe short tapering ventral part. Presumably the eye in basal tetanurans (i.e. non-coelurosaurian tetanu Monolophosaurus was very large. rans) into two major clades, the Spinosauroidea and Two new and undescribed specimens of Allosau the Allosauroidea. HOLTZ (1994) has three distinct rus show a condition intermediate between Sinrap clades of basal tetanurans, only one of which is tor dongi and those forms with elliptical orbits. The named (Allosauridae). CURR IE (1995) unites all ba first, MOR 693, is a nearly complete skeleton wi th a sal tetanurans into a single clade, the Carnosauria. _ superb skull from the Brushy Basin Member of the In addition, CURRIE (1995) incudes Ceratosaurus, Morrison Formation near Shell Wyoming. The sec Abelisaurus, and Carnotaurus in his Carnosauria, ond of these, DINO 11541, is a new species of Allo taxa which Sereno and Holtz consider to belong to saurus (CHURE, in prep.) from the Salt Wash the primitive theropod clade Ceratosauria. In spite of Member of the Morrison Formation in Dinosaur Na these differing views, all these authors exclude the tional Monument. Tyrannosauridae from basal tetanurans and place them in the Coelurosauria. Under any of the phylo The orbital shape in Allosaurus is somewhat vari genetic schemes of CURRIE (1995), HOLTZ (1994), able. It is always elliptical in shape, but in MOR 693 and SERENO et al. (1994, 1996) there is conver (Fig. 2B) and AMNH 600 (OSBORN, 1903b) the ven gence in the extreme shape where the orbit is nearly tral edge is rounded, in DINO 11541 (Fig. 1G) it is divided in two. This condition occurs in Abelisaurus, flat, and in DINO 2560 (the basis forthe skull restora Acrocanthosaurus, Carnotaurus, Tyrannosaurus, tion in MADSEN, 1976) it has a short tapering ventral and to a lesser extent in Carcharodontosaurus. This margin. However, in the latter specimen there is is not a function of size, as the smallest of these crushing in the orbital region and the shape may be skulls, Carnotaurus, is 48% the length of the largest, more elliptical than it appears. Tyrannosaurus bataar(TABLE I). In addition, some of The postorbital is concave anteriorly and does the taxa with constricted orbits, such as Carnotau not project into the orbit in Allosaurus. However in rus, have shorter skull lengths than taxa with uncon MOR693 and DINO 11541 there isa short projection stricted orbits, such as Sinraptor dongi (TABLE I). from the posterodorsal margin of the lacrimal into Taxa with constricted orbits do not constitute a the orbit (Fig. 2). This projection is slightly more pro monophyletic group under any of the phylogenetic nounced in MOR 693. This projection probably schemes cited above, and in one of them (HOLTZ, marks the anteroventral margin olthat part of the or 1994) they occur in widely disparate clades. Even bit occupied by the eye. Parts of sclerotic rings were within the monophyletic clade Tyrannosauridae a found in the left orbit of both MOR 693 and DINO constricted orbit occurs only in Tyrannosaurus, the 11541. In MOR 693 the sclerotic ring is collapsed other genera being more similar to the primitive upon itself as a jumble of plates. In DINO 11541 the theropod pattern. sclerotic ring is only partly visible (eight articulated lithe eye occupied only the dorsal part of the orbit plates) in the posterodorsal corner of the orbit in large headed theropods, then what occupied

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