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Home , Acrocanthosaurus, Alioramus, Allosauridae, Carcharodontosaurus, Carnosauria, Carnotaurus

GAIA N·15, lISBOAlLISBON, DEZEMBRO/DECEMBER 1998, pp. 233-240 (ISSN: 0871-5424)

ON THE ORBIT OF THEROPOD

Daniel J. CHURE 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 . 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), 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 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 , 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 constriction is due to both a posterior projection of bauri (COLBERT, 1989), lunensis (SERENO the lacrimal and an anterior projection ofthe postor­ et al., 1993), 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. Omitholestes (OSBORN, 1903A), 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 ( libratus Rus­ degree seen in . 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 remotus 1993), Syntarsus kayentakayae (ROWE, 1989), and (KURZANOV, 1976), nasicomis (GI L­ the ornithomimid samueli (PARKS, MORE, 1920), tanneri (BRITT, 1991), 1928) and the size of these rings strongly suggests 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 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 - 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 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. 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 tetanurans (i.e. non-coelurosaurian tetanu­ Monolophosaurus was very large. rans) into two major , the Spinosauroidea and Two new and undescribed specimens of Allosau­ the . 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 (). CURR IE (1995) unites all ba­ first, MOR 693, is a nearly complete skeleton wi th a sal tetanurans into a single , the . _ superb from the Brushy Basin Member of the In addition, CURRIE (1995) incudes Ceratosaurus, near Shell . The sec­ Abelisaurus, and in his Carnosauria, ond of these, DINO 11541, is a new of Allo­ taxa which Sereno and Holtz consider to belong to saurus (CHURE, in prep.) from the Salt Wash the primitive theropod clade . In spite of Member of the Morrison Formation in Dinosaur Na­ these differing views, all these authors exclude the tional Monument. from basal tetanurans and place them in the . 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 the (Fig. 2A). Preservation is such that it is difficult to de­ rest of the orbit? The eye in living is large and termine the pattern of plate overlap. Nevertheless, fills the orbit. There are no living terrestrial verte­ in both specimens the sclerotic plates are restricted brates with the unusual orbital shapes discussed in to the dorsal part of the orbit and in DINO 11541 the this paper. In a detailed study of cranial half or one-quarter circle of plates preserved indi­ pneumaticity WITMER (1997) suggested that the cates that the eye could fit within the area of the orbit ventral part of the orbit in Allosaurus fragilis is occu­ delineated by the lacrimal projection. In birds, the pied by the diverticulum suborbitale of the craniofa­ Ligamentum suborbitale is a thin fasciailligamen­ cial pneumatic system. However, it is not clear that tous band which stretches from the lacrimal to the there is any relationship between the presence of postorbital process and participates in forming the this diverticulum in the orbit and the various orbital ·ventrolateral wall of the orbit (BAUMEL & RAIKOW, shapes in large-headed theropods. Smaller thero­ 1993: 150, fig. 5.1A). Lacrimal and postorbital pro­ pods were probably similar to birds in that pneumatic cesses in theropods are probably manifestations of diverticula occupied only a small part of the orbit this ligament in theropods. (see WITMER, 1997: fig. 6).

235 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. 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 ON THE ORBIT OF THEROPOD DINOSAURS

TABLE I Skull length for large-headed theropods mentioned in text. Alioramus remotus, Cryolophosaurus ellioti, and Torvo­ saurus tanneri are excluded because insufficient cranial material exists.

SKULL LENGTH SPECIMEN SOURCE (mm)

Abelisaurus comahuensis 850 MC 11098 BONAPARTE & NOVAS (1985) Acrocanthosaurus atokensis 1325 no cat. no. pers. obs. Albertosaurus libratus 1050 AMNH 5434 MATTHEW & BROWN (1923) Allosaurus fragi/is 753 MOR 693 pers. obs. Allosaurus n. sp. 640 DINO 11541 pers. obs. Carcharodontosaurus saharicus "1600" • SGM-Din 1 SERENO et al. (1996) Carnotaurus sastrei 596 MACNCH 894 BONAPARTE et al. (1990) Ceratosaurus nasicornis 620 USNM 4735 GILMORE (1920) torosus 1040 NMC 8506 BAKKER et al. (1988) Monolophosaurus jiangi 670 IVPP 84019 ZHAO & CURRIE 1993 Nanotyrannus lancensis 572 CMNH 7541 BAKKER et al. (1988) Sinraptor dongi 900 IVPP 10600 CURR IE & ZHAO (1993) Sinraptor hepingensis 1040 ZDM 0024 GAO (1992) Tyrannosaurus bataar 1220 PIN 551-1 MALEEV (1974) Tyrannosaurus rex 1210 AMNH 5027 OSBORN (1912) Yangchuanosaurus magnus 1110 ChM V 216 DONG et al. (1983) Yangchuanosaurus shangyuensis 810 ChM V 215 DONG etal. (1983)

* "approximately 1.6m" in SERENO et al. (1996)

Most forms with a strongly constricted orbital va­ fairly complete skull. Be that as it may, why head­ cuity also have bony projections wh ich overhang the pushing would functionally necessitate the restric­ orbit dorsally. In Carnotaurus these projections take tion of the orbit is unclear. Furthermore, the cranial the form of laterally projecting frontal horns with flat architecture is strikingly different between Carno­ dorsal surfaces. PAUL (1988: 285) suggests that the taurus, Abelisaurus, Acrocanthosaurus, Carcharo­ postorbital projection dividing the orbit may have dontosaurus, and Tyrannosaurus. For example, been to reduce eye-damage during "-butting Carnotaurus is pug-faced with an extremely thin fights". The great width across the frontal horns and postorbital bar, whereas Carcharodontosaurus has their flat dorsal surface suggests that such "butting" a long and lightly built skull and moderate postorbital would probably be more in the form of pushing with bar, and Tyrannosaurus rex has a long, massive the dorsal surface of the head. Other forms with skull with a broad postorbital bar (Fig. 1 K-M). What greatly restricted orbits (Abelisaurus, Acrocantho­ functional reasons there could be for a constricted saurus, and Carcharodontosaurus) do not have orbital vacuity among such differently constructed horns, but do have shelf-like projections over the or­ sku lls is unknown. bit which might also indicate a head pushing behav­ TABLE II shows the size of the orbit as a percent­ ior like Carnotaurus. The exception to this pattern is age of skull length for selected theropods. In Coe/o­ Tyrannosaurus rex, which is reported to have a large physis bauri there is a growth series and, not supraorbital boss orrugosity (OSBORN, 1912). How­ surprisingly, the orbit is a relatively larger in juveniles ever, as noted by MOLNAR (1991), this rugosity is than adults (COLBERT , 1989, 1990). Theropods subject to considerable variation. This may suggest which had a small adult body size have an orbit that T rex was not a head-pusher. Conversely, there which is relatively larger than theropods with large may be more variation in the supraorbital structures adult body size, except, surprisingly, for adult Coe/o­ in Abelisaurus, Acrocanthosaurus, Carcharodonto­ physis, wh ich is closer to large theropods than other saurus, and Carnotaurus than we know, as each of theropods closer to it in body length, such as Orni­ these are on ly known from only one complete or tho/estes. TAB LE II shows thatthe orbit, and by infer-

237 D.CHURE

TABLE II Orbital length as a percentage of skull length in selected theropods discussed in text. Taxa are arranged in of increasing skull length.

SKULL ORBIT ORBIT AS TAXON LENGTH LENGTH % SKULL SOURCE (mm) (mm) LENGTH Coelophysis bauri largest 250 40 16% COLBERT (1989) smallest 68 20 29.4% COLBERT (1989) Compsognathus longipes 70 19 27.1% COLBERT (1989) Omilholestes hermanni 138 35 25.4% COLBERT (1989) Ceratosaurus nasicomis 550 77 14% pers. obs. (USNM 4735) Nanotyrannus lancensis 572 88' 15.4% BAKKER et al. (1988) Camotaurus sastrei 596 80 13.4% BONAPARTE et al. (1990) Monofophosaurus jiangi 670 85' 12.7% ZHAO & CURRIE (1993) Allosaurus fragilis 753 78 10.4% pers. obs. (MOR 693) Tyrannosaurus rex 1210 100 8.3% pers. obs. (AMNH 5027) Acrocanthosaurus atokensis 1325 100 7.5% pers.obs.""*

* Estimated from illustration. ** Cast of a privately owned specimen. ence the eye, becomes relatively smaller with ACKNOWLEDGMENTS increasing skull length, although in absolute terms I thank Ray Jones (Radiological Health Dept., the eyes are, in fact, larger. University of ) who used his gamma scintillator The implications of these observations for under­ to locate the still buried skull of DINO 11541 long af­ standing the paleobiology of theropods is uncertain. terwe had given up hope and abandoned the quarry. Most crepuscular and nocturnal birds have larger Ann Elder and Scott Madsen (Dinosaur National eyes than diurnal birds (WELTY, 1982: 92). RUSSELL Monument), and volunteers Rod Joblove and Rod & SEGUIN (1982) suggested that the small theropod Hopwood excavated the skull of DINO 11541 . Ann Troodon (= their Stenonychosaurus) was crepuscu­ Elder prepared the orbital region of the skull and the lar or nocturnal based in part of the relatively large sclerotic plates. Marcus Schmidt (Fire Management size of the orbit. In terms of relative size of the orbit Officer, Dinosaur National Monument) provided the (as a percentage of skull length), one might infer helicopter needed to lift the skull back tothe prepara­ niche segregation in theropods, with large-headed tion lab. I thank Jack Horner and Pat Leiggi (Mu­ forms being diurnal predators, and smaller forms be­ seum of the Rockies) for allowing me to study MOR ing crepuscular or nocturnal hunters. However, 693. Rich Cifelli (University of Museum) given what the record has left us this is a very and Ken Carpenter (Denver Museum of Natural His­ difficult hypothesis to test. tory) allowed me to study casts of the skull of Acro­ canthosaurus atokensis, the original of which is Much has been written in popular books about privately owned. This research is part of a larger the paleobiology of theropods. Unfortunately, most Ph.D. study currently underway on the systematics of this speculation is very difficult to formulate as of the Allosauridae. Bob Schiller (Grand Teton Na­ testable hypotheses. MOLNAR & FARLOW (1990: tional Park) and the National Park Service's Natural 210) provide a sobering review of biology, Resources Preservation Program provided funding in which they write: "These interpretations seem for that program under which I was able to study plausible, but it must be emphasized that the plausi­ MOR 693. bility of a hypothesis does not guarantee its correct­ ness, an unfortunate fact of life often overlooked." INSTITUTIONAL ABBREVIATIONS The wide range of orbit shapes in theropods reflects something in their biology, but what that is can not AMNH - American Museum of Natural History, yet be determined. New York City, N.Y., USA; ChM - Chongqing Mu­ seum, Chongqing, People's Republic of China;

238 ON THE ORBIT OF THEROPOD DINOSAURS

CMNH - Cleveland Museum of Natural History, GAO, Y.H. (1992) - Yangchuanosaurus hepingensis- a new speci­ es of carnosaur from Zigong, Sichuan. Vertebrata PalAsiati­ Cleveland, OH, USA; DINO - Dinosaur National ca, 30: 313-324. (in Chinese, English summary, pp. 323-324). Monument, Jensen, UT, USA; IVPP -Institute ofVer­ GILMORE, C.W. (1920) - Osteology of the carnivorous Dinosauria tebrate Palaeontology and Palaeoanthropology, in the National Museum, with special reference Beijing, People's Republic of China; MACHCH - to the genela - (Allosaurus) and Ceratosaurus. Museo Argentino de Ciencias Naturales, Chubut, U.S. Nat. Mus. Bull., 110: 1-159. ; MC - Museo de Cipolleti, Cipolleti, Argen­ HAMMER, W.R & HICKERSON, W.J. (1994) - A crested theropod from . Science, 264: 828-830. tina; MOR - Museum of the Rockies, Bozeman, MT. USA; NMC - National Museum of Canada, Ottawa, HOLTZ, T.R, JR. (1994) - The phylogenetic position of the Tyran­ nosauridae: implications for theropod systematics. J. 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