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 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 binocu lar vision in Tyrannosaurus and Nanotyrannus (PAUL, 1988). However, overlapping visual fields do not necessarily
imply stereopsis (MOLNAR & FARLOW, 1990; MOLNAR, 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 (SERENO & NOVAS, 1993), Syntarsus rhodesiensis (COLBERT, 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 RusSELL, 1970, Oaspletosaurus torosus RUSSELL,
1970, and Nannotyrannus lancensis BAKKER, WILLIAMS & 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 & NoVAS, 1985), Carcharodontosaurus saharicus (SERENO et al., 1996), Camotaurus sastrei (BONAPARTE,
1985), and Tyrannosaurus rex (OSBORN, 1912) (Fig.
1J-N). The condition is ontogenetically variable to
some extent in Tyrannosaurus bataar. In the type,
PIN 551-1 (MALEEV, 1974: fig. 48) there is a postorbital projection into the orbit. The smaller, referred
skul ls (PIN 551-3 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 . 1L). 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. 1F) 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 intermed iate between the circular and constricted orbita l shapes. The simplest of these is a
vertically elongated orbit, as in Alioramus remotus
(KURZANOV, 1976), Ceratosaurus nasicomis (GI LMORE, 1920), Torvosaurus tanneri (BRITT, 1991),
Yangchuanosaurus shangyuensis (DONG, ZHAO &
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 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 11541. 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
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 & CURRIE, 1993) (Fig. 1H) 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 wi th 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 11541 (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 11541 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
11541. In MOR 693 the sclerotic ring is collapsed
upon itself as a jumble of plates. In DINO 11541 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 11541 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
D.CHURE
A
B
Fig. 2 - Orbital reg ion in Allosaurus. A - DINO 11541, left orbit, rostra l 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
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)
Abelisaurus comahuensis
Acrocanthosaurus atokensis
Albertosaurus libratus
Allosaurus fragi/is
Allosaurus n. sp.
Carcharodontosaurus saharicus
Carnotaurus sastrei
Ceratosaurus nasicornis
Daspletosaurus torosus
Monolophosaurus jiangi
Nanotyrannus lancensis
Sinraptor dongi
Sinraptor hepingensis
Tyrannosaurus bataar
Tyrannosaurus rex
Yangchuanosaurus magnus
Yangchuanosaurus shangyuensis
850
1325
1050
753
640
"1600" •
596
620
1040
670
572
900
1040
1220
1210
1110
810
SPECIMEN
SOURCE
MC 11098
no cat. no.
BONAPARTE & NOVAS (1985)
pers. obs.
AMNH 5434
MOR 693
MATTHEW & BROWN (1923)
pers. obs.
DINO 11541
SGM-Din 1
pers. obs.
SERENO et al. (1996)
MACNCH 894
BONAPARTE et al. (1990)
USNM 4735
GILMORE (1920)
BAKKER et al. (1988)
NMC 8506
IVPP 84019
CMNH 7541
ZHAO & CURRIE 1993
BAKKER et al. (1988)
IVPP 10600
ZDM 0024
CURRIE & ZHAO (1993)
GAO (1992)
PIN 551-1
MALEEV (1974)
AMNH 5027
ChM V 216
OSBORN (1912)
DONG et al. (1983)
ChM V 215
DONG etal. (1983)
* "approximately 1.6m" in SERENO et al. (1996)
Most forms with a strong ly 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. 1K-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. TABLE 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 order of
increasing skull length.
SKULL
LENGTH
(mm)
ORBIT
LENGTH
(mm)
ORBIT AS
% SKULL
LENGTH
SOURCE
250
68
40
20
16%
29.4%
COLBERT (1989)
COLBERT (1989)
70
19
27.1%
COLBERT (1989)
Omilholestes hermanni
138
35
25.4%
Ceratosaurus nasicomis
550
77
14%
Nanotyrannus lancensis
572
88'
15.4%
BAKKER et al. (1988)
Camotaurus sastrei
596
80
13.4%
BONAPARTE et al. (1990)
ZHAO & CURRIE (1993)
TAXON
Coelophysis bauri
largest
smallest
Compsognathus longipes
COLBERT (1989)
pers . obs. (USNM 4735)
Monofophosaurus jiangi
670
85'
12.7%
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 .
ACKNOWLEDGMENTS
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 .
I thank Ray Jones (Radiological Health Dept.,
University of Utah) who used his gamma scintillator
to locate the still buried skull of DINO 11541 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 11541 . 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
ON THE ORBIT OF THEROPOD DINOSAURS
GAO, Y.H. (1992) - Yangchuanosaurus hepingensis- a new species of carnosaur from Zigong , Sichuan . Vertebrata PalAsiatica, 30: 313-324. (in Chinese , English summary, pp. 323-324).
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
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