Tyrannosauridae

KENNETH CARPENTER Denver Museum of Natural History Denver, Colorado, USA

yrannosaurids are large-headed theropod carni- this adaptation, broken teeth are found mingled T vores with short, two-fingered hands. They lived among the fossilized carcasses of their prey. 80–65 million ago during the Late . In allosaurids, kinesis allows the to ‘‘deform’’ Only a single , the , is recog- or absorb stress at joints between various skull bones. nized and only from and eastern . This allows the skull to accommodate larger bites. In Other reports of tyrannosaurids, such as Genyodectes tyrannosaurids, the skull was apparently akinetic or from Argentina, are now thought to represent other nearly so. Again, having a more rigid skull may reflect large theropod families (Molnar, 1990). an adaptation for a more powerful bite because en- Many of the adaptations seen in the tyranno- ergy is not lost or dissipated by ‘‘deforming’’ the saurids are centered around their role as top predator skull. Some intramandibular movement was proba- in the . As with most pred- bly possible between the dentary and the surangular ators, emphasis is on the mouth as the killing and and angular. Movement may have also been possible feeding organ. This adaptation follows a long trend between the two dentaries at the symphysis because in theropod evolution in which the size of the mouth, the bones are loosely joined by cartilage. hence the size of the bite, is increased, enlarging the The orbit is large in tyrannosaurids, as are both skull. However, a larger skull cannot be supported the olfactory and optic nerve tracts of the endocast on a long, slender neck, so the neck and body are (Osborn, 1912a). Thus, as with most predators, both concomitantly shortened. These changes may be seen smell and vision must have been very keen. What is by comparing the skull length to the length of the less certain is if tyrannosaurids had stereoptic vision presacral vertebrae. In the early, primitive , (). This stereoptical development is the skull is 24% of the presacral vertebral length; in best seen in in which the snout is nar- , an intermediate sized theropod, it is 40%; row in front of the orbit, allowing the eyes to see and in Tyrannosaurus it is 47%. forward. The overlapping fields of vision would The tyrannosaurid SKULL is a laterally compressed, allow the predator to judge distance to the prey. This rectangular box when viewed from the side (Fig. 1). overlapping vision was apparently not as well devel- The boxy look is due to the deep snout, which resists oped in the more primitive tyrannosaurid Gorgo- the strains placed on it during the bite. In contrast, saurus because the snout is not constricted in front of the snout is long and tapering and the teeth are small the orbits. However, it is possible that the eyeballs in the primitive Coelophysis. In this the power extended beyond the rim of the orbits, allowing some of the bite was proportionally less than that in the degree of overlapping vision; unfortunately, we will tyrannosaurids. The TEETH in tyrannosaurids are probably never know if this was the case. large, serrated blades arranged along curved jaws so The upper surface of the nasals is very rough, indi- that when the mouth is closed almost all the teeth cating that some sort of structure may have been engage the prey at the same time. With Coelophysis, present, such as a horn-like protuberance (Baird, per- the teeth are arranged along straight-edged jaws and sonal communication). In and Alber- the teeth engage from back to front as the mouth is tosaurus, a ‘‘horn’’ was also present just in front of closed. Tyrannosaurus has also gone one step further the eyes. This horn was developed from the lacrimal, than most tyrannosaurids by strengthening the teeth. just above the lacrimal fenestra. Interestingly, Tyran- This was accomplished by making the teeth thicker nosaurus does not have much of a lacrimal horn, al- so that in cross-section they are almost as wide from though the surface of the lacrimal is very rough like side to side as long from front to back. Even with the surface of the nasals. Perhaps their horn was

766 Tyrannosauridae 767

The FORELIMBS are short and could not have been used in locomotion. Their purpose is the subject of debate. Some paleontologists, such as Jack Horner, argue that the arms are too short to have any use (Horner and Lessem, 1993). I, on the other hand, ar- gue that the well-developed muscle scars indicate large, powerful muscles on the arms. Such well-mus- cled arms must have had some function, possibly to hold a struggling hadrosaur prey against the body prior to killing it (Smith and Carpenter, 1990; Carpen- FIGURE 1 Skull of the tyrannosaurid, Gorgosaurus libratus. ter and Smith, 1995). The hand has only two fingers rather than the functional three that characterize all other theropods. The phalanges of I have a twist formed from a cuticle-like material. The purpose of in them so that the claw angles away from digit II. horns in front of the eyes and possibly on the snout is This feature may have ensured that the struggling peculiar in a carnivore. Most probably the structures prey could not easily slip off of the claws. Instead, were for , either sexual or agonistic. once engaged the claws were locked into the flesh. The postcrania of tyrannosaurids indicate that the The hindlegs are large and powerful for carrying body was carried bipedally, with the tail counterbal- the body. The legs were most probably flexed so that ancing the body over the hindlegs (Fig. 2). The cervi- the projected somewhat forward in a - cal vertebrae are short and wide. The neural spines like manner. This flexure compensated for the greater are not the tall blades seen in more primitive thero- mass of the body in front of the . The feet, as pods but rather are short and wide for the attachment in most theropods, are raptor-like, ending in three of powerful muscles and ligaments. The bulldog-like sharp claws. These talons could hold the carcass neck would have supported the large, heavy head down while the neck pulled the head back, ripping with strong muscles for pulling flesh off the carcass. flesh from the carcass. Whether tyrannosaurids could The body of tyrannosaurids is deep to accommodate run very fast or not is debated among paleontologists. the internal organs in a foreshortened body. Rods Some, such as Edwin Colbert (1983), argue that the of bone, called GASTRALIA, support and protect the large size of tyrannosaurids, especially Tyrannosau- internal organs. They extend across the abdominal rus, precludes running. Instead, he argues that they cavity from the coracoids to the pubis. had a fast elephantine walk. Others, such as Greg

FIGURE 2 Skeletal reconstruction of the tyrannosaurid, Tyrannosaurus rex. 768 Tyrannosauridae

Paul (1988), argue that the bird-like hindleg indicates analysis indicates that, in fact, tyrannosaurids are gi- much faster locomotion, perhaps up to 70 km/hr. ant coelurosaurs more closely related (i.e., a ‘‘sister Certainly, changes in the upper foot and ankle group’’) to the ornithomimids than they are to allo- compared with that of Allosaurus would seem to indi- saurids (Holtz, 1994). cate a more stable foot for running. The ascending process of the astragalus is very tall and firmly See also the following related entries: attached to the with cartilage. This ensures that ARCTOMETATARSALIA ● ● COELU- the ankle remains steady over uneven ground. In ROSAURIA ● addition, the central metatarsal is constricted on its proximal end to a long slender splint. This splint is surrounded by the outer metatarsals, restricting the References amount of movement between the three bones. This Carpenter, K., and Smith, M. B. (1995). Osteology and adaptation, as well as the sturdy ankle, would cer- functional morphology of the forelimb in Tyrannosauri- tainly keep the foot from twisting on uneven ground; dae as compared with other theropods (Dinosauria). J. therefore, perhaps tyrannosaurids did run, but how Vertebr. Paleontol. 15(Suppl. to No. 3), 21A. fast no one knows. Colbert, E. H. (1983). : An Illustrated History, pp. Traces of SKIN impression are known for a speci- 224. Hammond, Maplewood, NJ. men of Gorgosaurus libratus (Day, personal communi- Horner, J. R., and Lessem, D. (1993). The Complete T. Rex, cation). These show small rounded or hexagonal pp. 239. Simon & Schuster, New York. scales on the tail. The pattern on the rest of the body Molnar, R. (1990). Problematic Theropoda: ‘‘carnosaurs.’’ is unknown, but if other skin impressions In The Dinosauria (D. B. Weishampel, P. Dodson, and H. (e.g., hadrosaurs; Osborn, 1912b) are any indication, Osmo´lska, Eds.), pp. 306–317. Univ. of Press, Berkeley. then the rest of the body was probably covered in Osborn, H. F. (1912a). Crania of Tyrannosaurus and Allo- hexagonal scales of different sizes. Impressions of saurus. Mem. Am. Museum Nat. History 1, 1–30. skin around a badly weathered skull of Tyrannosaurus ϭ Osborn, H. F. (1912b). Integument of the iguanodont di- ( ) bataar in showed the pres- nosaur Trachodon. Mem. Am. Museum Nat. History 1, ence of a wattle or bag of skin under the jaws (Mikhai- 33–54. lov, personal communication). Perhaps this bag of Paul, G. (1988). Predatory Dinosaurs of the World, pp. 464. skin enabled large chunks of prey to be swallowed. Simon & Schuster, New York. It is also possible that the skin was a brightly col- Smith, M. B., and Carpenter, K. (1990). Forelimb biome- ored dewlap similar to that seen in some lizards chanics of Tyrannosaurus rex. J. Vertebr. Paleontol. today. 10(Suppl. to No. 3), 43A. The origin of the family Tyrannosauridae is ob- scure because so few theropod remains are known from the early and middle part of the Late Cretaceous. They first appeared 80 million years ago, and per- Tyrrell Museum sisted for another 15 million years only to vanish in the great dinosaur extinction at the end of the of Palaeontology Mesozoic. Traditionally, it was thought that tyranno- saurids evolved from allosaurids of the (e.g., see ROYAL TYRRELL MUSEUM OF Colbert, 1983; Paul, 1988). However, recent cladistic PALAEONTOLOGY