Triceratops spoils or spoiled Triceratopsl

In 1917, L. M. Lambe ofthe Geological Survey ofCanada suggested that Gorgosaurus was not so much an aggressive predator, but instead maintained its sustenance by scavenging. The basis for his remarks was the apparent absence of heavy wear on the teeth of this theropod - these animals therefore must have fed primarily on the softened flesh of putrefying carcasses. This interpretation has appeared on and off again in discussions oftheropod diet and hunting behavior, frequently enough to be something like a cottage industry in anecdotal "knowledge" about these animals.

The notion oftheropod scavenging rests on the assumption that tooth wear was usually absent and that carcasses were readily available. It is further bolstered by the lack of a convincing account of why the forelimbs of these animals are so small. We take each of these in turn. Firstly, it turns out that tooth wear is present on the teeth of nearly all large theropods. This doesn't "prove" that tyrannosauroids and other large theropods had to have been active predators; both modern scavengers and active predators alike can have a high degree of wear on their teeth.

The commonness of carcasses, putrefied or otherwise, was probably dependent on the season - dry, stressful seasons probably claimed their share of dead hadrosaurids, ceratop-sians, sauropods, and the like. Interestingly, this potentially great contribution of carcasses would have been in the form of tough, dry flesh - dinosaur jerky, really - not the kind of soft, predigested carrion postulated by Lambe.

Finally, short forelimbs were likely used to slice and dismember prey, and are fully consistent with a head-first attack.

Recently, the suggestion oftyrannosaurids as carrion eaters has come from the observation that these theropods had surprisingly broad, bulbous teeth. Modern scavengers such as the hyena likewise have broad teeth, which are used to crush the bones of carcasses. This notion has been pooh-poohed by scientists who cannot imagine a dinosaur with the size and obvious carnivorous equipment of T. rex being a scavenger.

What is clear is that Tyrannosaurus teeth clearly exceed the size increase that might be predicted from its enlarged body. For this reason, T. rex leaves paleontologists with a mouthful of confusion.

In the end, we think it likely that animals like Tyrannosaurus would have been a more than adequate, indeed terrifying, active predator, yet one that wouldn't turn up its nose at a lunch of carrion. Indeed, it is probably better to view all theropods, from the exceptionally large tyrannosauroids down to much smaller troodontids, dromaeosaurids, and coelophys-oids, as equal-opportunity consumers, taking large herbivores, smaller flesh eaters, and even the occasional carcass.

Figure 9.23. Cannibalism in theropods. Above: the teeth of the large Madagascan theropod Majungatholus. Below: toothmarks on Majungatholus bone. Note that the spacing of the grooves in the bone matches that of the teeth; note also the scratches next to each groove, reflecting the serrations of the teeth.

Figure 9.23. Cannibalism in theropods. Above: the teeth of the large Madagascan theropod Majungatholus. Below: toothmarks on Majungatholus bone. Note that the spacing of the grooves in the bone matches that of the teeth; note also the scratches next to each groove, reflecting the serrations of the teeth.

evolution of Theropoda," below) and include such luminaries as Syntarsus (Zimbabwe, South Africa, and Arizona) and Coelophysis (New Mexico).

Other theropods, though, are also known in bonebeds: Allosaurus (Utah), Giganotosaurus and Mapusaurus (Patagonia). Could it have been that some theropods were gregarious, living in large family groups that perished together? Or perhaps each accumulation represents a communal feeding site? Do the bonebeds represent pack hunting? As new finds with one or two individuals get discovered, even that supposedly most solitary of killers -Tyrannosaurus - is a potential candidate for pack life. Theropods as gregarious beasts - even large theropods - is becoming a more and more likely prospect.

What can the skeletons of theropods tell us about how these animals related to each other socially? Like crests in hadrosaurids or frills and horns in neoceratopsians, quite a number of predatory dinosaurs - including Syntarsus, Dilophosaurus, Proceratosaurus, and possibly Ornitholestes, to Ceratosaurus, Cryolophosaurus, Alioramus, and Oviraptor - sported highly visible cranial crests (see Figures 9.12c and 9.24). Some are made of thin sheets of bone, while others are hollow, presumably part of the cranial air-sinus system. Beyond these, theropods such as Yangchuanosaurus, Allosaurus, Acrocanthosaurus, and the tyrannosauroids bore slightly elevated upper margins on the snout and the raised and roughened bumps over the eyes. These structures are believed to have been cores for hornlets (small horns) made of keratin, which must have given the face a punk-rock spiky look (Figures 9.12b, e, f and 9.13c).

The crests and hornlets assuredly functioned in display and - at least for the latter -may have been used occasionally in head-butting squabbles over territories and mates. If crests and hornlets functioned in visual display, particularly in those theropods that lived in large groups (see above), we might expect them to be species-specific and probably sexually dimorphic so as to signal a given animal's identity and sex. And likewise we might expect crests to show their greatest development in reproductively mature individuals; youngsters should have small, poorly developed crests and hornlets.

Are these expectations met in any thero-pods? To a degree; sexual dimorphism is found in the Syntarsus and Coelophysis. In these two theropods, one of the two morphs had a relatively long skull and neck, thick limbs, and powerfully developed muscles around the elbow and hip, while the other form has a shorter skull and neck, and slender limbs. In Tyrannosaurus, the case has been made for sexually dimorphic ornamentation. The larger, more robust morph was hypothesized to be the female.3 In the cases of other theropods, we just don't yet know. Figure 9.24. The skull of Cryolophosaurus, with its distinctive crest.

3. A female Tyrannosaurus was identified using soft tissue that was quite astoundingly well preserved over the 65.5+ million years that had elapsed since the animal's death. M. H. Schweitzer ofNorth Carolina State University found what she interpreted to be medullary tissue in the fossil bones - tissue that is diagnostic ofan animal that is producing eggs. Today medullary tissue is produced only by ovulating female birds; a clear indication that this T. rex, at least, must have been female - and ovulating (see Chapter 10).

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