Predation and Scavenging

Estimates based on fossil trackways suggest that medium-sized and large theropods walked at speeds of about 5-10 km h-1. This is quite as fast or even faster than large mammals of today. It would suggest that these flesh-eating dinosaurs were nomadic animals and ranged widely in search of food. Although speeds considerably faster than this have been proposed by several authors, including Bakker (1987), it does not seem that greater speeds would have been necessary, because most of the smaller dinosaurs on which they preyed would not have been able to move any faster (Farlow and Chapman 1997).

The relatively small size of the arms of the carnosaurs has often given cause for surprise and speculation as to their function. The extreme is reached in Tyrannosaurus (Fig. 122) in which the shoulder bone was relatively large while the limb was tiny and bore a small two-clawed hand. It has been postulated that this hand might have been used for grappling the partner during mating. An alternative suggestion is that the arm served to anchor the front of the body while the gigantic carnivore struggled onto its feet after resting on the ground (Norman 1985; Molnar and Farlow 1990). Quite possibly, both functions were employed. Even so, it is difficult to imagine why the front legs were reduced to such an extent.

When closing with their prey, the carnosaurs would have attacked with open jaws, grabbed the victim and shaken it violently. The shock of this would have caused death, as it does when animals are killed in this way by crocodiles, sharks and killer whales (Orcinus spp.). It is highly probable that, like top predators today, the carnosaurs were not only predators but took to scavenging when opportunities occurred (Currie 1997a). That they were also opportunistic cannibals, is proved by the presence of distinctive tooth marks on fossils of Majungatholus from Madagascar.

Carnosaurs were considerably larger than the predatory mammals that replaced them. The maximum sizes of predatory dinosaurs were the result of a conflict between two demands: population must be low enough to avoid overexploitation of food supply, but large enough to prevent chance extinction. Studies of fossil tracks suggest that heavy carnivores with narrow, bird-like feet, did not leave firm sandy surfaces, whereas herbivorous dinosaurs with much broader feet would have been able to browse in swampy areas beyond their reach. This would have helped to maintain the balance between herbivores and carnivores (Lockley 1997).

The stochastic probability of extinction increases as population size decreases ('stochastic'refers to patterns resulting from random effects). Flesh-eating dinosaurs solved the conflict by one or more combinations of the following: (1) They maintained larger population densities and/or faster rates of turnover than might be expected of their herbivorous prey, and probably higher than that of large contemporary mammals. (2) Their oviparity produced a larger number of viable offspring than viviparity would have achieved under similar environmental conditions. (3) They fed on different prey when they were young than when adult. (4) They consumed less food than would be expected for equally large meat-eating mammals. The most probable factors involved in enabling the larger carnosaurs to survive were that their mass-specific food consumption was lower than might be expected while, at the same time, their reproductive potential was higher than that of carnivorous mammals (Farlow 1993).

Valid genera of carnosaurs so far known number nearly 20, while there are almost double that number of dubious genera. The latter are based on fragments of fossilised bones (Norman 1985).

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