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251 13 The Biomechanics of a Plausible Hunting Strategy for Tyrannosaurus rex David A. Krauss and †John M. Robinson We present here a biomechanical analysis of a hunting strategy that Tyrannosaurus rex could have employed effectively. The modern analogy for this hunting strategy is “cow tipping,” in which reckless people ambush and tip cows over. Although this analogy seems odd, it is apt. Anatomical analysis of Triceratops indicates that, like a cow, if it were knocked over on its side it would have experienced difficulty in getting up. It seems likely that tyrannosaurs could have exploited this weakness in a hunting strategy. A series of physical analyses were conducted to test the hypothesis that this hunting method was possible for Tyrannosaurus rex. The results indicate that an adult tyrannosaur moving at moderate speed would have produced more than enough force to knock a large Triceratops off its feet. Further, it may only have needed to maintain pressure against the side of the Triceratops for as little as 2–3 seconds to tip it over. The most interesting aspect of this theory is that it explains most of the unique features of tyrannosaur anatomy. Specifically, their small arms seem to be an adaptation allowing them to grasp their prey’s back while pushing it with the pectoral region of their torso. Their large heads would have helped in tipping their prey over, and their large mouths and bone-piercing teeth would have made bites to the side more effective and lethal. According to this “ceratopsian-tipping” hypothesis, Tyrannosaurus would have ambushed its prey from cover, knocking it over and rendering it vulnerable, then killing it with a swift bite to the rib cage. Although generally described as a predator (Osborn 1905; Paul 1987, 1988; Molnar and Farlow 1990; Carpenter 1997; Carpenter and Smith 2001), Tyrannosaurus has also been described as a scavenger (Lambe 1917; Horner and Lessem 1993), and debate over its primary mode of feeding has persisted since the early 1900s. Most recently, the view of Tyrannosaurus as a scavenger has predominated based on structural (Horner and Lessem 1993; Farlow 1994) and bioenergetic (Farlow 1976, 1994; Ruxton and Houston 2003) arguments. Like many large carnivores, Tyrannosaurus may well have scavenged when the opportunity arose, but it is possible that it was primarily a hunter. What is missing from the argument in favor of tyrannosaur hunting is a viable strategy for an adult animal. Various hunting strategies for Tyrannosaurus have been proposed, including the Abstract Introduction Krauss and Robinson 252 quick strike of a land shark–like predator (Paul 1987) and a group hunting strategy, like that used by lions (Currie 1998), but neither of these hypotheses is entirely convincing. Here we present a modified version of the “land-shark” type of ambush hunting strategy that is not only physically possible for an adult T. rex but also explains some of the unusual aspects of its anatomy. Tyrannosaurs are unusual among theropods for several reasons. Most notable are their proportionally small arms and large head. Explaining the reduced arm size has long been a problem in paleontology. The hunting model put forth here presents one possible explanation. In this case, tyrannosaurs would use their arms to grapple with prey, not in the traditional sense of holding or slashing, but rather to grip the smooth back of a ceratopsian and prevent it from dislodging the attacking tyrannosaur. Short arms could be extended fully, thus reducing the risk of injury to the attacker. This hunting strategy would also place directional selection on large head size, which would provide a mechanical advantage in attacking ceratopsian prey, as will be seen below. A modern analog for the hunting strategy being proposed is the practice known as “cow tipping,” in which two or more people (usually intoxicated teenagers) sneak up on a sleeping cow and, by applying force to one side of the cow’s back, knock it over on its side. Once felled, the cow is incapable of getting back up because the structure of its legs does not allow it to place its feet under its center of gravity. A ceratopsian dinosaur would have been similarly incapacitated if knocked over on its side, and it seems plausible that tyrannosaurs evolved to take advantage of this difficulty. A ceratopsian should have been able to recover from a position of lying on its side by rolling back and forth as modern rhinoceroses do, but it would have taken several seconds during which time it would have...

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