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Monday, Feb. 26, 2001
The bite of a Jurassic killer
By ROWAN HOOPER
A combination of advanced medical scanning techniques and sophisticated data-analysis used in engineering has revealed the biomechanics of dinosaur feeding.
Allosaurus fragilis was a 1.4-ton, 3.5-meter-tall carnivorous bipedal dinosaur that lived and killed in the Jurassic, 150 million years ago, but little is known of its natural history. Emily Rayfield and colleagues at the University of Cambridge reported last week in Nature that Allosaurus probably had a slash-and-slice feeding technique similar to that seen today in the Komodo dragon (Varanus komodoensis).
Rayfield's biomechanical analysis of a fossilized Allosaurus skull shows that its maximal bite force was quite low for such a huge beast -- similar to that of much smaller mammals like today's wolves and jaguars. The bite force would have been much lower than that of its bigger cousin, Tyrannosaurus rex, which means, said Rayfield, that its feeding ecology was different.
T. rex had around 60 huge, robust teeth and is thought to have delivered massively powerful bites, hacking through flesh and bone and causing instantaneous death to its prey. Allosaurus, on the other hand, had about 80 saberlike curved teeth and was only capable of bites at least six times weaker than T. rex. Like today's giant lizards, Komodo dragons, Allosaurus probably inflicted several rapid slashing bites aimed at the soft tissues of the prey, which bled to death slowly.
Using computerized tomography (CT) scans and "finite-element analysis" similar to that used by engineers to measure stress on bridges, Rayfield and colleagues have shown that Allosaurus' skull was extraordinarily strong relative to its bite force. Like the old glass Coke bottle, it seems to be "over-built": too strong for its apparent function. The researchers argue, therefore, that the Allosaurus skull was adapted to absorb impacts with prey and from forces generated when it drew its teeth through that prey.
Allosaurus, known as "Big Al" to viewers of the recent BBC hit series "Walking With Dinosaurs," probably attacked its prey at high speed, impacting head-on. After this first strike, it would rapidly slice flesh from the prey. In contrast, T. rex, which lived much later, in the Cretaceous, is thought to have used its immense jaws primarily for dismemberment rather than high-speed impact.
The relatively low bite force of Allosaurus didn't seem to restrict its choice of prey: It is thought to have used its speed and mobility to capture light and agile prey such as the ornithopod dinosaurs. It would also have been able to feed on the armored stegosaurs and even on the largest land animals ever to exist -- the huge plant-eating sauropods such as Diplodocus and Apatosaurus. Rayfield and colleagues speculate that Allosaurus would ambush and fatally wound these giants before they could react to the attack.
Commenting on the Allosaurus skull model, Gregory Erickson of Florida State University, who has worked on T. rex, cautioned that the overbuilt skull might have had other functions, "such as accomodation of soft tissues or sexual display."
In more dinosaur news in Nature this week, Jose Sanz and researchers at the Universidad Autonoma de Madrid report on the contents of fossilized pellets coughed up by small theropod dinosaurs. Theropods (lit., "beast foot") are the carnivorous dinosaurs, which include Allosaurus and T. rex as well as smaller predators like Velociraptor.
According to analysis of the pellets, the smaller theropods fed on small birds and pterosaurs (flying dinosaurs). Digestion was quite extensive, but not as thorough as prey eaten by crocodiles. Like owls today, the dinosaurs digested what they could and coughed up the bones.