Paws and claws

As in modern birds, the grasping, powerful, clawed feet must have been an important part of the theropod arsenal (Figure 9.9). This character reached unparalleled sophistication in dromaeosaurids and troodontids, in which the claw on the second digit of the foot was especially huge, curved, and sharp, and capable of a very large arc of motion. During normal walking and running, it was held back or up, to protect it from abrasion or breakage. But, when needed, it could be brought forward and, with the powerful kicking motion of the rest of the leg, used to eviscerate the bellies of hapless prey, lethally disemboweling in one rapid stroke (Figure 9.10).

Figure 9.9. Typical theropod foot: (a) bones; (b) reconstructed doing what non-avian theropod feet did best.

Strong arms and dexterous, three-fingered hands characterized most theropods, particularly small- and medium-sized forms. The digits were long and capable of extreme extension, and tipped with powerful claws. The thumb could fold across the palm in a semi-opposable fashion; that is, somewhat like a human thumb. There is no mistaking the function of this hand: these are all adaptations for grasping.

Even highly specialized large theropods such as Tyrannosaurus, Tarbosaurus, and Carnotaurus, with their notoriously short arms (the hands could not reach the mouth), had stout, powerful bones and fingers, suggesting active use (note the hands and arms of Tyrannosaurus and Carnotaurus in Figure 9.11). But used for what? It has been suggested that perhaps the arms were short in order to balance an overly large head. In bipedal animals that have exceptionally large heads (like Tyrannosaurus, but not so much the case in Carnotaurus), increasing head size may have required downsizing other aspects of the front

(b)
Figure 9.10. (a) Left foot of Demonychus with its disemboweling second-toe claw. (b) Reconstruction of the feet of Demonychus in action.

half of the body to remain balanced with the back half at the hips. It has also been suggested that the arms helped to push the dinosaur up from a resting pose.

Yet the size and design of the arm bones suggests some more compelling function. Indeed, it has been calculated that the arm of Tyrannosaurus could have lifted 300 kg. It is clear from the robust bones and large, stout claws at the tips of the strong fingers that the forelimbs aspired to greater purposes than weight reduction up front. Tearing flesh? Gripping prey? Lifting? Nobody knows.

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  • zahra
    Do Dinosaurs have paws or claws?
    9 years ago

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