Scavengers and Hunters, Part II

As mentioned in last week’s article, the fact that Tyrannosaurus rex has small arms has sometimes been advanced (although never in peer-reviewed literature) as supporting the idea that it was an obligate scavenger. (As can be seen here, Carnotaurus sastrei has similarly small, if quite differently structured, forelimbs).

In fact, arms are completely unnecessary for most hunters. Among today’s predators, cats (Felinae) and sometimes bears (Ursidae) do use their forelimbs to bring down prey. How many predators don’t? Dogs (Canidae), weasels (Mustelinae), cetaceans, amphibians, lizards and snakes (Squamata), crocodylians, predatory birds (Aves)—not to mention all fish (non-tetrapod Craniata) and most invertebrates. Each and every one of these uses their mouths, and forelimbs are secondary (if even present at all). There is no reason to think most theropods were any different. In fact, most were probably not even capable of moving their hands further forward than their jaws (Carpenter 2002).

When you have jaws over a meter long lined with teeth like railroad spikes, you don’t need arms.

Other problems with the “obligate scavenger” hypothesis come up when you look at modern animals. How many extant vertebrates actually are obligate scavengers? The only examples are some species of vulture (Accipitridae, Cathartidae). Even notorious scavengers like hyenas (Hyaeninae) spend large amounts of time hunting as well (usually nocturnally). The only reason vultures are able to live without hunting is because they can scan huge areas of land using barely any energy as they ride, wings spread, on thermals. Hardly an option for T. rex, or any non-avialan dinosaur.

Furthermore, there is fossil evidence for T. rex as a hunter: an Edmontosaurus annectens specimen with healed bite marks (Carpenter 1998). The bite radius matches that of a T. rex, but not any other carnivore from that environment (see Part I, previous week). Since the wound partially healed, the animal was clearly not being scavenged. (Unless, as some have jokingly suggested, it was asleep and the T. rex bit it by mistake.)

Did T. rex and C. sastrei scavenge? Almost certainly. Did they hunt? Almost certainly.

References:

• Carpenter, K. 1998. Evidence of predatory behavior by carnivorous dinosaurs. Gaia 15: 135-144.
• Carpenter, K. 2002. Forelimb biomechanics of nonavian theropod dinosaurs in predation. Senckenbergiana Lethaea 82: 59–76.
• Farlow, J. O. and T. R. Holtz, Jr. 2002. The fossil record of predation in dinosaurs. pp. 251–266, in M. Kowalewski and P. H. Kelley (eds.), The Fossil Record of Predation. The Paleontological Society Papers 8.