Sunday, October 17, 2010

Its What's For Dinner

The tyrannosaurids belong to a group of carnivorous dinosaurs called theropods within the Saurischia ("reptile-hipped") dinosaurs. They lived during the Cretaceous, 85-65 million years ago. They are characterized by massive skulls with short but deep jaws containing large sharp teeth, elongate hindlimbs, small eyes, and highly reduced forelimbs. The Tyrannosauridae taxon includes such creatures as Albertosaurus, Gorgosaurus, Daspletosaurus, Tarbosaurus, and of course Tyrannosaurus rex. These were the dominant large carnivores during the Late Cretaceous in North America and Asia. The T. rex is the last and largest of the terrestrial carnivores. Their fossils are actually relatively common in North America, particularly their teeth. These teeth, with their large banana shape, leave recongnizable markes on the bones of other dinosaurs. You would think that this would make it easy to piece together the feeding habits of these dinosaurs, but it is still difficult to even broadly classify the animal as predator, scavenger, or both. We all grew up thinking of T. rex as a mighty hunter, but the evidence against the predator hypothesis includes the animal's small eyes (most predators have large eyes to help find and follow prey), small arms (difficult to hold struggling prey with dainty little forearms), and large legs (seem fast but are actually slow). Add to it the fact that there is evidence of T. rex teeth scoring bone but not actually killing the prey, bones have healed. Scavaging, however, uses all of these traits as a plus and includes supporting evidence such as large olfactory lobes (smelling foul, dead meat well helps you find it). However, if you look at extant, large predators they are opportunists, scavanging when it is available, and if you look at animals such as birds, sharks, and snakes you will notice that little arms aren't that much of a disadvantage. See how there is some confusion?

In a study published in PLoS ONE this week, palentologists discuss a different type of T. rex feeding habit. During a study of Maastrichtian (latest age of the Late Cretaceous) dinosaurs one of the researchers found a large theropod pedal phalanx (foot/toe bones) that had tooth marks on it that were made by a large carnivorous dinosaur. Considering the geographic region (North America) and the time period, T. rex was the only large carnivore known and so a very likely candidate for inflicting the bone damage. They examined this bone as well as other dinosaur specimens. They found a total of 17 specimens bearing tooth marks made by Tyrannosaurus. The deep U- and V-shaped gouges and shallower scores resemble those found on the pelvis of a Triceratops, and closely resemble the furrowed ‘puncture and pull’ traces that have previously been attributed to T. rex. Four of the examined specimens represent Tyrannosaurus. That's right, we're talking cannabalistic Tyrannosaurus rex!

Figure 2. Tyrannosaurus rex bones bearing
 tooth marks made by Tyrannosaurus rex.
The authors argue that the marks on the bones are the result of feeding rather than fighting. They say that the traces would be difficult to inflict on a live animal, and that, in the case of one bone, the marks are the result of two or three bites. Considering that most animals probably wouldn't sit still while a Tyrannosaur repeatedly bit them, that seems likely. Additional evidence includes where the bites occurred - on the toes (tightly bound in life and not the targets during a fight) rather than the vulnerable head or flanks. Finally, there is no bone remodeling (healing) evidenced, suggesting that the animal died.

Put this together and it gives more evidence to the hypothesis that Tyrannosaurus was an indiscriminate, opportunistic feeder. It didn't just go for herbivorous dinosaurs but also other Tyrannosaurs. The bone marks found in this study were probably the result of scavenging. However, you can't rule out that the Tyrannosaur killed its prey and fed on it over a long period of time.

Cannibalistic Tyrannosaurs, whew, that's a visual right? But, if you think about it, cannibalism isn't all that uncommon in nature. I mean, its been seen in bears, alligators, hyenas, etc. So it seems a good arguement for T. rex as well.

Very cool.

Read more in the paper (its free access):
Longrich Nicholas R., Horner John R, Erickson Gregory M., and Currie Philip J. (2010) Cannibalism in Tyrannosaurus rex. PLoS ONE: 5(10), e13419. (DOI: 10.1371/journal.pone.0013419)

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