Was Tyrannosaurus rex a predator or a scavenger?

The idea that T. rex might have been a scavenger has been around since the early 1900s. The most famous proponent of the theory today is John R. Horner of Montana State University. According to Horner and other paleontologists, there are lots of reasons why T. rex may not have been a predator.

­One argument plays off the theory that dinosaurs evolved into birds. Some of the world's largest flying birds, such as condors, are scavengers -- they eat what they find instead of what they kill. By this logic, really big dinosaurs might have been scavengers, just like their really big avian counterparts today.

This is mostly speculative, but some aspects of T. rex anatomy suggest that it was a scavenger. Its nasal passages, for instance, are huge, potentially perfect for smelling faraway carrion. A tyrannosaurus's teeth and jaw are made for biting -- hard. When a T. rex closed its mouth, the lower teeth met the inside of the upper teeth, concentrating lots of force upward from the inside and downward from the outside. This force could break a bone just like you could break a stick if you bend it with two hands. Paleontologists have also analyzed a coprolite, or a pile of fossilized T. rex dung, and found bone fragments inside. This may mean that the dinosaur relied on picked-over bones for nourishment. To some, the presence of lots of broken teeth also suggests that T. rex chewed its way through bones out of necessity, damaging its teeth in the process.

The scavenger theory applies to a tyrannosaur's body, too. Calculations made by paleontologist James Farlow suggest that T. rex was so massive that it would have sustained life-threatening injuries if it fell while running [source: Hecht]. There's also the matter of T. rex's almost comically undersized forelimbs, which would not be of use in breaking a fall or helping the animal regain its footing.

Scientists also debate whether the dinosaur could run at all, and it would need to run to capture prey. There's no footprint, or trackway, evidence to suggest that it did, but most trackways aren't big enough to encompass the stride a T. rex would have used when running. Several studies analyzing different aspects of tyrannosaur physiology suggest that T. rex's speed topped out at about 10 meters per second, or 22 mph, but others suggest that it may have run much faster. Researchers John Hutchinson and Mariano Garcia suggest that tyrannosaur legs couldn't have supported enough muscle to allow fast running [source: Fitzgerald].

According to Graeme D. Ruxton and David C. Houston of the University of Glasgow, T. rex could have supported itself by eating only dead animals if the ecosystem had about the same amount of carrion as today's Serengeti. The two came to this conclusion by studying the energy it would take to find food compared to the energy the food would provide the dinosaur [source: Ruxton and Houston].

So it was possible that T. rex was a scavenger -- but was it likely? Next, we'll explore some of the most common counterarguments.

One possible discovery could offer proof that Tyrannosaurus rex was a scavenger -- evidence of healed-over tyrannosaur bites in other fossils. Most fossilized dinosaurs died from events unrelated to being eaten, so it should be easy to tell whether they survived past attacks from predators. Unfortunately, there are few fossils that show clear evidence of past T. rex bites.

John W. Happ of Shenandoah University suggests a Triceratops whose skull was discovered in 1997 lived for years after being bitten by a T. rex [source: Perkins]. An Edmontosaurus fossil shows evidence of vertebral spines that re-grew after a bite that may have been inflicted by a tyrannosaur [source: Carpenter]. But there are few such examples, and the cause of the bone damage is difficult to prove conclusively. There's also the question of whether any animal would survive long enough for its T. rex bite to heal.

Some of the same evidence used to suggest that T. rex may have been a predator can be interpreted differently. For example, T. rex had tiny arms, especially compared to the prehistoric raptors. But some researchers point out that armlike forelimbs aren't a prerequisite for being a predator -- snakes, for instance, manage without them. One study also defies the idea that these limbs were useless. Kenneth Carpenter of the Denver Museum of Natural History suggests that based on the size, shape and position of the arm bones and shoulder blades, these apparently puny limbs were powerful.

Then there's the matter of the abundance of broken dinosaur teeth some interpret as evidence of eating abandoned bones. If T. rex was a predator, it might have used one of many methods to attack prey, and each could have resulted in broken teeth. Tyrannosaurs might have:

Plus, an animal as large as T. rex may have needed bones for nourishment regardless of whether it killed an animal or found a corpse.

This counterargument even gets down to the defense mechanisms of T. rex's potential prey. Was the neck frill on a Triceratops for protection or sexual display? And did it use its horn for defense against predators or combat with intruders on its territory? Since we have no way to observe the behavior of these animals, we don't really know. The most logical answer may be that Tyrannosaurus rex, like many large predators, was an opportunist, catching fresh meat when possible and eating carrion when necessary.