Normally, Goodwin said, top predators are one-third or one-fourth as abundant as their prey, because of the larger energy needs of carnivores. Opportunistic hunters like the hyena, however, can be twice as abundant as the top predators.
"If you count the lions and the leopards and the cheetahs in the Serengeti, the number still does not equal the number of hyenas, because hyenas have a much wider food source," Horner said. "Cheetahs, for example, only go after things that are really fast. They don't eat turtles. But a hyena will eat a turtle, or anything else that it can catch or is dead."
Similarly, T. rex was eating anything it could, he said. "There's no evidence that T. rex could run very fast, so it wasn't out there being a cheetah. If it could get a sick animal, it would."
Horner suggests that juvenile and young adult T. rex may have been primarily flesh eaters, while the older adults, which developed proportionally larger, bone-crushing teeth as they aged, also consumed the bones and marrow of their prey.
Horner and Goodwin, together and separately, have been digging for dinosaurs in Eastern Montana for decades. The fossils date from a time when the area bordered an inland sea, which periodically advanced and withdrew over coastal plains, depositing sediment that was later exposed and heavily eroded. When Horner started his census of dinosaurs in the Hell Creek Formation around Fort Peck Lake in 1999, he teamed up with Goodwin to re-examine some of the dinosaurs discovered in the area.
Since then, through lab analysis and annual summer digs, they have shown that one named species, Torosaurus, was just a big, aged Triceratops; two dome-headed dinosaurs, Dracorex and Stygimoloch, were merely younger members of the genus Pachycephalosaurus; and the so-called Nanotyrannus was just a juvenile T. rex.
Once these fossils had been properly iden
|Contact: Robert Sanders|
University of California - Berkeley