Tyrannosaurus rex may have towered over its Cretaceous competition, but for their first 80 million years, most tyrannosaur species were small-timers—no bigger than humans, researchers say.

Recent fossil finds—including six new tyrannosaur species last year alone—suggest that T. rex's genus had a mysterious growth spurt relatively late in its lineage, according to a review of tyrannosaur fossils in tomorrow's issue of the journal Science.

"Ten years ago we only knew about five or six different tyrannosaur species, and they were all very similar—giant apex predators like T. rex," said paleontologist Stephen Brusatte, a Ph.D. student at Columbia University affiliated with New York's American Museum of Natural History.

"Now we have 20 tyrannosaurs, spanning a hundred million years through the Jurassic and Cretaceous," said Brusatte, who co-authored the new review.

"They range in size from small dogs all the way up to T. rex," which could reach 40 feet (12 meters) from nose to tail tip.

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Tyrannosaurs originated in the Middle Jurassic period, about 165 million years ago. Though they generally remained small for 80 million years, early tyrannosaur species resembled T. rex in that they were bipedal predators with "incisor like" teeth (prehistoric time line).

But significant physical differences are also in evidence. Some animals had longer arms, for instance, or relatively small heads.

"There is quite a difference between the oldest species and T. rex," Brusatte said. "But there were a hundred million years of evolution to play with."

University of Maryland tyrannosaur expert Thomas Holtz, Jr., added: "I like to call [early tyrannosaurs] the jackals of the Jurassic and Early Cretaceous.

"They were tough little guys, but they were little guys, sort of hanging out in the wings and taking out young dinosaurs and small dinosaurs but leaving the big prey to things like Allosaurus," said Holtz, who was not involved in the new review.

Tyrannosaur Growth Spurt

It's not known exactly why or how the tyrannosaurs surged in size, study co-author Brusatte said.

"About 80 million years ago, they became not just huge in a physical sense but also in an ecological sense," he said. "They became dominant, apex predators."

Unfortunately for paleontologists, this relatively sudden evolutionary shift took place during a 15-million-year Middle Cretaceous period that's poorly represented in the fossil record.

What scientists do know is "that for the first 80 million years there were other groups of large, carnivorous dinosaur predators," said Brusatte, referring to the allosauroids and megalosauroids.

"So for most of their history, the tyrannosaurs were kept in check. Then, for some reason, most of these groups went extinct, and tyrannosaurs had the opportunity to flourish."

Raising Dinosaurs From the Dead

Even as new tyrannosaur fossil finds have provided precious evidence about the lineage, high-tech analyses have produced a much clearer picture of what T. rex and its evolutionary cousins were like in life.

For example, massive T. rex, some say, was likely slow moving but possessed of sharp senses of smell and hearing and—of course—the ability to deliver a bone-splitting bite. (Video: T. Rex's Bone-Shattering Bite Filmed.)

Tyrannosaur abilities and behaviors are being reconstructed with imaging techniques such as CT scans, which reveal brain size and inner ear structures. Also, computer-driven biomechanical models are incorporating masses of data into programs that determine muscle strength or reveal how a dinosaur might have used its legs.

Fossils from some species, including T. rex, have also been uncovered in various stages of life. This information—fed into software that correlates body sizes and age data—is allowing scientists to virtually watch the animals grow and mature the way biologists do with living species.

It all adds up to a much clearer understanding of a genus of dinosaur that is not quite as familiar as we once thought.