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Washington, Sep 17 (ANI): Tyrannosaurus rex might be known as those giant-sized dinosaurs, but for their first 80 million years, they were small-timers-no bigger than humans, say researchers.

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.

“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,” National Geographic News quoted paleontologist Stephen Brusatte, a Ph.D. student at Columbia University affiliated with New York’s American Museum of Natural History, as saying.

“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,” she said.

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. But there were a hundred million years of evolution to play with,” said Brusatte.

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.

However, Brusatte added that it is not known exactly why or how the tyrannosaurs surged in size.

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

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.

The study has been published in the journal Science. (ANI)