By Judith Burns
Science reporter, BBC News
A 3m-long dinosaur fossil from China which predates T. rex by 60 million years is a blueprint for the mighty carnivore, say researchers.
They tell Science magazine that the fossil displays the same features as T. rex but in miniature.
The new species, Raptorex kriegsteini , would have weighed around 65kg; its descendants were 90 times as massive.
Scientists believe it could be the "missing link" between earlier species of dinosaur and T. rex.
The 125-million-year-old specimen suggests that T. rex's characteristic big head with enhanced jaw, relatively small forearms and huge back legs were inherited from this much smaller dinosaur, and that the body type changed little over millions of years except in size.
Long-armed relatives
In the past 10 years, fossils from earlier and more primitive species have been discovered in rocks between 100 and 176 million years old (Middle Jurassic to Early Cretaceous Periods) in Europe, North America and China.
“ In short, much of what we thought we know about tyrannosaur evolution turns out to be either simplistic or out-and-out wrong ”
Stephen Brusatte, American Museum of Natural History
These previous finds have confirmed that the Tyrannosaur family of dinosaurs is descended from small-bodied, long-armed predecessors.
But there has been little evidence of a direct link between these relatively small dinosaurs and the massive fossils of T. rex.
The team believes that Raptorex kriegsteini is that link. It looks identical to a big T. rex , even down to the shape of the skull with its enlarged olfactory bulbs, which would have given it the same acute sense of smell as T. rex.
Lead researcher Dr Paul Sereno, from the University of Chicago, said: "It's as close to the proverbial missing link on a lineage as we might ever get for T. rex.
"From the teeth to the enlarged jaw muscles, the enlarged head, the small forelimbs, the lanky, running, long hind-limbs with the compressed foot for hunting prey: we see this all, to our great surprise, in an animal that is basically the body weight of a human or 1/90th the size that ultimately this lineage would reach in T. rex at the end of the Cretaceous."
Theory overturned
The team believes that the new fossil completely overturns accepted opinion on the evolution of tyrannosaurs.
Until now it had been thought that their strange body shape evolved as a consequence of their large size.
The fossil record bears out the previous theory that as tyrannosaurs developed truly giant size over time, they needed to modify their entire skeletons so they could continue to function as a predators as they grew.
According to co-author Stephen Brusatte of the American Museum of Natural History in New York: " Raptorex, the new species, really throws a wrench into this observed pattern.
"Here we have an animal that's 1/90th or 1/100th of the size of T. rex, about my size, but with all the signature features - the big head, the strong muscles, the tiny little arms - that were thought to be necessary adaptations for a large body predator.
"So really we can say that these features did not evolve as a consequence of large body size but rather that they evolved as an efficient set of predatory weapons in an animal that was just 1/100th of the size of T. rex and that lived 60 million years before T. rex. "
The fossil record tells us that tyrannosaurs only grew to huge sizes during the final 20 million years of the Cretaceous.
Dr Brusatte said: "So that means that for most of their evolutionary history, about 80% of the time that they were on Earth, tyrannosaurs were small animals that lived in the shadow of other types of very large dinosaur predators.
"In short, much of what we thought we know about tyrannosaur evolution turns out to be either simplistic or out-and-out wrong."
...
Read more here/Leia mais aqui.
