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Although its evolutionary relationship to Tyrannosaurus Rex is a bit overstated--after all, it lived about 150 million years before its more famous descendant--the early theropod Tawa still counts as a major discovery. This small, bipedal dinosaur lived 215 million years ago on the supercontinent of Pangaea, which later split into North America, South America and Africa. Based on an analysis of its remains, Tawa appears to have originated in South America, though its bones were found farther north, near the famous Ghost Ranch cite in New Mexico that's yielded countless Coelophysis skeletons.
Will Tawa really cause paleontologists to rewrite the book of dinosaur evolution, as some breathless accounts surmise? Well, it's not as if bipedal, South American, meat-eating dinosaurs were rare on the ground--witness, for example, Herrerasaurus, which we already know lay at the root of the dinosaur family tree, not to mention those numerous (though native to North America) Coelophysis specimens. Like the Asian Raptorex, another recent discovery, Tawa is being described as a miniature T. Rex, though this seems to be a gross oversimplification.
Over and above its presumed resemblance to T. Rex, what's important about Tawa is that it helps to clear up the evolutionary relationships, and ultimate origins, of the earliest theropods. With this missing piece of the fossil puzzle in place, the discoverers of Tawa have concluded that the very first dinosaurs evolved in South America in the early to middle Triassic period, then radiated out worldwide over the ensuing tens of millions of years.