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Tawa hallae
Image courtesy Jorge Gonzalez
The discovery of a dog-size T. rex ancestor may rewrite dinosaur evolutionary history, a new study says.
Measuring about 6 feet (180 centimeters) long--tail included--the 215-million-year-old Tawa hallae was found by hikers who noticed some small bits of bone at New Mexico's fossil-rich Ghost Ranch.
The dinosaur bears a mix of characteristics, such as air sacs, that link Tawa to older dinosaur species found in South America, researchers say.
The connection boosts the theory that--at a time when the continents were still linked as a single supercontinent, Pangaea--the earliest dinosaurs arose in what is now South America, according to the study, which was funded in part by the National Geographic Society's Committee for Research and Exploration and will be published December 11 in the journal Science. (The National Geographic Society owns National Geographic News.)
The dinosaurs then diversified into three lineages and migrated out to the rest of the world, scientists say.December 10, 2009
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Tawa hallae
Image courtesy Jorge Gonzalez
Telltale imprints of air sacs in Tawa fossils suggest that the new dinosaur species' neck and braincase had those features—just like modern birds. Birds are thought to have descended from theropod dinosaurs--two-legged carnivores that included Tawa (artist's conception above) and T. rex.
"It's the most primitive animal that we've seen [air sacs] in," said paleontologist Sterling Nesbitt of the University of Texas at Austin, co-author of the December 10, 2009, study.
In addition to its typically theropod traits, Tawa shares characteristics with the most primitive carnivorous dinosaurs. "The key thing is that it links this group of really early dinosaurs very nicely to later theropods," said paleontologist Hans-Dieter Sues, of the U.S. National Museum of Natural History, who was not part of the new study.
The mix of traits suggest that Tawa descended from early species found in what is now South America, such as Herrerasaurus. This would support the much disputed claim that Herrerasaurus is in fact a theropod—and suggests that all theropods came from South America.December 10, 2009
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Tawa on the Family Tree
Image courtesy Zina Deretsky, National Science Foundation
Based on Tawa's links to earlier dinosaurs, the new study suggests that the first dinosaurs evolved in what is now South America. There, they quickly differentiated into ornithischians (represented above by Triceratops), sauropodomorphs (Apatasaurus, also seen above) and theropods before migrating out in successive waves more than 220 million years ago.
Tawa's fossils were found alongside those of two other known theropod species that inhabited New Mexico at roughly the same time. The three species, though, were only very distantly related.
Instead, "each of the lineages has its closest relatives coming from South America," said Nesbitt, co-author of the December 2009 study—key evidence that theropods diversified in South America and then migrated out.
The natural history museum's Sues added, "When you think about it, it's not all that surprising. You had a gigantic supercontinent, Pangaea, and there weren't really any major obstacles to [dinosaurs] disseminating themselves around the world."December 10, 2009
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