Dinosaur find in New Mexico tells of greater migration, evolution

New Mexico find sheds light on early dinosaur dispersal
Tawa hallae » Triassic-era meat-eater has primitive South American kin
By Brian Maffly, The Salt Lake Tribune - Updated: 12/11/2009 07:26:24 AM MST

A team of young paleontologists has identified a new species of predatory dinosaur that sheds new light on early dinosaur evolution and movements before the continents separated 200 million years ago.
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Randall Irmis, the Utah Museum of Natural History's curator of paleontology, helped excavate the bones in northern New Mexico while a graduate student at Berkeleyand published the study with lead author Sterling Nesbitt, of the University of Texas at Austin.
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Irmis and Nesbitt's team includes co-authors Nathan Smith of the Field Museum of Natural History, Chicago; Alan Turner of Stony Brook University in New York; and Mark Norell of the American Museum of Natural History, New York City. Also named is Alex Downs, the curator of the paleontology museum at Ghost Ranch, a 21,000-acre spread near Abiquiu made famous by painter Georgia O'Keeffe. But Ghost Ranch is also famous as the world's richest trove of fossil remains of coelophysis, another small theropod.

Downs said a visiting group in 2004 came across a hip and thigh that did not belong to a coelophysis. He notified Irmis and his colleagues, all graduate students then under 30, known to have an interest in early dinosaur evolution. The group converged on New Mexico the following year and struck pay dirt in the Chinle's colorful clay deposit at Ghost Ranch.

This formation is common across southern Utah, presenting as talus slopes beneath Windgate sandstone cliffs. At the time the formation was deposited, the Colorado Plateau was near the equator and its climate was warm and wet.

The scientists recovered five to seven sub-adult specimens that together built a complete skeleton of a previously unknown creature. Scientists believe these individuals were swept up in a flood and quickly interred together with other Triassic animals and plants.
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"The preservation of tawa is exquisite. Tawa was in soft rock and very easy to prepare," Nesbitt said. The "nearly pristine" detail on the articulated bones was so fine, the team could make out muscle scars, construct a complete skull and see how ligament attached to bone and how thighs engaged with hips.

As a result, Tawa is now among the most completely known Triassic theropods and a real gift to science, Irmis said. With such a clear view of this animal's anatomy, the team could perform reliable morphological comparisons with other theropods from around the world. This is where things got interesting.

The specimens were found in the same deposits as two other early predators, coelophysis and chindesaurus. But these three are not each others' closest relatives. Tawa's next-of-kin appears to be a South American proto-theropod called herrerasaurus.
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The team was most intrigued with what this suggests about the trans-Pangea movements of dinosaurs.

"The discovery of multiple dinosaur species in one place that emigrated from elsewhere got us wondering whether other Late Triassic reptiles show similar patterns," said Irmis. "It turns out a variety of other reptile groups made multiple trips from the northern and southern continents [then parts of Pangea] and back again during the Late Triassic, including other dinosaurs."
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Why were theropods dispersing into North America, but not their plant-eating cousins? Perhaps differences in plant life made this terrain uninhabitable for early vegetarian dinosaurs, Irmis speculates.

"Because so many other reptile groups were crisscrossing Pangea just fine, it suggests there were no big physical barriers like mountain ranges," he said. "Instead, the absence in North America of plant eating dinosaurs during the Triassic suggests that the barriers related to climate."