New Mexico find sheds light on early dinosaur dispersal

Published December 10, 2009 12:04 am

Tawa hallae » Triassic-era meat-eater has primitive South American kin.
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A team of young paleontologists has identified a new species of predatory dinosaur, pulled from the Colorado Plateau's Chinle formation, that sheds new light on early dinosaur evolution and movements before the continents separated 200 million years ago.

This 6-foot, long-snouted theropod could provide an evolutionary link between primitive dinosaurs found in South America, where scientists believe dinosaurs first appeared, and more evolved meat-eaters found in North America. The team, co-led by a University of Utah geologist, will report their findings Friday in the journal Science.

The 213-million-year-old specimens also indicate early dinosaurs freely roamed far across the Earth's mega-continent, Pangea, before North and South America separated, said Randall Irmis, the Utah Museum of Natural History's curator of paleontology. This period represents the Triassic, the first phase of dinosaurs' long reign spanning 230 to 69 million years ago.

Irmis helped excavate the bones in northern New Mexico while a graduate student at Berkeley. Irmis' team dubbed the new dinosaur Tawa hallae to honor the region's rich Native American heritage. Tawa is the Hopi word for the Puebloan sun god.

The study contends the 213-million-year-old specimen provides evidence early dinosaurs roamed far across the Earth's megacontinent Pangea, before North and South America separated.

Tawa was a bipedal animal with long claws and serrated teeth that dined on the fish, amphibians and reptiles whose fossils were recovered in the same bone bed, said lead author Sterling Nesbitt, of the University of Texas at Austin.

The scientists suspect tawa had avian features, such as "proto-plummage" and lungs permeating the neck and skull. The tawa specimens are the oldest animal remains that show evidence of spinal air sacs, standard equipment on advanced theropods like T. rex and modern birds, Irmis said.

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.

"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.

"Tawa has a mix of features. The upper jaw looks like [the less evolved] herrerasaurus, but the snout look like [the more evolved] coelophysis. It fits the morphological gap," Nesbitt said.

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."

Meanwhile, scientists remain puzzled by the complete absence in North America's Triassic formations of the two plant-eating dinosaur lineages, massive long-necked sauropods and armored and duck-billed ornithischians, such as triceratops.

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."

bmaffly@sltrib.com" Target="_BLANK">bmaffly@sltrib.com

Tawa hallae exhibit Saturday

Where » Utah Museum of Natural History, University of Utah, Presidents Circle

When » Saturday, noon to 4 p.m.

What » Fossil specimens from this newly named species of meat-eating dinosaur will be on display in the lobby of the museum starting Saturday. Curator of paleontology Randall Irmis helped excavate and identify this 213-million-year-old creature recovered from Chinle formation in New Mexico's Ghost Ranch. You can hear Irmis discuss the discovery and dinosaur science Saturday afternoon.

More info » The National Science Foundation, which supported Irmis' research, posted a video and other materials at http://www.nsf.gov/news/special_reports/tawa" Target="_BLANK">here.



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