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Rare Fossil Sheds Light On Dinosaur Evolutionary Tree

December 10, 2009

A team of paleontologists has unearthed a previously unknown meat-eating dinosaur from a fossil bone bed in northern New Mexico.

The discovery settles a long-standing debate about early dinosaur evolution, and reveals a period of explosive diversification.  It also hints at how dinosaurs spread across the supercontinent Pangaea.

A description of the new species, named Tawa after the Hopi word for the Puebloan sun god, appears in the Dec. 10 issue of the journal Science.

The paleontologists said the 213-million-year-old fossils were the best preserved dinosaur skeletons ever found from the Triassic Period.

Tawa is part of a group of dinosaurs known as theropods, which includes T. Rex and Velociraptor.  These dinosaurs primarily ate meat, walked on two legs and had feathers.  And while most went extinct by 65 million years ago, some lineages survived to spawn modern birds.

The paleontologists recovered the fossil bones of several individual dinosaurs, but the type specimen is a nearly complete skeleton of a juvenile that stood about 28 inches tall at the hips and was 6 feet long from snout to tail.   Its body was roughly the size of a large dog, but with a much longer tail.   

One of Tawa’s most important contributions to science involves what it says about another dinosaur, Herrerasaurus, the center of a contentious debate since its discovery in Argentina in the 1960s.

Herrerasaurus had some traits in common with theropods, such as large claws, carnivorous teeth and certain pelvic features.  However, it lacked other theropod traits such as pockets in vertebrae for airsacs. Some paleontologists claimed it was so atypical that it was outside the evolutionary tree of theropods, or even of dinosaurs. Others placed it among the earliest theropods.

“The question was did those carnivorous traits arise in Herrerasaurus and in theropods independently or were they traits from a recent common ancestor that got passed down,” said Sterling Nesbitt, a postdoctoral researcher at The University of Texas at Austin’s Jackson School of Geosciences.

Nesbitt, the lead author of the report, conducted the research with his colleagues while a graduate student at Columbia University’s Lamont-Doherty Earth Observatory and the American Museum of Natural History.

“We had so few specimens of early theropods that it was hard to answer that question. But now that we have Tawa, we think we have an answer,” he said.

The researchers found that Tawa had a mix of Herrerasaurus-like characteristics (such as in the pelvis) along with features found in firmly established theropod dinosaurs (such as pockets for airsacs in the backbone).

The characteristics that Herrerasaurus shares uniquely with theropods such as Tawa confirm that these features didn’t arise independently and that Herrerasaurus is indeed a theropod.

“This new dinosaur Tawa hallae changes our understanding of the relationships of early dinosaurs, and provides fantastic insight into the evolution of the skeleton of the first carnivorous dinosaurs” said Randall Irmis of the Utah Museum of Natural History and University of Utah, a co-author of the study.

The firm placement of Herrerasaurus within the theropod lineage brings to light an interesting fact about dinosaur evolution: once they appeared, they very rapidly diversified into the three main dinosaur lineages that persisted for more than 170 million years.

Herrerasaurus was found in a South American rock layer alongside the oldest members of two major lineages””the sauropods and the ornithischians.

“Tawa pulls Herrerasaurus into the theropod lineage, so that means all three lineages are present in South America pretty much as soon as dinosaurs evolved,” Nesbitt said.

“Without Tawa, you can guess at that, but Tawa helps shore up that argument.”

Tawa skeletons were found beside two other theropod dinosaurs from around the same period. 

“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.”

According to Nesbitt, each of the three is more closely related to a known dinosaur from South America than they are to each other. This suggests the three species each descended from a separate lineage in South America, rather than all evolving from a local ancestor, and then later dispersed to North America and other parts of the supercontinent Pangaea. It also indicates there were multiple dispersals out of South America.

Because so many different groups with different life modes were able to move freely across Pangea, the researchers concluded that during the Late Triassic, there were no major physical barriers, such as large mountain ranges, to the movement of reptiles between parts of Pangea that later separated into distinct continents.

However, this presented a paradox to the team.

“We wondered: if reptiles, including dinosaurs, were able to freely move around Pangea during the Late Triassic, then why aren’t there any sauropodomorph and ornithischian dinosaurs in North America during the Triassic?” said Irmis.

“Our conclusion is that climate, possibly related to latitude, controlled the distributions of some reptile species.”

“We think that all the major dinosaur groups had the ability to get to North America [part of Pangea] during the Late Triassic, and may have even passed through, but for some reason, only the carnivorous dinosaurs found the North American climate to be hospitable during this time,” said Irmis.

The first Tawa fossils were discovered in 2004 by volunteers taking a paleontology seminar in Abiquiu, New Mexico. That dig site, known as Hayden Quarry, is in a hillside on Ghost Ranch made famous by the painter Georgia O’Keefe.

Alex Downs, an instructor for the course, contacted Nesbitt and a colleague to see if they were interested in viewing the fossils, which included a thigh bone, part of a hip and what later turned out to be some unrelated vertebrae.

“When we saw them, our jaws dropped,” said Nesbitt.

“The specimens are unusual because they are so well preserved,” said Irmis.

“Because dinosaur bones are hollow, they are usually broken and crushed, but those of Tawa are nearly pristine.”

Nesbitt agreed.

“A lot of these theropods have really hollow bones, so when they get preserved, they get really crunched. But these were in almost perfect condition,” he said.

Nesbitt was surprised by how much material was preserved at the New Mexico site.  He and his colleagues began a full-scale excavation in 2006, and have continued to unearth new material every summer since then.  The fossil bone bed extends for tens of yards along the hillside, offering the hope of additional discoveries.

In addition to Nesbitt and Irmis, authors of the study included Nathan Smith of the University of Chicago and the Field Museum of Natural History, Alan Turner of Stony Brook University, Alex Downs of the Ruth Hall Museum of Paleontology and Mark Norell of the American Museum of Natural History.

Image 1: A reconstruction of the newly discovered Triassic, carnivorous dinosaur, Tawa hallae. Credit: ©Artwork by Jorge Gonzalez.

Image 2: Based on an analysis of how Tawa relates to other early dinosaurs, researchers hypothesize that dinosaurs originated in what is now South America, and soon after diverged into ornithischians (like Triceratops), sauropodomorphs (like Apatosaurus) and theropods (like Tyrannosaurus rex), before dispersing across the Triassic world more than 220 million years ago. The theropods evolved into modern-day birds, although Tawa split off from the ancestral branch early on and was not a direct bird ancestor. Credit: Zina Deretsky, National Science Foundation

Image 3: A reconstruction of the Tawa hallae skeleton. Credit: Sterling Nesbitt

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