Published online 10 December 2009 | Nature | doi:10.1038/news.2009.1137

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Dinosaurs diversified before spreading around the world

Fossil finds suggest that South America was the wellspring of the dinosaurs.

dinosaurAn artist's conception of the early dinosaur, Tawa hallae.Jorge Gonzalez

Fossils found in the US state of New Mexico are providing strong evidence that dinosaurs originated in what is now South America, and had already evolved into three main groups before spreading around the world.

The fossils — of a new species, named Tawa hallae — back more than 200 million years, to when Earth's land masses were joined together as the supercontinent Pangaea. They retain features from the earliest dinosaur specimens, found in South America.

Dinosaurs are thought to have first evolved about 230 million years ago, their populations yo-yoing until a cataclysmic event wiped them out about 65 million years ago.

The find suggests that the early dinosaurs — classified as theropods, sauropods and ornithischians — migrated from South America around the rest of Pangaea roughly 220 million years ago. The animals then settled in the most suitable climates at different latitudes, says palaeontologist Sterling Nesbitt, of the University of Texas at Austin, who led the study.

Nicknamed Tawa, the two-legged creature studied by Nesbitt's team lived about 215 million years ago, and shares key features with the earliest known theropods, including air sacs in bones, hips with open sockets, and teeth and claws that suggest they were meat-eaters.

At least five Tawa specimens were found together in the Hayden Quarry on the Ghost Ranch, Abiquiu, once the retreat of American artist Georgia O'Keefe. One specimen is a near-complete skeleton of a juvenile — which stood 70 centimetres high at the hips and was about two metres long. There is also another larger, near-complete skeleton, thought to be an adult.

Quality find

The first Tawa was found in 2004, when people attending a seminar at the Ruth Hall Museum of Paleontology in Abiquiu were on a hike in the region and found protruding remains of the dinosaur. A team of four graduate students studying with the museum's curator, Alex Downs, then excavated the fossils.

mountainsThe hills of Ghost Ranch, New Mexico - home of the Tawa specimens.Sterling Nesbitt

The specimens were miraculously pristine, with most of the bones still in a natural three-dimensional form, not crushed like typical fossils of their age. This also allowed for better identification of the air sacs in the vertebrae, a feature typical of theropods. Team members are still trying to work out the function of these cavities — which are like those found in today's birds, relatives of theropods.

Surmising that the Tawa individuals probably died and were quickly buried, says Nathan Smith, a co-author from the Field Museum in Chicago. "We were lucky to find skeletons nearly intact."

Randall Irmis, a palaeontologist at the University of Utah in Salt Lake City and co-author of the study, says that their conclusions about the migration patterns are supported by the presence in the Hayden Quarry of multiple dinosaur species known to have originated much farther south.

"We think all the major dinosaur groups had the ability to get to [what is now] North America during the late Triassic," says Irmis. "Only the carnivorous dinosaurs found the North American climate to be hospitable."

Migratory origins

An important aspect of Tawa's discovery is the context it provides for a long-debated, mysterious specimen from South America, called Herrerasaurus ischigualastensis. First identified in southern Brazil by the Argentinian palaeontologist Osvaldo Reig, H. ischigualastensis had a mosaic of traits that meant it was difficult to define as a theropod, or even to be sure it was a dinosaur. It had the typical carnivore traits, but no air sacs in its bones, according to later reports on more defined specimens from Argentina2. But other similarities with Tawa mark out H. ischigualastensis as a theropod, says Nesbitt.

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Kevin Padian, a palaeontologist at the University of California, Berkeley, who mentored both Nesbitt and Irmis and who has long argued that H. ischigualastensis may not be a theropod, isn't convinced. "I'm not disagreeing on Tawa — it's a great step — but the book is not closed," he says, adding that he is looking for more primitive creatures to settle the question.

Researchers expect a renewed hunt for the first dinosaurs — a trail likely to go to the Valley of the Moon in the province of San Juan, Argentina, the area where the oldest dinosaur fossils to date have been found. 

  • References

    1. Nesbitt, S. J. et al. Science 326, 1530-1533 (2009). | Article | ChemPort |
    2. Sereno, P. C. & Novas, F. E. Science 258, 1137-1140 (1992). | Article | PubMed
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