11 December 2009

New dinosaur solves evolutionary riddle

By
Cosmos Online
Newly unearthed fossils of a previously unknown species of carnivorous dinosaur has answered some long-standing questions about early dinosaur evolution.
Tawa hallae dinosaur

A fleshed-out reconstruction shows the newly discovered Triassic, carnivorous dinosaur, Tawa hallae. Credit: Jorge Gonzalez

SYDNEY: Newly unearthed fossils of a previously unknown species of carnivorous dinosaur has answered some long-standing questions about early dinosaur evolution.

Fossil bones of several individuals of the new species, Tawa Hallae, as well as one complete skeleton of a juvenile, were uncovered in Ghost Ranch, a renowned palaeontological site in northern New Mexico, USA. They were described in the latest issue of the U.S. journal, Science.

“The most exciting part about discovering Tawa isn’t the well-preserved fossilised skeletons the team found at Ghost Ranch,” said Sterling Nesbitt, lead author of the paper and palaeontologist from the University of Texas. “It’s what the fossils tell scientists about the history of dinosaur evolution.”

The Tawa, which lived about 215 million years ago during the late-Triassic period, is thought to have been between two to four metres long, and a metre to a metre and a half tall at the hips.

It had a long tail and a long and flexible neck and is part of a group of dinosaurs known as theropods, which includes the T. Rex and Velociraptor.

Argentinian mystery

However, the significance of the discovery of Tawa is the characteristics it shares with another dinosaur, the controversial Herrerasaurus ischigualastensis, discovered in the 1960s in Argentina.

While Herrerasaurus had some traits in common with therapods – such as large claws and carnivorous teeth – it lacked others, such as the hollow sacks in the neck vertebrae, which link the theropod to modern birds.

As a result, debates arose as to whether the Herrerasaurus was an early theropod or from a separate evolutionary tree altogether.

Now, with the discovery of the Tawa, which shared a mix of characteristics with both the Herrerasaurus and established therapod dinosaurs, it can be confirmed that the Herrerasaurus traits didn’t arise independently, and that it is in fact, like the Tawa, an early theropod.

Early evolution of dinosaurs

The discovery is significant for two reasons, according to Alex Cook, a palaeontologist from the Queensland Museum in Brisbane. For one, it tells a lot about the early diversification of dinosaurs across the supercontinent Pangaea, and secondly, it reveals that the bone structure characteristic of later therapods and modern birds were evolved very early on.

“It’s great to find such intact material from that late-Triassic period,” Alex said, pointing out that the only evidence of dinosaurs from that period in Australia to be uncovered to date are footprints. “It really adds a lot to the knowledge that we have of dinosaurs from that time.”

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