Saturday, July 21, 2007

Coexisting with dinos

Dinos were not alone. New fossil findings suggest the earliest dinosaurs, who were not as large as the ones we are used to, lived with their primitive ancestors for close to 20 million years

The coexistence of dinosaur precursors Dromomeron romeri, lower left,and a Silesaurus-like animal, bottom centre, and the dinosaurs Chindesaurus bryansmalli, top centre, with a crocodylomorph in its mouth, and a Coelophysoid theropod, upper right, indicates that the rise of dinosaurs was prolonged rather than sudden

The ascent of the mighty dinosaurs to the throne of the animal kingdom may have been more gradual than previously believed, scientists said on Thursday.
New fossil discoveries dating from about 215 million years ago showed some of the earliest dinosaurs lived for millions of years side-by-side with related animals long seen as their ancestors and precursors, the scientists said.
Many scientists had thought these reptiles – very much like dinosaurs, but more primitive – died out around the time of the appearance of the first true dinosaurs, which were dog-sized beasts and not giants, roughly 230 million years ago.
“When dinosaurs first evolved, they were not very common and they were pretty small,” said Randall Irmis of the US-based University of California-Berkeley, who worked on the study.
“So they’re not the dominant creatures on land at all during most of the Triassic period. And it’s only until the Jurassic when they really explode in diversity and reach these huge sizes we’re so familiar with,” Irmis added.
Scientists had previously hypothesised that the first dinosaurs quickly out-competed their more primitive cousins, known as ‘basal dinosauromorphs,’ condemning them to extinction. But the new findings indicate that any such competition was prolonged.
The newly found fossils from New Mexico, dating from the Triassic period, showed that the first dinosaurs co-existed with these animals – “dinosaur wannabes,” as one scientist called them – for perhaps 15 to 20 million years.
“For the first time, we’re finding the earliest dinosaurs and their closest relatives together,” said one of the researchers, palaeontologist Kevin Padian of the University of California-Berkeley.
“That tells us that the transition to the beginning of the age of dinosaurs was not a very rapid affair and that, therefore, it wasn’t instant competitive superiority.” Irmis said these dinosaur precursors are not thought to have been direct ancestors of the dinosaurs, but rather having shared a close common ancestor.
The scientists discovered new dinosaur precursors including one 3 to 5 feet long called Dromomeron and another unnamed one about three times larger that walked on four legs and ate plants with a beaked snout.
Relatively small bipedal carnivorous dinosaurs also were found, including Chindesaurus, which measured about 6 feet long, as well as remains of an apparent close relative of the well-known Triassic dinosaur, the carnivore Coelophysis.
The fossils were found at the Hayden Quarry at Ghost Ranch, USA, a site that over the decades has yielded many exquisite fossils. For example, hundreds of Coelophysis fossils were found in the 1940s at Ghost Ranch, thus making it among the best documented of all dinosaurs.
These early Triassic dinosaurs were not the bullies and behemoths that later appeared in the Jurassic period, which started around 200 million years ago.
In fact, they were mere pipsqueaks next to some of their nasty neighbours. The scientists found remains of crocodile-like phytosaurs up to 25 feet long, and a relative of the equally long and vicious four-legged predator Postosuchus.
At the time these animals lived, the New Mexico site was a lush environment with a river system, flood plains and forests with towering large conifer trees.

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