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Dinosaurs' Rise Was Slow, Not "Lucky Break," New Fossils Suggest

Susan Brown
for National Geographic News
July 19, 2007
 
A new species of dinosaur ancestor is among a fossil trove recently uncovered in New Mexico that suggests the rise of the dinosaurs was a gradual process.

The find counters the theory that dinos came to dominate the landscape suddenly as the result of an evolutionary "lucky break."

Until now, fossils of dinosaur precursors had been found only in rocks more than 230 million years old. The first true dinosaurs were found in much younger deposits.

(Related: "T. Rex's Oldest Ancestor Discovered in China" [February 8, 2006].)

This lack of overlap led many experts to conclude that dinosaurs had burst onto the scene after intense competition or a dramatic extinction event wiped out their predecessors.

But the latest bounty of bones from late Triassic rocks—between 210 million and 220 million years old—includes fossils of several different kinds of dinosaur relatives alongside those of early true dinosaurs.

The mixed assembly led the paleontologists who found the fossils to conclude that the two groups lived side-by-side for 15 million to 20 million years.

"For the first time anywhere in the world we found a variety of these dinosaur precursors with true dinosaurs," said Randall Irmis, a paleontologist at the University of California, Berkeley, who helped lead the dig.

"That suggests that the rise of the dinosaurs was gradual, rather than sudden."

Irmis and colleagues describe the newfound fossils in tomorrow's issue of the journal Science.

Mixed Bag

Hikers first noticed fossil bones a few years ago at the base of a hillside on Ghost Ranch in Abiquiu—once frequented by painter Georgia O'Keefe—which lies about 40 miles (64 kilometers) north of Española (see a map of New Mexico).

With partial funding from the National Geographic Society, Irmis and his colleague Sterling Nesbitt of the American Museum of Natural History in New York excavated at the site last summer.

(National Geographic News is part of the National Geographic Society.)

Among their finds was a new species of dinosaur precursor that they named Dromomeron romeri.

The creature was very small, no more than three to four feet (about a meter) long, Irmis said. D. romeri's only known close relative was found in Argentina in rocks at least 15 million years older.

Another as yet unnamed dinosaur relative from the site is related to Silesaurus, a fossil found in Poland four years ago.

Paleontologists initially thought Silesaurus was a dinosaur, but it has since been demoted to dinosaurlike status.

Within the same 10-by-20-foot (3-by-6-meter) plot, the team also found bones of true dinosaurs.

One unnamed dino species is closely related to the well-known Ceolophysis, tiny dinosaurs that were at most 9 to 10 feet (3 meters) long, were bipedal, and were probably carnivorous, Irmis said.

Dino Boom

Despite the dino diversity, most of the bones found at the site were from fish, amphibians, and a different group of reptiles.

This fits the picture of the known Triassic landscape, which was dominated by large carnivorous beasts called rauisuchians and the ancestors of crocodiles.

But researchers have long wondered why this branch of reptiles withered to just one line of early crocodiles by the beginning of the Jurassic, about 200 million years ago.

The scientists hope the fossils will finally shed light on why the dinosaurs of the Triassic and their relatives, which were small and rare by comparison, eventually took over.

"If you'd gone back to that time, you wouldn't have believed [dinosaurs] were going to rule the Earth," said Michael Benton, a paleontologist at Bristol University in England who was not part of the expedition.

The latest find will help make clear how dinosaurs and their close relatives fit into the Triassic world, Benton said.

"That gives us a more truthful picture of the origin of the dinosaurs."

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