Jul 20, 2007
UC Berkeley scientists, digging deep into a remote New Mexico hillside, have discovered a trove of fossil bones that they say is evidence that dinosaurs and their early relatives lived side by side for tens of millions of years before the relatives slowly died off and left the dinosaurs to dominate the ancient world.
Until now many scientists had thought that dinosaur "precursors" -- perhaps their ancestors -- disappeared suddenly long before the dinosaurs themselves rose to prominence, but the bones dug up by Berkeley paleontologists show evidence of a different story.
The discovery of a wide variety of creatures all mingled together in layer upon layer of rocks dating from Earth's late Triassic period between 235 million and 200 million years ago, they say, shows that the strange relatives of the dinosaurs remained on the scene while the dinosaurs evolved into truly dominant creatures during the Jurassic period, between 120 million and 200 million years ago.
Until now, many scientists have argued that the early close relatives of dinosaurs must have disappeared abruptly in an early "mass extinction" about 215 million years ago that has never been clearly explained. Others have thought that the true dinosaurs, whether carnivores or plant eaters, simply outcompeted their relatives for dominance in the ancient environment and quickly drove them to extinction.
But the new findings show clearly that the disappearance of what noted Berkeley paleontologist Kevin Padian calls "the dino wannabes" was a long, very slow process.
The scene of this latest dinosaur discovery is New Mexico's fabled Ghost Ranch -- a modest cluster of buildings in a spectacular landscape of mountains, cliffs and canyons where Georgia O'Keeffe once lived and drew inspiration for her paintings from the red-rock mesas and the stark, bleached animal skulls strewn about the desert floor.
Two of Padian's graduate students, Randall Irmis at Berkeley and Sterling Nesbitt, who is now working at the American Museum of Natural History, led the dig for the past two years, and have recovered more than 2,300 fossil specimens, from dinosaur thigh bones a foot long to microscopic fish scales. A report on the team's discoveries is being published today in the journal Science.
The fossils date from a time when all the continents of the world were massed into one "supercontinent" now called Pangea -- long before the process of continental drift began splitting Pangea into separate land masses -- and the fossil site was then located at the equator.
"This is the first time anywhere that we've found dinosaurs together with their closest relatives," said Padian, "and the dinosaurs obviously lived with those guys for a long, long time."
Although Padian and his colleagues will not say that his wannabes are direct ancestors of the dinosaurs, Anthony Fraser, a paleontologist at the Virginia Museum of Natural History, thinks that's possible.
Fraser was not connected with the Berkeley fossil-hunting team, but in an interview, he praised their work and the fossils they found. "Those guys are surely on their way to being dinosaurs," he said of the mysterious dinosaur precursors. "They're in the same lineage, at least, but we may never know who's an ancestor of what."
Among Padian's wannabes -- collectively known as "basal dinosauromorphs" -- are the bones of a species that has never been seen before, but is clearly at least an early relative of the dinosaur. The team has named it Dromomeron romeri, and it may have been, according to Irmis, a two-legged animal and probably a swift runner.
Another precursor relative puzzles the team completely because its fossilized remains are so fragmentary. It may be a long-gone creature named silesaurus, which was first identified when its bones were dug up in Poland about seven years ago. Researchers believe it must have been a large reptile about 7 feet long with a big, toothless beak, indicating it was probably a plant eater. "It's very bizarre," says Irmis of his team's Ghost Ranch find.
Mixed with those fossils in the same rock layers are the bones of several species of small true dinosaurs, none much larger than 6 feet long, the team reports. Among them was one known as chindesaurus, a meat eater that ran swiftly on two legs, and another related to the carnivorous dinosaur coelophysis -- both reminiscent of the much later velociraptors, the vicious pack hunters of "Jurassic Park."
All the fossils at Ghost Ranch are curious, if not bizarre, and among the varied mix are the remains of amphibians that must have looked like frogs crossed with crocodiles; beasts called "eagle lizards" with coats of heavy armor plates; and the four-legged evolutionary ancestors of today's crocodiles that are known to share a common ancestry with dinosaurs.
All these and more have come from the cutaway side of a hill at Ghost Ranch called the Hayden Quarry, and the entire area around the site is marked by other quarries where other scientists have found hundreds upon hundreds of dinosaurs and their precursor relatives in different layers -- but never so abundantly and so clearly side by side.
In fact, virtually all the Southwest, including much of New Mexico and Arizona, is underlain by rocks called the Chinle Formation, where sedimentary rocks were laid down by ancient rivers between 250 million and 200 million years ago.
The Chinle Formation's rocks are a rich burial ground for countless groups of long-extinct animals -- including mammals, lizards, crocodiles, turtles and frogs -- and, pieced together, their bones will surely lead to more insights into the evolution of many modern animals -- including the "modern dinosaurs" alive today, known more prosaically as birds.
Irmis kept field notes of the team's most recent work at Ghost Ranch, which ended only a month ago. "Relatively few people have concentrated on the origin of the dinosaurs," he wrote. "Where did they come from? How did they diversify? Why were they more successful than some of their early contemporaries? When did dinosaurs first get big?"
These questions will continue to puzzle the team during next year's dig at Ghost Ranch, and for many years to come.