Rare 225-Million-Year-Old Fossils Unearthed : Science: The find confirms speculation that mammal-like reptiles co-existed with dinosaurs and throws some light on a supercontinent puzzle.

Researchers have discovered a stunning, 225-million-year-old collection of rare fossils, unique in North America, in a suburb of Richmond, Va.

The discovery, to be announced today by the National Geographic Society, confirms speculation that mammal-like reptiles called cynodonts co-existed with dinosaurs, “barely hanging on” for 140 million years until the dinosaurs suffered a cataclysmic demise 65 million years ago, according to paleobiologist Hans-Dieter Sues of the Smithsonian Institution.

Only when the dinosaurs were wiped out, perhaps by an asteroid that created winter-like conditions throughout the world, were the tiny mammals able “to get a lucky break” and proliferate extensively, Sues said.

Among the fossils recovered by Sues and his colleague, geologist Paul E. Olson of Columbia University’s Lamont-Doherty Geological Observatory, are those of at least four previously unknown lizard-like animals and a strange reptile, four or five feet long, that was armored with big, bony back plates and large, rigid spines.

The researchers have already identified fossils of more than 200 different species from the deposit, Sues said, “and so far we have just gone through and gotten the obvious things.”

The new discovery solves a puzzle that has confused paleontologists for many years. During the late Triassic period from which the fossils date, all the land masses of the Earth were combined in one so-called supercontinent known as Pangaea.

Researchers have found fossils believed to be from the late Triassic in Argentina, Brazil and South Africa. But previously, only a cynodont jaw and a few bone fragments had been found in the United States, leading researchers to speculate that the animals may have inhabited only a limited region.

“We thought it had something to do with geography,” Olson said. “Now, by finding so many mammal-like reptiles and other creatures common to . . . the Southern Hemisphere, we can assume that the same creatures probably roamed throughout Pangaea.”

The discovery “makes a nice connection between various parts of the world at that time,” according to paleontologist James Hopson of the University of Chicago, a cynodont expert.

The fossil discovery is particularly valuable, Hopson said, because “the quality of the material is excellent.”

“It’s beautifully preserved,” he said.

Among the fossils Sues has identified are 11 jaws of cynodonts. Five of them are minuscule--one less than a centimeter long--suggesting that they come from hatchlings. The adults were also not large, perhaps the size of shrews, which they probably resembled.

These cynodonts, which eventually evolved into mammals, might have had fur, Sues believes, and their bone and teeth structure look more mammalian than that of their simultaneously evolving reptilian cousins.

According to Hopson, it is unusual to find a broad size range that includes juveniles and adults.

“That gives us some ideas about how dentition changes as the animals grew up,” he said. “That’s very useful because (the cynodonts) show the transition from a reptilian type of tooth structure to a mammalian type. . . . If we have a growth series, it helps to explain what is going on.”

Sues also noted that if the four previously unknown lizard-like specimens are confirmed to be lizards, “they will be the oldest found anywhere in the world.”

The location of the fossil group was discovered by Olson when he stumbled over a cynodont jaw fragment while surveying for oil and gas in the area. He showed it to Sues, who began excavating the site a little more than a year ago.

So far, most of the fossils have come from a hole 30 feet long, 3 feet wide, and about 8 feet deep. The fossils were found in a layer about 10 inches thick at the bottom of the hole.

The large number of fossils they have obtained from the dig is “amazing when you consider that the only places in the world that have produced comparable numbers of fossils from this period have covered many square miles,” Sues said.

Sues has also found other, smaller pockets of fossils in the area and noted that much larger numbers of fossils were probably destroyed by weathering.

At the time the fossils were deposited, the area that is now Virginia was located much closer to the Equator, probably at a latitude comparable to that of central Mexico. The area was undoubtedly covered with lush, subtropical vegetation, ponds and perhaps a large lake or waterway. Sues speculates that a shallow pond may have drawn the animals to the site.