When armored tanks walked on four legs.

When armored tanks walked on four legs

The fossilized bones of an armor-plated dinosaur have surfaced in an unlikely spot -- about 20 million years out of place.

Amateur and professional paleontologists on a dig in Colorado last summer found the specimen in 142-million-year-old rocks from the late Jurassic period, making it the oldest known ankylosaur in North America, says excavation leader James Kirkland, who announced the discovery this week. The next-oldest known ankylosaur from North America lived about 20 to 30 million years later.

The Colorado ankylosaur belongs to the family Nodosauridae--passive, plant-eating animals built like fortresses. Thick, bony armor covered their backs and heads, protecting them from the sharp teeth and claws of predators. Some nodosaurs also possessed the biological equivalent of barbed wire -- horn-like spikes jutting out from their sides. The spikes may have helped when a large attacker attempted to flip the creature over to reach its unprotected belly, says Kirkland, a paleontologist with Dinamation International Co. in Fruita, Colo.

The new find fills a major gap in the nodosaur record. The earliest specimens date back over 175 million years to the early Jurassic. With the exception of a few nodosaur fragments found in Europe, the next-oldest specimens have come from the early Cretaceous period. Until now, scientists had virtually no information on nodosaurs from the intervening span of at least 40 million years.

"That's an enormous amount of time. And the problem has been: What was going on with these animals?" says Kenneth Carpenter of the Denver Museum of Natural History. The new specimen "falls between those two intervals, so it provides us with a missing link," he says. Carpenter is currently studying the relationships among the known ankylosaurs, a suborder that includes Nodosauridae and Ankylosauridae.

Kirkland and his co-workers recognized the animal as a nodosaur after finding distinct pieces of armor and large, protective side spines. Their discovery came on the last day of the excavating season, and they plan to search for more parts of the animal this year. Kirkland says the skull may still lie buried in the quarry. From the length of the side spines, he estimates that the animal stretched about 12 feet from head to tail.

Part of the excitement arises from the fossil's origin in a late-Jurassic rock layer known as the Morrison Formation. "The Morrison is what really put dinosaurs on the map," says Kirkland, noting that it has yielded some of the best-known American dinosaurs over the past century. Through all those years of digging, however, America's Jurassic nodosaurs went undiscovered. Kirkland says last summer's find demonstrates that even in the most intensively studied sites, important fossils remain hidden, waiting to shed new light on the reptiles that once ruled the Earth.