Utah scientific: Drought may have killed San Rafael dinosaurs

Published: Monday, April 30 2007 12:06 a.m. MDT

A cast of an allosaurus towers above visitors at the Cleveland-Lloyd Dinosaur Quarry visitors center, which was rededicated Saturday.

Kelly Rigby, BLM

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A University of Utah graduate student may have found the answer to one of the big mysteries in paleontology: Why did scores of dinosaurs die in the San Rafael Swell?

The site is the Cleveland-Lloyd Dinosaur Quarry about 30 miles south of Price. Most of the allosaurus skeletons in the world's museums come from this desert quarry.

On Saturday, the Bureau of Land Management rededicated the quarry's visitor center, filled with new exhibits and double the size of the original 1968 structure. About 110 attended, despite the necessity of traveling the final 12 miles on a dirt road.

"It was a fantastic ceremony," said Terry Gates, who is in line to receive his doctorate in geology at the U. this Friday. "It was beautiful weather."

Gates analyzed the site's bones, attempting to determine what caused the mass death of the great creatures.

About 20,000 dinosaur bones have been found at Cleveland-Lloyd, and undoubtedly more are waiting to be excavated. By far most are those of the meat-eating allosaurus, a two-ton, up to 35-foot-long monster with serrated teeth. But also found are bones of the carnivorous ceratosaurus; camptosaurus, a lumbering elephant of a plant-eater; and camarasaurus, a long-necked vegetarian.

What happened to these animals 147 million years ago?

"It's not a flood deposit, it's not the remnants of a swampy bog, it's not a river deposit," said Michael Leschin, the BLM employee responsible for operations at Cleveland-Lloyd.

What the site does represent, he added, is "the densest concentration of Jurassic dinosaur bones ever found." The bones are found disarticulated, that is, not connected in skeletons. But they are in good shape, indicating they didn't tumble downstream in massive floods.

For his master's degree research, Gates performed what he calls "a kind of prehistoric forensic analysis report." (His doctorate thesis is on another dinosaur subject.)

For the forensic study, he and others carried out new digging in two spots adjacent to the earlier excavation, still inside the quarry building.

"These were the first broad-scale excavations that took place in the quarry in decades," he said. "We collected about 400 bones from our excavation."

They carefully measured the bones' orientation, examined how close they were to each other, mapped them and took sediment samples. The bones were prepared at the Utah Museum of Natural History on the University of Utah campus, where Gates works.

He studied the fossils for signs of modification. "Were these bones stepped on back in the Jurassic before they were buried? Were they being scavenged? Were there tooth marks" on them from the jaws of other dinosaurs?

This data set was compared with modern groupings of animal bones to check what sort of process today "closely mimics" the Utah deposit.

One of the closest analogies he found is "an African water hole during drought."

Many animals gather around water holes in a drought. Elephants will eat all the food in the vicinity and then ingest "dirt and logs and rocks" to keep their stomachs full, without leaving the water hole, he said.

At the end of a bad drought, "you get numerous animals dying." As animals die, carnivores move in and feed on the carcasses. They tear up the remains.

The African water hole site contained a large "disarticulated mass of bones" — similar to what scientists are finding at Cleveland-Lloyd.

Gates thinks the long-necked dinosaurs may have died first in the drought, each leaving "a ton of meat" for the allosaurs.

The allosaurs scavenged and they too died in the drought. Still more of the meat-eaters moved in, apparently not above cannibalism.

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