Dinosaur bones point to mass die-off

PROVO — A vast collection of broken dinosaur bones unearthed in southeast Utah indicates they were smashed underfoot by other dinosaurs shortly after they died, according to paleontologists.

Brigham Young University scientists have spent years analyzing more than 4,000 bones from a quarry just west of Arches National Park.

They say the bone collection — which includes at least 67 dinosaurs representing eight species — suggests a mass die-off, likely from drought.

After the die-offs, other plant-eating dinosaurs stomped among the carcasses as they passed through, snapping most of the bones at the site, according to BYU professor Brooks Britt, lead author of a recently published study of the bones.

Many bones would have been crushed easily under dinosaurs weighing 20 tons or more with feet larger than tires, Britt said. More than 95 percent of bones studied at the site were broken.

"Some of them were just pulverized," Britt said.

Utah holds some the country's most well-preserved and numerous dinosaur quarries and is a hotbed for researchers looking for clues to ancient life. The site analyzed by Britt and his team offers insights into the lesser-known lives of dinosaurs and other lifeforms some 124 million years ago.

It's also a fresh reminder that digging up dinosaur bones isn't as easy as it's often depicted. The site is a complex mix of bones that were scattered and rescattered in prehistoric floods and sometimes trampled more than once by other dinosaurs, Britt said.

Britt and BYU's Earth Science Museum curator Rod Scheetz, a co-author on the study, said large numbers of dinosaurs keeled over, likely during drought cycles, in an area near modern-day Moab that once was likely within sight of a receding lake.

They theorize that, after they died, other dinosaurs — including long-necked sauropods and herbivores called iguanodons — tromped through, grinding the bones into the mud and snapping many like twigs.

"That means the big boys were stepping on those things," Britt said. "Those would have been audible, big snaps."

Researchers spent years picking through the site and hauling fragments, some as small as two centimeters (less than an inch), back to the lab to be pieced together. They were at first puzzled by the cause of the bone fractures. Looking closer, they found that the angled breaks were similar to breaks in fresh bones that have not yet become brittle.

Many were also ravaged by thumb-size insects, Britt said.

Though trampled dinosaur bones have been found elsewhere, Scheetz said the site near Moab helps fill in gaps about the early Cretaceous period, spanning roughly 145 million to 99 million years ago.

"Now we're getting a little better picture," Scheetz said.

Researchers have only investigated a fraction of the site near Moab and more work is expected.

Meanwhile, some of the bones are on display at BYU's museum and results from the study have been published in the journal Palaeo.