The Cleveland-Lloyd Dinosaur Quarry (CLDQ) was first discovered in the 1920s and excavations soon showed it was the densest deposit of Jurassic theropod dinosaurs ever found. They also showed the fossil bed had an unusually large proportion of Allosaurus fossils—a 28- feet-long predatory dinosaur that lived in North America 155 to 150 million years ago.
The question of how so many dinosaurs ended up in such a small area has baffled scientists ever since. Some theories suggest there was a catastrophic event—potentially the dinosaurs were poisoned, died in a severe drought, or became trapped in the thick mud that would have been present at the site.
Another idea is that the site represents numerous events that would have brought dinosaurs to the site, either alive or dead, over different periods of time.
In a study published in the journal PeerJ, a team of U.S. researchers tried to reach a consensus about how the fossils ended up at the CLDQ by analyzing the processes that affected the remains as they were fossilized. As well as looking at the dinosaur fossils, the team examined the minerals in the sediments to better understand the environment of the period.
This allowed them to develop a reconstruction showing the history of the deposits.
Study author Jonathan Warnock, from the Indiana University of Pennsylvania, tells Newsweek that before they began their research they did not know which result—evidence of single or multiple death events—to expect.
Their findings showed the dinosaurs accumulated at the site over several different periods, with carcasses being transported there during flooding events. When floods came, the dinosaurs would have washed in and rotted in what was then a pond.
Excavation work at the Cleveland-Lloyd Dinosaur Quarry. Joe Peterson
He says it could be because there were a lot of smaller predators like Allosaurus in relation to fewer, but far larger herbivores, like sauropods: “[This] might have been the Jurassic norm. We hope to be able to understand Jurassic population dynamics.”
They also need to better understand the size of the floods. This will help them find out if snapshot dinosaur population had washed into the pond, or if a smaller flood brought in a comparatively larger number of Allosaurus. To work this out, researchers plan to look at the energy required to move the bone fragments and pebbles.
“Most of the paleo-detective work to date has focused on the detail of the large number of carnivorous dinosaurs, trying to explain the disproportional number; however, my work, over 10 years ago, and now the new study looks at the CLDQ as a mystery of how over 75 dinosaurs ended up becoming buried in one small location 147 million years ago.”
“Peterson’s new study is innovative because it focuses on parts of the bone deposit that largely have been overlooked for a hundred years,” he says. “Indeed, the information Peterson received from chemical analyses of the bone fragments tell us that the place where the dinosaurs were buried was likely a pond that dried up every year like we see on the African savannah. They are also suggest that the bones arrived at the quarry through annual flood events.
“This interpretation of how the bones arrived at the quarry is in contrast to my prior work suggesting droughts congregated dinosaurs to the site. The chemical data support both the drought hypothesis and the newly proposed flood hypothesis, leaving us with more information on the nature of the environment, but still on a scientific quest to discover what happened to these dinosaurs so long ago.”