About 150 million years ago, the landscapes within the modern-day San Rafael Swell in Utah's Emery County looked much different than they do today. In place of the sparsely distributed junipers and pinyon pines, there were lush forests filled with ferns and ginkgo trees. In place of the vast, open desert, there was an inland sea. In place of small, scurrying lizards, there were mighty dinosaurs. (Read: A Deep Dig into Utah's Deep Time)
Origins of the Cleveland-Lloyd Dinosaur Quarry
Before the days of Tyrannosaurus rex, during the Late Jurassic period (around 145 million years ago), the Allosaurus fragilis was the top dog.
These fierce predators had serrated, blade-like teeth that could tear through the flesh of their prey, which included almost every other dinosaur of the Morrison Formation ecosystem in which it lived. Juvenile Allosauruses hunted en masse, helping each other to chase down their dinners.
One fatal day, a troop of Allosaurus followed some Stegosauruses stenops down to a muddy flood pond — and into a so-called “predator trap.” They all got stuck for good, thus creating what would one day become known as the Cleveland-Lloyd Dinosaur Quarry, one of the biggest known ancient dinosaur graveyards in the world.
Or so some theories suggest. In truth, the questions of why so many dinosaur remains have been uncovered at the Cleveland-Lloyd Dinosaur Quarry in Jurassic National Monument — more than 12,000 individual bones, belonging to at least 74 individual dinosaurs — and why so many of those bones belonged to Allosaurus in particular, are still mysteries. Other hypotheses include that the bones concentrated here after floating downstream from somewhere else and even the possibility of poisoned water.
However it came to be that the bones were deposited there, visiting the Cleveland-Lloyd is an opportunity to learn about these prehistoric animals. And, as it is a working quarry, a visit could also be a unique chance to see some paleontologists at work — and perhaps even leave with some paleontological theories of your own.