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A Dinosaur Murder Mystery at Jurassic National Monument

A dramatic exhibit at Jurassic National Monument includes a full-scale cast of an Allosaurus skeleton. Note the many wide-spaced teeth. (Photo by Andrew Gulliford)

Over 15,000 dinosaur bones, the densest deposit of Jurassic dinosaur fossils ever located, have come from multi-colored soils in Utah. How they got there, who was eating whom and what killed the dinosaurs remains a 152-million-year-old mystery. I had to go visit.

The bones come from eleven different species, but 75% of them are carnivores and most of the fossils are from the long, serrated-toothed and fast-moving predator Allosaurus fragilis. At the Cleveland-Lloyd Quarry site south of Price, in the San Rafael Swell, millions of years ago a seasonal pond became a death trap. Recovered bones prove that dinosaurs chewed on the flesh and bones of their dead companions. A seasonally dry depression “became a stagnant pond laden with dinosaur corpses, toxic with algae, and relatively high in heavy metals,” according to Utah Friends of Paleontology.

Exactly what happened and why remains a mystery, but thanks to the 2019 Dingle Act, the world-famous quarry is now the Bureau of Land Management’s Jurassic National Monument with recent exhibits, trails to hike, and the first visitor center in the BLM.

Scientists continue to research disarticulated bonebeds using modern geochemistry. Apparently, some of the dinosaurs died elsewhere and were washed into an ephemeral pond, but why so many Allosaurus among the other carcasses? Were they attracted to the wetland during drought and died of poisoning? Why have most of the recovered bones been from juvenile dinosaurs? A Smithsonian Magazine article titled “What Killed the Dinosaurs in Utah’s Giant Jurassic Death Pit?” doesn’t have any solid conclusions, either.

A cast of Allosaurus vertebrae makes for a lively children’s play area near a shade shelter with picnic tables.

The history of the site, which became a National Natural Landmark in 1966, is easier to understand. Out in the adobe badlands of the San Rafael Swell, sheepherders and cowboys moved their herds always looking for grass and water far from the small Mormon communities struggling to survive on the desert’s edge. Large bones kept appearing as winds washed over the soft, sandy soils.

Rancher W. Lee Stokes from Cleveland, Utah, knew about the fossils. While attending Princeton University he told about his finds to his professors who were eager to verify his claims. In 1927, the Department of Geology at the University of Utah led a team to the site and excavated 800 bones. From 1939-1941 crews from Princeton arrived led by Stokes. Financed by Philadelphia lawyer and Princeton alumni Malcolm Lloyd, the site became known as the Cleveland-Lloyd Dinosaur Quarry named after the closest town and lawyer Lloyd who as a scientific benefactor generously donated $10,000 for the 1941 dig. Princeton teams, working seasonally in the hot sun and cursed by early summer’s cedar gnats and no-see-ums managed to dig up 1,200 bones and what would become a complete composite Allosaurus skeleton.

Stokes graduated from Princeton and returned home to become known as the “Father of Utah geology” and to continue excavations. Paleontologist James Henry Madsen, Jr. discovered a new dinosaur in 1974 and christened it Stokesosaurus clevelandi for his mentor. By the late 1980s, scientists from Brigham Young University dug up a rare fossil dinosaur egg. During the decades, thousands of bones and casts from the bones have gone to 65 museums worldwide including collections in Canada, Australia, France, Germany, Great Britain, Mexico, Taiwan, Turkey and Switzerland.

The quarry site itself, covered by two modest metal buildings with a few windows and skylights, will soon have a new, more modern facility. It may not be as fancy as the vertical wall of dinosaur bones displayed at the National Park Service’s Dinosaur National Monument near Jensen, Utah, but Jurassic National Monument, open a few days a week, has its own attractions including only 50 to 60 visitors daily. When I went, I had just missed talking to paleontologists from the University of Pennsylvania and the University of Wisconsin-Oshkosh, but I had the facility all to myself.

BLM staffer Terri McKendrick delights in greeting visitors at Jurassic National Monument, which is only open on certain days of the week. She especially enjoys questions from children.

BLM staffer Terri McKendrick delighted in telling me about the quarry site. I thought that Tyrannosaurus Rex was the big, bad, bully on the block, but she explained that the 25 to 35-foot-long Allosaurus were equally tough to contend with, and they were not dainty eaters.

“They could not chew their food,” she told me. “Their jaws were not as strong as a mountain lion’s so they took their meat in hunks. Gross, huh?”

McKendrick said that plant-eating dinosaurs had teeth like spoons, while meat-eaters or carnivores with serrated teeth ripped and tore their protein. Just thinking of all the blood and gore, I remembered teaching science to fourth graders and how fascinated 10-year-olds became with the paleontological past. They could visualize early mornings and late evenings at the mud pond as some dinosaurs came down to drink and others gathered for a feast only to find themselves trapped in the sinking, stinking, ooze. Allosaurus traveled in packs so some members got stuck as did other, larger, heavier theropods arriving to scavenge carcasses. It must have been a beggars’ banquet of brawling, screaming reptiles.

On queue, as I went to look at the metal buildings with plaster casts of bones in their original locations 60 million years ago, a tiny lizard ran across the path in front of me. I’m glad I didn’t meet any of his older, bigger cousins. One crushed femur and tibia of a single, young Allosaurus had been stepped on after death by a larger dinosaur. The quarry site shows all the researchers’ work tools – buckets, trowels, string, paper towels, dust pans. Visitors envision the tedious tasks of finding dino bones and then carefully, cautiously, removing millions of years of sediments and deposits while focusing on intensive mapping and other techniques to keep even the smallest bones and chips. Bones are then cleaned, glued and cast to make replicas.

I saw the back fin or dorsal plate and tail spike of a stegosaurus. Work at the quarry has also helped to prove that dinosaurs had colored skin with various patterns. Bones alone do not tell the story.

“All details found within the deposit, including geology, vertical and horizontal bone placement and orientation are studied,” an interpretive sign says.

“The bones are found in the Late Jurassic-age Morrison Formation, which is prime hunting grounds for the giant dinosaurs in the American West,” says Dr. Julia McHugh, curator of paleontology at the Museum of Western Colorado in Grand Junction. “What makes Cleveland-Lloyd Dinosaur Quarry so unique is that most of the fossils recovered from the quarry belong to just one genus: the predator Allosaurus. We still don’t know why so many fossils accumulated in one place, but research into how the site was formed continues today.”

To see if any ancient dinosaurs happened to be lurking about, I climbed the Raptor Point Trail and then took the Rim Trail for striking 360-degree views across the San Rafael Swell. I saw summer clouds building to the west and virga, or rain which never hits the ground, to the east, but no dinosaurs. I came back to the visitor center where Terri McKendrick was helping young children earn their BLM junior ranger badge.

She asked which plants they had seen that day – cliff rose, stinking milkvetch, San Juan onions or green Mormon tea – and which lizards – whiptails or fence lizards that have the blue bellies.

“What’s blooming?” she asked the children. Had they seen twinpod mustard, budsage, spring parley, evening primrose, silvery townsendia and Indian paintbrush? She encouraged all visitors to fill out the BLM’s anonymous visitor questionnaire.

At the Museum of Western Colorado’s Dinosaur Journey Museum in Fruita, a vivid display portrays snack time in dinosaur land. Though adults might find this exhibit “gross,” children love to point at the blood and gore and are entranced by the age of dinosaurs. Yes, paleontologists now believe that dinosaurs had feathers.

“I love it here because I like to learn from people,” she told me. She added that just recently campers had discovered a new dinosaur bone five feet long and 16 inches wide. Scientists had yet to excavate it. Paleontology is part of our nation’s heritage with valuable sites on BLM land. Jurassic National Monument is part of the larger system of National Conservation Lands. The public can collect invertebrates, but fossil remains of animals with backbones cannot be acquired without a permit.

I left still not knowing how the ancient predator trap had worked and what had caused mass deaths millions of years ago. What had been a wet, mud-filled pond bottom was now yellowish sand and dirt lifting in an afternoon wind. I was encouraged by the children, filling out their forms, asking questions, drawing dinosaurs. What secrets still hidden on America’s public lands will they discover and reveal? When William Stokes attended Princeton and told his professors what he had found, how could he have known that he had located the largest Jurassic dinosaur boneyard in the world?

Andrew Gulliford is an award-winning author and editor and a professor of history at Fort Lewis College. He can be reached at gulliford_a@fortlewis.edu.

A photo of the local topography at Jurassic National Monument shows the exposed geology of the Jurassic Period that produces dinosaur bones from 200 to 145 million years of age.