Researchers investigate how thousands of dinosaur bones were buried at Wyoming bonebed

Finding, identifying and cataloguing 13,000 dinosaur bones deposited in a single bonebed in Wyoming requires a detective team. 

Dr. Keith Snyder, chair of biology at Southern Adventist University, collaborated with Dr. Matthew McLain, assistant professor at the Masters University, Dr. Jared Wood, director of the Dinosaur Science Museum at Southwestern Adventist University, and Dr. Arthur Chadwick, director of the Dinosaur Excavation Project from Southwestern Adventist University, to develop the number one online depository for hadrosaur specimens in the world.

How did the bones get there? Why are they so exquisitely preserved? These are two of the questions answered by this 20-year research project in an article published in the journal PLOS One in May 2020. 

These researchers examined 13,000 bones of the duck-billed dinosaur Edmontosaurus annectens, collected between 1996 and 2016. All the bones come from the Hanson Ranch bonebed in the Lance Formation, a sandstone/siltstone deposit in the Powder River Basin of eastern Wyoming. 

The bonebed includes other animal fossils, but dinosaurs make up more than 95% of the specimens, and E. annectens bones comprise about 94% of the dinosaur bones.

How did the Hanson Ridge bonebed originate? The researchers surmise from the evidence that a large number of E. annectens died in a catastrophic drowning event and were subsequently washed in a viscous flood of silt and clay to this site. 

"It seems most reasonable to assume that a catastrophic event such as drowning, killed a large hadrosaur [duck-billed] herd, then they bloated and floated for a while before coming to rest on a shoreline," Lead author Dr. Keith Snyder, a biologist at Southern Adventist University in Collegedale, Tennessee, told Current Science Daily. "Within a year a second water event transported the bones in a thick matrix of clay particles to the location we are now quarrying."

Large but lighter bones are most abundant. The researchers suggest this is because denser bones, such as the braincase and sacrum, may not have moved as far. Also bony elements that were still attached (articulated) such as vertebrae may have been too dense to move far.

As to why certain types of bones are represented, Snyder said the reason is unknown.

"We still don’t have the final answer," he said. "Our bonebed is basically a lens-shaped structure. However, probably half of the bonebed has eroded away over time. Thus we can’t get a really complete picture.

"Our suggestion is that the elements that are least represented in the bonebed have either dropped out before the debris flow when the bones came to a stop, or they were carried farther than the main bonebed. It just so happens that those two sides of our bonebed are eroded away, limiting our ability to fully understand these fluvial processes."

The researchers hypothesize that the Hanson Ranch bonebed is of a herd "just prior to seasonal breeding" because the bones are largely adult or those of a subadult, more than a year old. They speculate that the youngsters of the herd were in a separate place, and therefore not included in the same transport.

Almost all the bones have "little to no abrasion, weathering and breakage," Snyder said. The team says there is some evidence that initially the E. annectens carcasses were scavenged by larger dinosaurs, as some shed dinosaur teeth are among bones in the bonebed. About 4% of the bones have teeth marks in them. Some bones also have a type of fracture related to trampling, which may have been minimized because the carcasses were lying in a fine-grained bedding. 

The researchers attribute the good condition of most of the bones to the short time that they were exposed to the air before they were transported and buried. The exposure to air "was limited to a period of time long enough (weeks or months) to result in decay-mediated disarticulation of the skeletons but was not long enough [months to years] for weathering to affect the bones," the team said.

Other bonebeds with dinosaur remains are in western North America with similar findings to those of this in-depth study of the Hanson Ranch bonebed. "The Hell Creek formation in North Dakota and South Dakota are often thought of as the same formation," Snyder said. "There are a huge number of similarities." 

As for the next step, Snyder told Current Science Daily, "We have so many plans that it is tough to limit my comments. We plan to continue using the main bonebeds as a training grounds for citizen scientists. We have already found many smaller pockets of dinosaur bones across the ranch, and we plan to continue to search for more quarries. We are trying to figure out how these smaller quarries compare to the main bonebed.

"Most outlying quarries are in a sandy matrix and are primarily triceratops and turtles with a smattering of a whole bunch of other things. We need to do more microfossil categorization since we have just found a thin layer with a large number of troodon/pectinodon teeth in it. We have approximately 30,000 elements in the collection at this point, and are looking for collaborators to help us work on publications."

"... We highly prize the ability to do good science in this field of study," he said. "We have developed the number one online depository for hadrosaur specimens in the world."

All the Hanson Ranch bones are catalogued and housed at Southwestern Adventist University in Keene, Texas. They can be viewed online, in 3D. A short film of the dinosaur project appears here.

Snyder became involved in dinosaur research because of his son's interest in going out to dig with the project when he was 9. 

"It is a joy that we work well together and have made this part of our lives," Snyder said. "It is always fascinating to try and figure out puzzles like this one."