Due to the continuing corona virus situation, this meeting will be held online through Zoom.

IN THE LAND OF RAINBOWS AND UNICORNS:

Forensic Science (Taphonomy) of a 76.4-million-year-old

Tyrannosaur Mass Mortality

Dr. Alan Titus

Paria River District Paleontologist for the Bureau of Land Management

Sunday, April 18, 2021           2:00 P.M.

THIS TALK WILL BE ONLINE

In 2014 a fossil site was discovered inside Grand Staircase-Escalante National Monument, Utah that may reveal new insight on the gregarious or social behavior of large predatory tyrannosaur dinosaurs in North America.

So far, the site, nicknamed the Rainbows and Unicorns Quarry, has yielded the 76.4-million-year-old remains of seven species of turtles, multiple fish and ray species, three kinds of dinosaurs and the nearly complete skeleton of a juvenile (12-foot-long) Deinosuchus alligator. Among the dinosaurs were a treasure trove of tyrannosaur fossils, representing a minimum of four individuals of the 4,000 lb, 29-foot-long T.rex cousin named Teratophoneus. It is the first and still the only tyrannosaur mass grave ever found in the southern USA. Only two other such sites like it have been described from North America, one in Montana and the other in Alberta, Canada.

Both the Montanan and Canadian sites have been used to argue that tyrannosaurs were gregarious animals, possibly hunting in packs like wolves or lions. Since the Monument tyrannosaur species represents a completely different lineage from either species found up north, the research team headed by Dr. Alan Titus, Paria River District Paleontologist for the BLM, was keen to look for evidence of possible gregariousness in Teratophoneus.

Unfortunately, the site’s ancient history is complicated, with the bones appearing to have been exhumed and reburied by the action of a river, destroying the original context within which they lay. However, all was not lost, and a team of researchers from the Bureau of Land Management (BLM), the Denver Museum of Nature and Science, the Field Museum in Chicago, the University of Arkansas, and James Cook University (Australia), examined the evidence at the site and ran batteries of tests and analyses on the vestiges of the original site, now preserved as small rock fragments and fossils in their final resting place, sandbar deposits from the river. From simple mapping, element frequency counts and observations on weathering, to much more sophisticated techniques involving scanning electron microscopes and mass spectrometry, everything indicated the original burial of the fossils was in a lake, not a river. They also suggested the tyrannosaurs arrived at the site together within a very short period if not at the same time.

As the details of the site’s history emerged, the team was able to conclude the tyrannosaurs died together during a seasonal flooding event that washed their carcasses into the lake, where they sat, largely undisturbed until the river later churned its way through the bone bed. It also became clear that there were also many similarities with the Canadian site, which has yielded the most plausible evidence for tyrannosaur gregariousness. As a result, the Utah site might also have resulted because tyrannosaurs habitually lived in groups.