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Earth Notes: Rainbows & Unicorns Quarry

a fossilized dinosaur skull
Alan Titus/Bureau of Land Management-Utah
Tyrannosaur skull

In 2014, paleontologists found an incredible fossil site in Grand Staircase-Escalante National Monument in southern Utah. It contained hundreds of tyrannosaurs, hadrosaurs, freshwater turtles, fish, flying reptiles, and more, locked in rock 76 million years old.

The huge continuous bone bed turned out to be the largest mass death site from the Late Cretaceous in Utah.

Alan Titus, paleontologist for the monument, gets excited by such finds, so much so his coworkers named it the Rainbows and Unicorns quarry.

Most exciting were the remains of five tyrannosaurs of varying ages from yearlings to full adults. They drowned together in a flood, were buried, and then exhumed in river deposits, at a time when the land was a wet, forested coastal plain.

The grouping of the tyrannosaurs suggested to Titus that they were social pack animals, hunting much like wolves do today. It’s a controversial interpretation because prevailing scientific thought holds that these top-of-the-line carnivores were loners.

At the site, overlying dirt and rock are carefully removed, the remains mapped and photographed, and the fossils put in plaster jackets. They’re then taken to the Bureau of Land Management lab in Kanab, Utah, where they are painstakingly prepared… and researched for clues as to whether the social hypothesis is true.

The size and sheer volume of the Rainbows and Unicorns site has kept excavators busy for the past eight years, and likely will for years to come. And, says Titus, it points to the importance of public lands as storehouses of many more amazing discoveries.

Rose Houk is a Flagstaff-based writer and editor, specializing in natural history and environmental topics.  Rose was a founding contributor of KNAU's Earth Notes and has written nearly 200 scripts for the series. She is also the author of many publications about national park and monuments, along with audio productions. 

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