How do you kill a whole group of prehistoric predators? Utah State palaeontologist Jim Kirkland has been pondering this question ever since leading the excavation of several carnivorous dinosaur skeletons buried together not far from Utah's Arches State Park. The dinosaurs were all killed in the same place some 125 million years ago, and after studying the geology of the site, Kirkland and his team think they've identified the culprit. 

The unfortunate predators appear to be Utahraptor, a deadly species closely related to Velociraptor, but even larger than the supersized stars of Jurassic World. That impressive size, however, proved no match for the ensnaring power of quicksand. 

An artist's reconstruction of how Utahraptor may have appeared in life. Art by Emily Willoughby via Wikimedia Commons.

At least ten skeletons were buried at the site, from a fully grown five-metre (16ft) adult to tiny one-metre (3-foot) babies, but it was tricky for Kirkland and his team to come up with a precise number because the fossils have been so difficult to excavate. 

"The bones were so tightly packed, and many of them small and delicate, that we could not get around them to make the block smaller without damaging them," he told me via email.

The solution? Instead of digging out each bone individually, the team decided to pull out the entire group of skeletons in one huge hunk of rock. It took many months of digging over a period of ten years, and the end result was a massive block of rock and bone three metres (9ft) across and weighing nine tonnes.

The dig crew stands proudly around the massive block of fossils, wrapped safely in protective plaster. Image: Jim Kirkland.
Image: Don DeBlieux

The site's geology, the scientists found, was very similar to areas where quicksand forms today. But why did so many of these carnivores end up stuck? The answer may lie with the other remains identified within the rock – bones belonging to small plant-eating dinosaurs called iguanodonts. It's easy to imagine one such hapless herbivore getting stuck and struggling in the sticky sand. The commotion attracts a predator on the lookout for an easy meal – but what looks like harmless mud turns out to be a deathtrap.

"Predator traps" like these are known from other fossil sites, such as the La Brea tar pits in California, which preserve an unusually high abundance of Ice Age carnivores for the same reason. But according to Kirkland, this new dinosaur find would be "the first published attribution of a predator death trap due to quicksand."

And palaeontologists are excited about this site for more than the quicksand. Finding so many dinosaurs of one species can be a big deal, notes palaeontologist Mike D’Emic, who wasn't involved in this excavation. What's more, Utahraptor is a dromaeosaur, a type of dinosaur only rarely found in North America, let alone in such large numbers. "That kind of sample would allow scientists to ask a number of questions, such as how long it took Utahraptor to grow up or how its bones and body proportion changed as it grew," D'Emic told me.

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The first Utahraptor jaw from the site. Image: Jim Kirkland

One of the biggest questions on Kirkland's mind, though, is whether these predators were hunting together. Movies love to depict such dinosaurs as pack hunters, but there's actually very little direct evidence for this. Were these quicksand-bound dinosaurs moving together as a family group? Or were they all drawn in separately? The answers may lie deeper within the big block, but we may have to wait a while – it's going to take scientists a long time to expose them.

"The preparation will be quite challenging even in the lab," Kirkland told me. "You move a centimetre off the bone in a day," he added in discussion with KUER in Utah. "It's very slow, meticulous work – all has to be done under microscopes on big mounts because it's such a massive specimen."

And until that immense task is complete, we can't draw too many conclusions, cautions D'Emic. "Only when the fossils are brought back to the lab and carefully prepared, as the research team is doing now, can they be positively identified," he told me. "It will be interesting to see what else comes out of the block once it is fully prepared."

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Top header image: Charles Peterson, Flickr