A nine-ton block of sandstone that was pulled from a Utah mountain late last year holds the biggest fossil trove ever found of the giant predatory dinosaur known as Utahraptor. Covered in feathers, with a huge sickle claw on each second toe, Utahraptor looked like a pumped-up version of the Jurassic Park star Velociraptor.
The fossils might help resolve a long-standing debate about whether these predators hunted in groups. In the Jurassic Park films, velociraptors were shown cooperating to chase down prey, an idea based at the time on several predators that had been found alongside an herbivore. The new fossils may help confirm whether the silver screen got it right. (See also: "Jurassic World Dinosaurs Stuck in the 1980s, Experts Grumble.")
Scientists have found the remains of six Utahraptor dinosaurs in the rock so far, and more may be trapped there.
If the dinosaurs died together, they might provide some long-sought evidence of group hunting. The densely packed dinosaurs (in some places, fossils are stacked three feet thick) may have died at different times as they blundered into quicksand, or perhaps they died together in a social supper gone horribly wrong.
The remains were excavated in a massive effort that has spanned more than a decade, led by Utah state paleontologist James Kirkland. (See video above for more on the excavation effort.)
The recent finds include never-before-seen bones that are already changing scientific views of the Utahraptor anatomy.
"We're really going to have a different view of this guy," Kirkland says. Part of the emerging picture is that while young Utahraptor dinosaurs were lightly built and turkey size, Kirkland explains, the adults were heavily muscled, "Arnold Schwarzenegger" versions.
Kirkland was tipped off to the site in 2001 by a geology student who had found what first looked like a human arm bone while scouting eastern Utah's early Cretaceous rocks. All the Utahraptor fossils are contained within a large blob of sandstone that appears to have once been what geologists call a "dewatering feature," or in common terms, "quicksand," Kirkland says.
After relocating the site, Kirkland found that the hollow bone was actually part of a dinosaur foot. And there was more.
By chipping off smaller pieces of the block, Kirkland and his team uncovered bones from a 16-foot-long adult Utahraptor, four juveniles, and a baby that would have been only about three feet long from snout to tail.
Other bones at the site belong to a beaked, bipedal herbivore called an iguanodont. The remains of these dinosaurs may have been what attracted the Utahraptor group to the site in the first place.
But these bones are only the bits and pieces that paleontologists have been able to chip off so far. The vast majority of the bones reside in a massive block of sandstone, which gave Kirkland a clue as to why so many raptors became buried in one place.
Dinosaur Death Trap
Kirkland thinks that the Cretaceous drama played out like this: An unwary iguanodont stumbled into the quicksand, bellowing and struggling. If this didn't attract carnivores, then "that nice meaty smell would have," Kirkland says, like flies to flypaper. One Utahraptor after another tried to nab an easy meal and only ended up getting stuck, adding to the deathly aroma.
"We believe it's going to be the first example of dinosaurs trapped in quicksand en masse in the fossil record," Kirkland says.
In order to investigate this idea, though, Kirkland and his team had to get the block out of the field for delicate preparation. "We didn't want to take out a nine-ton block," he says, but "every time we tried to cut in, we kept hitting legs and vertebral columns." (Read about hunting for fossils in "Digging Utah's Dinosaurs" in National Geographic magazine.)
Airlifting the dinosaur-filled rocks with heavy-lift helicopters was too expensive, so Kirkland and his team worked with hands and heavy machinery to get the dinosaurs off the hill and to their temporary home at Salt Lake City's Department of Natural Resources. (See video above.)
Other researchers are paying close attention as the work proceeds. "Any time you find multiple dinosaurs together it's a big deal," says University of Edinburgh paleontologist Stephen Brusatte, "and even more so if they are dromaeosaurs, some of the most iconic but also the rarest dinosaurs in the North American fossil record."
The Family That Preys Together...
Even more tantalizing is the question of whether the block represents a Utahraptor pack or various raptors joining the smorgasbord at different times.
Thanks in no small part to the Jurassic Park movie, raptors of all sizes are often depicted as pack hunters. Yet the evidence for social raptors is relatively scant.
The best evidence so far, says University of Maryland paleontologist Thomas Holtz, Jr., is a trackway from China that "really does appear to be a group-hunting track with Utahraptor-like dromaeosaurids pursuing an iguanodontian, pretty much Jim's scenario."
The block could add new evidence through the preservation of the bones. "If the skeletons show some interweaving," with arms and legs tangled together, Holtz says, "that would be a good clue" of pack behavior. The degree to which the bones were weathered—or damaged by sun and exposure before burial—could also help show whether they were buried together or at different times.
Brusatte agrees that context will be key. "If a careful study of sedimentology supports the idea that this was a predator trap, and the dromaeosaur bones are all found fossilized in a similar condition and a position that indicates that they were mired, then I think the team will have a solid case that this is more than just a jumble of bones, but evidence that some dromaeosaurs did live and hunt together socially."
It will take years to reveal the block's full tale. The first step in doing so is finding a place where the nine-ton block can be prepared and studied. Because Kirkland's facilities at the Department of Natural Resources aren't large enough, he's on a search for a venue where the dinosaurs can be brought out of the rock bone by bone, in public view.
After waiting 125 million years to spill their secrets, the bones can wait a few more. For now, they're safe within their shell of plaster and wood, locked in the positions in which they were engulfed by the Cretaceous sand.