It may be the first known case of a family that preyed together.
At a remote cliffside in eastern British Columbia, a team of scientists has uncovered the preserved footprints of three tyrannosaurs – large meat-eating dinosaurs – all heading in the same direction.
Because the prints were made under similar conditions, researchers say, there’s a good chance the creatures were walking together, a sight that would surely strike fear in any time traveller that happened on the scene.
The find marks the first time the footprints of more than one tyrannosaur have been found in close proximity and it suggests that the most fearsome predators of all time were social rather than solitary creatures.
“It’s probably about the strongest evidence you can get that these animals travelled in groups,” said Richard McCrea of the Peace Region Paleontology Research Centre who led the excavation near Tumbler Ridge, B.C.
The first two prints, made by the same animal, were spotted in October 2011 by Aaron Fredlund, a local guide and outfitter. Five more prints, including those of two other tyrannosaurs have been uncovered since then, spread across 10 metres of a rock formation that dates back 70 million years.
The tracks were left in what would have been soft mud, possibly sediment deposited by a waterway overflowing its banks. Given the ephemeral nature of the sediment the tracks likely didn’t sit for very long before they were covered with the volcanic ash that protected them for eons. They were only recently exposed by the steady erosion of the cliff where they were found.
While the three-toed footprints are unmistakably those of tyrannosaurs, it’s not clear which species the ambling creatures belonged to. The era predates the appearance of Tyrannosaurus Rex in the fossil record, but it was a time when other large tyrannosaurs, including Albertosaurus and Daspletosaurus, are known to have roamed the eastern slopes of the Rockies.
All the footprints were made by adult tyrannosaurs, with the largest measuring half a metre long – about the size of a serving platter. This corresponds to an animal that measured roughly 10 metres from head to tail.
In a paper published Wednesday in the open-access journal PLOS ONE, the team notes that the footprints of other dinosaur species can be seen in the same formation. But while those tracks are oriented in random directions, the tyrannosaur prints run parallel to each other. The unlikelihood that the tracks of three similar predators, rare to being with, would point in the same direction by chance led the team to conclude the animals were together.
The find raises the hair-raising possibility that tyrannosaurs were pack hunters, working together to bring down large herbivores of similar or greater size.
“There’s an advantage in numbers. You can take down large prey if you’ve got three versus one,” said Michael Ryan, curator of vertebrate paleontology at the Cleveland Museum of Natural History, who was not involved in the find.
Robert Bakker, a paleontologist with the Houston Museum of Natural Science who specializes in dinosaur behaviour, said that if the tyrannosaurs were fellow travellers then there’s good reason to think they were part of a family group, not unlike a pride of lions on the Serengeti.
“Top predators are the most quarrelsome cannibalistic category in the ecosystem,” Dr. Bakker said. “It’s impossible that these [tyrannosaurs] would clump together in a common cause unless they were sharing genes.”
The find is consistent with other evidence that some tyrannosaurs spent at least part of their time together. In some cases, the bones of several individuals have been found in the same place, as though they were living as a group.
But skeptics have pointed to other scenarios that can cause the bones of unconnected animals to accumulate in one place. The tracks provide an important new piece of evidence since they were likely to have been created in a narrow window of time, even if it can’t be definitively proved that the animals were walking together rather than walking in the same place several hours apart.
The low probability that the tracks are unrelated doesn’t obviate the need for caution in interpreting the find, said Jack Horner, a professor of paleontology at Montana State University in Bozeman.
“You’ve basically got one stride. One stride doesn’t tell us much,” said Dr. Horner, who added that he did not object to the idea that tyrannosaurs were gregarious.
It’s possible that stronger evidence will emerge from the site said Phil Currie, a professor of paleontology at the University of Alberta and a co-author of the find.
Since the tracks lead straight into the cliff, further excavations could reveal more of what looks to have been a common journey
“I would love to go into the cliff and follow those trackways even deeper,” said Dr. Currie. “It may just tell you exactly the same thing, that you’ve got three animals moving side by side… but it might also show that they interacted with each other. You never know.”
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