An immaculately preserved dinosaur leg uncovered in North Dakota may be a relic from the day a massive asteroid slammed into Earth, bringing the age of the nonavian dinosaurs to an end, scientists claim. That said, not all experts are convinced that the dino actually died on that fateful day 66 million years ago — or at least, they're witholding judgment until more data is available for review.
"We need the whole story," Kirk Johnson, the Sant Director of the Smithsonian National Museum of Natural History in Washington, D.C., told Live Science.
A team led by Robert DePalma, a doctoral student at the University of Manchester in the United Kingdom, uncovered the fossilized leg, which still has skin attached, and suggested that the dinosaur died and became buried during the famous asteroid impact, BBC News reported (opens in new tab). The specimen has not yet been described in a peer-reviewed scientific journal.
Johnson said he's frustrated about the way this discovery and previous ones from DePalma and his colleagues have been presented, with a "big media splash" preceding the release of any detailed, published data. This approach has made many scientists wary of any discoveries made at the fossil site in North Dakota, known as Tanis, he said. "It looks like it's an amazing site, and the way it's been rolled out has increased the controversy and doubt about the site," he said.
According to Paul Barrett, a merit researcher at London's Natural History Museum, the newfound dinosaur leg belongs to Thescelosaurus, an herbivorous dinosaur whose name means "wonderful lizard" in ancient Greek. "It's from a group that we didn't have any previous record of what its skin looked like, and it shows very conclusively that these animals were very scaly like lizards," Barrett told BBC News. "They weren't feathered like their meat-eating contemporaries."
Related: What happened when the dinosaur-killing asteroid slammed into Earth?
Based on his examination of the fossil, Barrett said the dinosaur's leg was likely ripped off very quickly, and the limb bears no signs of disease or having been picked apart by scavengers. Barrett examined the fossil on behalf of BBC One, which will soon premiere a documentary (opens in new tab) about Tanis, where the specimen was recovered.
"It's a cool fossil, if it's what it looks like," Johnson said. From the BBC photos and videos, it appears that the dinosaur leg has been mummified. "I don't think we've ever seen a mummy of a Thescelosaurus before," he told Live Science.
BBC One also called in Steve Brusatte, a vertebrate paleontologist and evolutionary biologist at the University of Edinburgh in Scotland, as an outside consultant on the project. Brusatte told BBC News that he's skeptical of the idea that the Thescelosaurus perished on the exact day the dino-killing asteroid came whizzing through the sky and punched a huge hole, known as the Chicxulub crater, into the Yucatán Peninsula.
It's possible that the Thescelosaurus and other animals discovered at the North Dakota site died days or years before but were violently uncovered during the asteroid impact and then reburied along with debris from the planet-rocking event, Brusatte said.
The Tanis site has drawn similar skepticism in the past, Science magazine reported (opens in new tab) in 2019.
That year, Robert DePalma, then a graduate student in paleontology at the University of Kansas, and his colleagues reported finding at the site fossilized fish whose gills were riddled with small glass spheres called spherules. These freshwater fish included sturgeon and paddlefish and were found jumbled together in a 4.2-foot-thick (1.3 meters) deposit, surrounded by scattered remnants of tree trunks and thick mud speckled with more glass spheres, according to Science.
In their 2019 study, the team determined that these glass spheres were about 65.8 million years old and theorized that they formed from molten rock that was flung into the sky during the Chicxulub impact. They suggested that the fossilized animals at Tanis were initially deposited there by violent seismic waves that radiated from the impact site, some 1,860 miles (3,000 kilometers) away, Science reported.
That violent upheaval might explain why, at Tanis, marine fossils can be found jumbled next to the remains of land animals, Johnson noted. That said, all these animals didn't necessarily get corralled together and buried on the exact day the asteroid struck. It's possible that heavy rains washed the carcasses into the same depression after the impact, along with lingering debris from the day of the event. "That could happen over days, months, years," Johnson said, and could mean that not all the creatures actually died during the impact itself.
"Those fish with the spherules in their gills, they're an absolute calling card for the asteroid," Brusatte told BBC News. "But for some of the other claims — I'd say they have a lot [of] circumstantial evidence that hasn't yet been presented to the jury."
Since a full description of the Thescelosaurus leg hasn't yet been published, "I have a ton of unanswered questions about the fossil, and look forward to seeing all of the data being published and available," Andrew Farke, the director of the Raymond M. Alf Museum of Paleontology in Claremont, California, told Live Science in an email.
In particular, "geological context — how the dinosaur leg was positioned within the rock of the area — is going to be key," Farke noted. Johnson said that he's also curious as to whether there's any sediment trapped in the leg, between the skin and the bone. If the dinosaur died prior to the asteroid impact, the flesh of its leg may have started to decay or been scavenged upon; sand and mud could have then slipped into the space where the flesh had been. If the dinosaur died and became buried on the day of the impact, it's less likely that sediment would be found between the skin and bone, he said.
Related: How did cockroaches survive the dino-killing asteroid strike?
In addition to the glass-filled fish and dino leg, the team has reported finding the fossilized remains of a turtle and small mammals; the skin of a Triceratops; a pterosaur embryo locked inside an egg; and a fragment of what might be part of the impact asteroid itself at Tanis, according to BBC News.
"For some of these discoveries, though, does it even matter if they died on the day or years before?" Brusatte said. "The pterosaur egg with a pterosaur baby inside is super-rare; there's nothing else like it from North America. It doesn't all have to be about the asteroid."
Some of these discoveries have been described in scientific journals, but to date, the team hasn't published a complete description of Tanis or how and where these fossils were found in relation to each other, Johnson said. "It's a cool site, no question about that," he said. But "it's been tremendously frustrating — they haven't laid out the whole story."
"I don't know if there is controversy about the interpretation of the site, so much as an eagerness to see the full story presented in detail," Farke told Live Science. "I think everyone, regardless of their opinion on the site, has questions about the discoveries that will hopefully be answered by additional peer reviewed publications."
Editor's note: This story was updated on April 8 with comments from Kirk Johnson and Andrew Farke. The article was first published on April 7.
Originally published on Live Science.