Evrim Yazgin
Cosmos science journalist
A fossilised Thescelosaurus leg has sparked controversy. Some scientists claim the well-preserved fossil represents an individual killed by the asteroid which caused the end of the geological era dominated by dinosaurs. But not everyone in the scientific community is convinced.
A small, herbivorous non-avian dinosaur, Thescelosaurus lived at the end of the Cretaceous period. The limb, which includes the animal’s skin, was found in a layer of rock just below the distinct boundary between the Cretaceous and the post-dinosaur age. This boundary is made up of the sediment deposited by the Chicxulub asteroid impact 66 million years ago in what is now the Gulf of Mexico.
The fossil’s location and likely fast preservation are suggestive. However, the lack of a peer-reviewed study published in a scientific journal has led some experts to doubt whether the dinosaur in question was necessarily the victim of cosmic bad luck, falling directly victim to the asteroid.
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Found on a dig in Tanis in the US state of North Dakota, the fossil is set to feature among other discoveries from the site in an upcoming BBC documentary. The crew spent three years filming at Tanis, roughly 3,000km from the Chicxulub crater. The documentary, Dinosaurs: The Final Day, hosted by Sir David Attenborough, will be broadcast on BBC One on April 15.
Professor Paul Barrett, a researcher at the UK’s Natural History Museum in London, examined the leg for the BBC program. He told the BBC, “It is a stunning fossil … Skin preservation like this is still relatively rare.”
He added: “While it is plausible that this Thescelosaurus was killed on the day of the strike, it’s also possible it was exhumed by the asteroid impact, and then mixed together with everything else in the aftermath. But the fact that it is so well-preserved suggests to me that even if the animal didn’t die as a result of the events that caused the deposit, it must have died very close in time to it.”
The leader of the Tanis dig, Robert DePalma, a graduate student from the University of Manchester, UK, told the BBC that the Tanis site paints a picture of the dinosaurs’ final days. “We’ve got so many details with this site that tell us what happened moment by moment, it’s almost like watching it play out in the movies,” he said. “You look at the rock column, you look at the fossils there, and it brings you back to that day.”
Other fossils found at Tanis include an extremely well-preserved embryonic pterosaur –flying reptiles which soared over the world of the dinosaurs – and fish with small particles in their gills. Once chemically analysed, it was determined that the particles are molten rock from the asteroid impact breathed in by the fish.
“All the evidence, all of the chemical data, from that study suggests strongly that we’re looking at a piece of the impactor – of the asteroid that ended it for the dinosaurs,” said Professor Phil Manning, DePalma’s PhD supervisor.
Peer-reviewed articles have been published on the site, including a 2019 paper on the glass spherules inhaled by the fish. More are promised. Until then, however, some claims remain open to doubt.
Professor Steve Brusatte from the University of Edinburgh, UK, another consultant for the BBC program, is sceptical. For now.
“Those fish with the spherules in their gills, they’re an absolute calling card for the asteroid,” he said. “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. “For some of these discoveries, though, does it even matter if they died on the day or years before?” he added. “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.”