Paleontologists Find Fossilized Remains of a Dinosaur Possibly Killed in Earth’s Fifth Mass Extinction Event

The leg bone is one of many other specimens uncovered at the North Dakota site

A dinosaur leg bone fossil mounted on a frame and ready to be analyzed in a lab.
The dinosaur leg fossil will be featured in an upcoming BBC documentary special premiering April 15.  BBC One

Paleontologists claim to have found a fossilized leg belonging to a dinosaur that may have perished when an asteroid struck Earth 66 million years ago, reports BBC's Jonathan Amos. The well-known impact event is often linked to the decimation of non-avian dinosaurs, which ushered in the rise of mammals. Very few dinosaur bones date to the final few thousand years before the impact, so having a dinosaur that could be direct evidence to the Cretaceous-Tertiary extinction would be astounding, per the BBC.

The fossilized leg, dotted with impressions left behind by scaly skin, was found in the Tanis fossil site in North Dakota and belonged to a Thescelosaurus dinosaur, report Marianne Guenot and Alia Shoaib for Business Insider. Details on the discovery at the Tanis dig site are set to air on April 15 in a BBC documentary narrated by Sir David Attenborough called "Dinosaurs: The Final Day." The new findings have yet to be published in a peer-reviewed journal, but scientists are wonderstruck by the find, reports Nicole Karlis for Salon.

"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. You look at the rock column, you look at the fossils there, and it brings you back to that day," says paleontologist Robert DePalma, a University of Manchester graduate student who leads dig projects at the Tanis site, to the BBC.

Dinosauria's doomsday was brought on by a 7.5-mile-wide asteroid, about the size of Mount Everest, that smacked into the Gulf of Mexico. The 90-mile-wide impact site, called the Chicxulub crater, is widely expected  to be the origin point for the mass extinction event that wiped out all non-avian dinosaurs on Earth, Salon reports.

Although the impact occurred about 1,860 miles away from what is now North Dakota, researchers dated the limb to the extinction event from the presence of debris that rained down after the collision. Shockwaves triggered by the massive asteroid impact likely rocked the ancient river system where the Tanis dig site now lays. Water and sediment from the river rapidly mixed with falling debris, creating ideal conditions for a mass fossil-preserving burial event, reports the Telegraph's Sarah Knapton.

Both land and aquatic creatures were fossilized together. Per the BBC, debris was found on the leg and in an ancient fish entombed near the limb. Tiny, glass shard–like pieces of molten rock kicked up by the asteroid were found lodged inside the fossilized fish's gills, per Business Insider. The debris lodged into the fish's gills, called spherules, has been linked to the impact location in Mexico's Yucatan peninsula through radiometric dating, per the BBC. 

"We were able to pull apart the chemistry and identify the composition of that material. 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," says paleontologist Phil Manning, who is DePalma's supervisor at the University of Manchester, to the BBC.

Particles found in preserved tree resin were also of extraterrestrial origin. The particles were not enriched with calcium and strontium, which would be present if the particles were from Earth. Instead, they had high levels of iron, chromium, and nickel, which are present in asteroids, reports Kenneth Chang for the New York Times.

The dinosaur leg appeared to have been ripped off quickly, with no traces of disease or any evidence that the leg was scavenged, says Paul Barrett, a dinosaur expert from London's Natural History Museum, to the Telegraph. Other remains found nearby include a fossilized turtle impaled by a wooden stake, small mammals in their burrows, skin belonging to a triceratops, a fossilized pterosaur embryo inside its egg, and a possible fragment from the devastating asteroid, the BBC reports. X-rays determined that the pterosaur egg may have been buried by the mother in the sand, similar to the way modern turtles bury their eggs, per the BBC.

However, some paleontologists are skeptical. Steve Brusatte, a vertebrae paleontologist at the University of Edinburgh, tells the BBC that he remains apprehensive about the dinosaur findings because they have not yet been peer-reviewed or published in a scientific journal, Business Insider reports. Brusatte also explains some of the dinosaurs may have died before the impact. When the asteroid sent shockwaves across the planet, the remains may have been exhumed and buried again under debris. Regardless of the timing of Earth's fifth mass extinction event, Brusatte states the fossilized remains overall are rare finds.

"For some of these discoveries, though, does it even matter if they died on the day or years before? 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," Brusatte tells the BBC.

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