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Dinosaur that broke its wrist may have fallen while mating

A dinosaur bone unearthed in eastern Russia shows evidence of an injury the plant eater sustained when it fell awkwardly

Life 2 March 2022

dinosaur

An artist’s reconstruction of Amurosaurus

Andrey Atuchin

A four-legged duck-billed dinosaur that lived 68 million years ago – in what is now eastern Russia – probably broke its wrist after falling from an upright position during mating or while reaching for leaves.

“It was standing on its heels for maybe eating, mating, perhaps just [passing] time, when it fell,” says Filippo Bertozzo at the Royal Belgian Institute of Natural Sciences.

Bertozzo and his colleagues used a form of X-ray imaging to make the discovery, by analysing a dinosaur lower foreleg bone – the ulna – that was uncovered in Blagoveshchensk in south-east Russia.

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“This bone is usually very slender, smooth and regular,” says Bertozzo, but in this particular case, the bone had a huge swelling at its wrist end. “I was wondering, what did that to this poor animal?”

Through inspection of the curvature of the bone and shape of the elbow, the team confirmed that the bone belonged to a herbivorous hadrosaur (Amurosaurus riabinini), which would have lived in herds of dozens to hundreds of individuals. The animal was a subadult and around 5 metres long and 2 metres high.

By analysing a digital reconstruction of the bone, and comparing its structure to ulnas belonging to uninjured A. riabinini individuals from the same fossil site, the team revealed that the dinosaur probably fell from an upright position, which caused a diagonal fracture at the end of the ulna. This led to overgrowth of the bone during the healing process.

“The fracture is completely surrounded by this structure that is protecting the bone during the healing process,” says Bertozzo.

Based on the extent of healing in the bone, the dinosaur probably limped around for at least four months before its death. “We can know for sure that the injury didn’t immediately kill the dinosaur because, clearly, when you die, your body cannot heal,” says Bertozzo.

“It is impressive to think about how resilient these animals were. With a trauma like that, to walk and run would have been quite difficult,” adds Bertozzo. He speculates that the hadrosaur may have been protected by hiding in the middle of herds, but establishing whether its herd members helped to care for it will be very challenging.

Journal reference: Historical Biology, DOI: 10.1080/08912963.2022.2034805

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