An artist's impression of Edmontosaurus

The herbivorous dinosaur Edmontosaurus is known from the final epoch of the Mesozoic era. Image © Die Infografen UG (haftungsbeschränkt), licensed under CC BY-SA 4.0 via Wikimedia Commons

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Mummified dinosaur could show signs of scavenging by ancient crocodiles

Tooth and claw marks in the preserved skin of a mummified dinosaur from the Late Cretaceous have been revealed by a new study.

It proposes that scavengers helped preserve the soft tissue of the ancient reptiles by removing flesh and fluid from the body shortly after death. 

Evidence of scavenging from over 70 million years ago may have been found in a well-preserved fossil.

American researchers studying an Edmontosaurus specimen discovered in North Dakota, USA, identified bite marks across its bones and skin. They claim it is the first soft tissue evidence of carnivore activity at or near the time of death of a dinosaur, with an ancient crocodile believed to have torn open the carcass to access more nutritious flesh inside.

Their paper also suggests that dinosaur skin can be preserved even after a carcass was targeted by scavengers, rather than needing to be buried quickly as has previously been thought.

Dr Clint Boyd, who co-authored a paper detailing the discovery, says, 'Not only has Dakota [the dinosaur mummy] taught us that durable soft tissues like skin can be preserved on partially scavenged carcasses, but these soft tissues can also provide a unique source of information about the other animals that interacted with a carcass after death.'

The study was published in the journal Plos One

The head of a mummified Edmontosaurus on display at the American Museum of Natural History

Dinosaur mummies have occasionally been discovered by palaeontologists, preserving soft tissues in great detail. Image © Skye McDavid, licensed under CC BY-SA 4.0 via Wikimedia Commons

How is dinosaur skin and other soft tissue preserved?

Fossils form in a variety of ways, with teeth and bone the most commonly preserved tissues. These hard tissues are more resistant to decomposition, allowing them to survive being buried while the process of fossilisation takes place. 

Soft tissue such as skin and flesh, however, often rot away or are eaten before they can be preserved. It is commonly believed that dinosaur specimens with fossilised skin were rapidly buried, which prevented decomposition from taking place.

This rapid burial can be caused by a variety of reasons, such as dinosaurs sinking into mud pits or being covered during flooding. Natural disasters can also be responsible, with a fossilised leg discovered elsewhere in North Dakota suggested to have been buried by the asteroid which wiped out the dinosaurs.

As the tissue dissolves away over time, impressions of the remains, including the skin, can be formed in the soft sediment. As the resulting void is infiltrated by water carrying minerals a cast of the remains, known as an impression fossil, is formed.

Another type of skin preservation forms compression fossils, when heat or pressure preserves the skin as a thin layer surrounding bones.

The third, and rarest form, is when the skin becomes partly mineralised. This preserves the organ's structure and occasionally organic molecules as well.

Some of the most well-preserved specimens are so detailed they are known as dinosaur mummies, with the carcasses believed to have dried out for an extended period before being buried.

How these dinosaurs managed to dry out without being found by scavengers has been the subject of debate. The researchers behind the current study hoped that examining a well-preserved specimen would help to shed some light on the subject. 

A portion of the tail of the Dakota Edmontosaurus specimen

The tail of Dakota has tooth and claw marks from an unknown large reptile. Image © Drumheller et al., licensed under CC BY 4.0 via Eurekalert!.

What is Edmonotosaurus, and what does the Dakota mummy reveal?

Edmonotosaurus was a herbivorous dinosaur that lived during the Cretaceous between 76 and 66 million years ago in what is now Canada and the USA

A number of mummified Edmonotosaurus have been found, including some which contain possible gut contents such as seeds and twigs. 

The specimen used in the study, known as Dakota, was discovered in part of the Hell's Creek formation in 2000 and is now owned by the State of North Dakota. Millions of years ago, it would have roamed a forested coastline before meeting its untimely end.

By examining the skin and bone fossils, the researchers revealed a number of marks that appear similar to bite and puncture wounds. None of the wounds showed evidence of healing, suggesting that they occurred during or after Dakota's death.

The marks on the right forelimb appeared to be similar to feeding marks of modern crocodiles, with evidence that the skin had been pulled away as a flap to access the flesh underneath. 

While it is possible that the bite marks to the skin and bone were caused by different species, the researchers think it is likely that one ancient crocodyliform, such as Brachychampsa montana, used vigorous head movements to tear open the skin of Edmonotosaurus before pulling back the flap to feed on the more nutritious flesh inside.

While bite marks on the tail can't be as easily assigned to a particular animal, the researchers have suggested that any medium or large carnivore, such as a young Tyrannosaurus rex, could have inflicted the wounds.

As at least some of these wounds were likely caused after death, the study shows that dinosaur mummies can form even if they are not isolated. The researchers suggest that the carnivores' preference for meat would lead to them tearing open the skin to access the internal organs leaving the skin itself relatively intact.

Over time, smaller scavengers and decomposers such as insects and microbes would be able to access the body through the wound and break down a dinosaur's insides, while allowing gases and liquids to escape.

This would help the skin to dry out and shrink around the bones to give it a greater chance of being preserved. 

The researchers have suggested that previous theories about how dinosaur mummies form should be modified to allow for three main pathways – rapid burial, dessication and deflation, and aqueous anoxia (where tissues can be preserved in low oxygen bodies of water).

Studies of Dakota will continue as more of the mummy is carefully prepared over the coming years, potentially offering further insight into the scavengers of the Late Cretaceous.