Borealopelta: The Mummified Nodosaur Found in Alberta in 2011

Borealopelta is an armoured dinosaur from the Early Cretaceous of Canada, famous as one of the best-preserved dinosaur fossils ever found. A heavy equipment operator uncovered it on 21 March 2011 at an oil sands mine near Fort McMurray, Alberta. Preserved in three dimensions with its armour plates, skin and traces of pigment still in place, it has revealed how a large plant-eating dinosaur was camouflaged and what it ate before it died.

Background: nodosaurs

Nodosaurs were four-legged, heavily armoured plant-eaters closely related to the club-tailed ankylosaurs. Their backs were covered in bony plates called osteoderms, and many had large spikes over the shoulders. Usually, however, their armour is found scattered, because the plates are embedded in skin and fall apart when the body decays.

Alberta is famous for Late Cretaceous dinosaurs from sites such as Dinosaur Provincial Park. The oil sands region of the north is much older and marine in origin, so dinosaur fossils there are extremely rare.

The discovery in 2011

On 21 March 2011 a heavy equipment operator was working with an excavator at Suncor’s Millennium Mine when he struck unusually hard rock containing what looked like patterned plates. Mine staff stopped work in the area and called the Royal Tyrrell Museum of Palaeontology in Drumheller.

Museum scientists, among them the curator Donald Henderson and the technician Darren Tanke, travelled to the mine and worked with mine staff to recover the fossil. Part of the specimen had already broken away during digging, and the rock was fragile. The block broke into several pieces during recovery, which were encased in plaster, moved to the museum and later reassembled.

The preparation was undertaken by the museum technician Mark Mitchell, who spent more than 7,000 hours over about five years carefully removing the hard rock around the armour and skin. The specimen, which preserves the animal from the snout to the hips, went on public display at the Royal Tyrrell Museum in 2017.

Naming and preservation

In 2017 Caleb Brown and colleagues described the fossil in Current Biology and named it Borealopelta markmitchelli. The genus name means “northern shield”, and the species name honours Mark Mitchell.

The rocks belong to the Clearwater Formation, laid down in an inland sea about 112 to 110 million years ago. The animal is thought to have died on land or near the coast, then been carried out to sea, possibly by a flooded river. Its bloated carcass may have floated before sinking, belly-up, to the sea floor, where minerals rapidly formed around it.

Because it was buried quickly in the marine sediment, the armour and skin stayed in their original positions. The animal is estimated to have been about 5.5 metres long and to have weighed around 1,300 kilograms in life, with large spines over its shoulders about half a metre long.

Camouflage and colour

The researchers detected chemical traces of the reddish-brown pigment pheomelanin in the skin. Its distribution indicated that the animal was darker on its back and lighter on its belly, a pattern called countershading.

In living animals, countershading helps conceal the body by cancelling out the shadows cast by overhead light. Today it is common among smaller animals that need to hide from predators, while very large herbivores such as elephants rarely show it. The finding suggests that even a heavily armoured dinosaur of over a tonne faced serious danger from large predators of its time.

The last meal

In 2020 Brown and colleagues reported on a mass of material preserved in the stomach area. It consisted mostly of fern leaves, together with smaller amounts of other plant material and charcoal.

The selection of ferns suggests that Borealopelta was a selective browser rather than eating any plants within reach. The charcoal indicates that it fed in an area recovering from a recent wildfire, where young ferns often grow quickly. Features of the plant remains suggested that the animal died in late spring or summer.

Timeline

• c. 112–110 million years ago: Borealopelta lives near the coast of an inland sea in what is now Alberta.
• 2011, 21 March: A mine equipment operator uncovers the fossil at the Millennium Mine near Fort McMurray.
• 2011: The Royal Tyrrell Museum excavates the specimen and moves it to Drumheller.
• 2011–2017: Mark Mitchell spends more than 7,000 hours preparing the fossil.
• 2017: The species is named Borealopelta markmitchelli, and the fossil goes on display.
• 2020: A study of its stomach contents reveals a last meal of ferns.

Why it matters

Borealopelta shows what an armoured dinosaur actually looked like, with its plates and skin in place rather than reconstructed from scattered bones. Its pigment and stomach contents add rare evidence about dinosaur colour, camouflage and diet, joining discoveries such as Sinosauropteryx in turning fossil colour into a subject of real scientific study. The find also shows how industrial workers can play a vital part in palaeontology.