Fossil fans may remember that earlier this year, a newly discovered armoured giant named Borealopelta made a big splash as one of the best-preserved dinosaurs of all time. After being swept out to sea more than 100 million years ago, the animal was interred belly-up on the seafloor and ultimately transformed into a beautiful, statuesque fossil.

Borealopelta, on display at the Royal Tyrrell Museum in Alberta, is a stunning fossil ankylosaur (a nodosaurid, more specifically) that was buried upside-down underwater, a very common position for these armoured dinosaurs. Image: David Moscato

Borealopelta may be one of a kind, but it follows a puzzling trend of ankylosaurs (the group of dinosaurs characterised by coverings of bony armour) being entombed upside-down. For decades, palaeontologists have mused that an inverted resting pose isn't just common in these dinosaurs – it seems to be the norm!

But if that's true, it begs the question: Why?

"The idea that ankylosaurs are typically preserved upside-down is found in the scientific literature dating back to 1933, if not before," said Jordan Mallon of the Canadian Museum of Nature in an email. "It's still commonly repeated in textbooks, although no one has ever done the work of actually testing it."

This isn't just a question of curiosity. How a dinosaur is buried affects how it's preserved, which can in turn change what palaeontologists are able to learn about it. "[T]here's no doubt that upside-down ankylosaurs lose much of the anatomy of the undercarriage (e.g., the palate) to erosion," Mallon explained, "whereas right-side-up ankylosaurs lose features along the top side (e.g., armour)."

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Another upside-down ankylosaur, out in the field in the position it was found. Image: Canadian Museum of Nature

Mallon compiled notes on more than three dozen ankylosaur fossils known from Alberta, and sure enough, almost three-quarters of them were buried and fossilised on their backs. The next step was to figure out why this was the case, and to do that, he brought together a team of colleagues, including one computer modeller and – intriguingly – two armadillo experts.

"There have been many hypotheses put forward to explain why ankylosaurs are so often inverted," Mallon said. Together, the researchers tested three major suggestions: carnivore activity, bloating and flipping on land, and bloating and flipping in the water.

Testing whether or not these dinosaurs were being flipped by hungry scavengers or predators (perhaps to get to their soft bellies?) was simple enough: look for bite marks, which are often easy to find on chewed-up dinosaur bones. But a search revealed that only one of the 37 fossils examined had any signs of predation. It seems hungry carnivores are probably not responsible for these overturned fossils.

The second suggestion – that the dinosaurs may have turned over as they decomposed on land – is inspired by the same thing supposedly happening with modern armadillos. So Mallon's team – particularly those armadillo experts – got very familiar with roadkill armadillos (nearly 200 of them!) and discovered that, regardless of various decomposition processes, armadillo carcasses simply don't end up on their backs more often than on their bellies or sides. Another unlikely explanation for our upside-down dinosaurs.

Palaeontologists often turn to modern animals for insights into extinct ones. In this case, they examined whether dead armadillos follow the same upside-down patterns as ankylosaurs. Image: Jin Loughry and Colleen McDonough

The third hypothesis turned out to be much more promising. Even today, it's fairly common for dead animals to end up in ponds, rivers or oceans, and to bob along the surface, inflated like gross balloons by the gases of decomposition (a phenomenon that scientists have dubbed "bloat and float"). Would a dead, gas-filled ankylosaur be prone to capsizing?

According to computer simulations, yes! The researchers built virtual ankylosaur corpses and examined their stability as they virtually expanded – and it turns out that as the floating bodies bloated, their centre of mass and centre of buoyancy gradually separated until the body was unstable right-side up, causing it to flip! Then, when the body finally ruptured and released the gas inside, the remains would sink, armour-down, below the water.

Computer models of ankylosaurid (with tail club) and nodosaurid (no tail club) ankylosaurs. These simulations helped show how the animals' stability in water would change as they bloated. Image: Donald Henderson

This phenomenon also explains why upside-down ankylosaurs, like Borealopelta, tend to be buried in aquatic environments. "[W]e show that upside-down ankylosaurs are usually preserved in river channel or marine deposits (i.e., aqueous settings) in North America," Mallon said, "whereas right side-up ankylosaurs are often found in dune collapse deposits (i.e., terrestrial settings) in Asia."

Donald Henderson, curator of dinosaurs at the Royal Tyrrell Museum and a member of this research team, suspects this process of bloat, float, flip and sink is what allowed Borealopelta to become so beautifully preserved. "It went to the seabed upside-down, and because it was pressed into the seabed fine mud, that protected it," he explained. "It sealed it away from the oxygenated water, and so there were no scavengers, no bacterial activity. In that situation, being on its back was a really good way to preserve all that detail."

The thick, armoured skin of these dinosaurs may also be an important part of this puzzle. Sometimes, a bloat-and-float carcass will drop bits of itself as it decomposes at the water’s surface. In the case of the ankylosaurs, Henderson said, "[We argue that] the skin is so thick and so strong, it could hold the high gas pressure, [and the body] could get well off-shore before it finally goes pop and goes down like a stone."

All extinct species come to us through the filter of the fossilization process. For ankylosaurs, it seems being thick-skinned and top-heavy has a strong effect on what kinds of remains they left behind for us. And they may not be the only ones: the researchers suggest the same thing may be true of glyptodonts – the Volkswagen-sized armadillo cousins of the Ice Age – which are also said to be commonly inverted. Perhaps they had more in common with ankylosaurs than just their armour!