Ankylosaurs were terrible swimmers. It’s hard to see how they wouldn’t be. Nothing about their anatomy says or even whispers “aquatic,” least of all the rows of bony ornaments that would have made them about as buoyant as a brick. So why, then, do their remains keep turning up in rock that used to be ancient seabed?

This is more than just a curious little happenstance. More than one new species of ankylosaur has been found in marine sediments. Aletopelta, found in southern California, was previously unknown until paleontologists excavated the vestiges of the armored dinosaur that inadvertently became a temporary reef. And Borealopelta, named just this past year, is the most exquisite ankylosaur ever found – pointed scutes so delicately fossilized that experts were able to determine this trundling herbivore was colored a rosy red. 

And the mystery goes deeper still. Whether in strata laid down by oceans, rivers, or floodplains, ankylosaurs are often found upside down. This, it seems, may be the crucial clue to the mystery of the ankylosaurs that went out to sea. The fact that so many ankylosaurs are found belly up, paleontologist Jordan Mallon and colleagues argue, is a sign of what happened to these dinosaurs during the earliest part of their afterlives.

In the science of taphonomy – roughly, what happens to an organism between its death and eventual discovery – experts are familiar with a phenomenon called “bloat and float.” Keeping our focus on dinosaurs here, the basic idea is that any body not quickly buried will eventually bloat with gases to the point where it can actually float in water (should it find itself swept up or jostled into the aquatic environment somehow). In time, as the body decomposes or is chewed on by scavengers, the gases are released and the carcass settles for its ultimate burial. 

Bloat and float had to be responsible for the likes of Aletopelta and Borealopelta. These dinosaurs must have died inland and somehow their bodies were swept along waterways to the sea, where they floated for an unknown tally of days before dropping to the bottom. The new study by Mallon and colleagues helps explains how this actually happened.

The initial inspiration for the new study was the fact that many of North America’s Cretaceous ankylosaurs were often preserved upside down. The famous fossil hunter Charles M. Sternberg, for example, once wrote that almost all ankylosaurs he excavated from the Late Cretaceous rock of Alberta were found this way. When Mallon and colleagues checked, it turned out that Sternberg was right – 26 of 36 specimens were buried upside down. 

There’s more than one way to flip an ankylosaur, of course. Over the past century other experts have suggested that carnivores might have toppled the armored dinosaurs while they scavenged, that postmortem bloat in terrestrial environments might tip ankylosaurs over, and even that the likes of Ankylosaurus were so clumsy that they accidentally rolled down hills to find themselves helplessly on their backs. But the best fit, the researchers found, was the bloat and float model.

Working from computer models of the ankylosaurs Euplocephalusand Sauropelta, Mallon and coauthors estimated how the still-living and dead, bloated bodies of these dinosaurs would have behaved in water. The two models weren't exactly the same. While Sauropelta or a similar dinosaur would have been awful in the water – tipped over with the slightest disturbance alive or dead – the Euoplocephalus model proved to be surprisingly stable to going belly up. Still, when either form was tipped over they would have stayed in that orientation.

The resulting scenario goes something like this. Postmortem bloating made ankylosaur carcasses unstable in water, Mallon and coauthors propose, “apt to overturn with only a slight bump of a sandbar or a disturbance in the water.” The dinosaur would then stay like this, “with the ballooned belly floating near the surface, and the dorsum weighted down by the heavy armour.” More than that, the slight difference in pre-flip stability might help explain why certain ankylosaurs became preserved.

Paleontologists know two flavors of ankylosaur from the Late Cretaceous of North America – nodosaurids like Sauropelta and ankylosaurids like Euplocephalus. But there seem to be more nodosaurids found in marine rocks than ankylosaurids. Their instability in the water might explain why. Nodosaurids were more likely to tip, and so a live nodosaurid finding itself in water would be more likely to drown. Nodosaurids also lacked tail clubs, and, while the notion is speculative, Mallon and colleagues suggest that this prevented them from becoming entangled in near-shore vegetation or other obstacles that could have hung up their ankylosaurid relatives. 

Every dinosaur skeleton – every dinosaur fossil – presents us with a puzzle. In many circumstances, understanding what happened between the animal’s death and its excavation may never be known with much clarity. But discoveries like Aletopelta and Borealopelta offer us a window to an unseen sequence of events, a series of chance occurrences that must have happened in order for such rare parts of prehistory to be preserved. These dinosaur balloons might not have been good for a party, but they give paleontologists plenty to celebrate.