Ankylosaurs are odd-looking, even by dinosaur standards: They’re squat and fat, with armored backs and, usually, tail clubs. But for many scientists, there’s another reason these creatures stand out—most are fossilized upside-down. The reason for this strange orientation was a mystery for decades, but thanks to an unusual collaboration between paleontologists and armadillo experts, we may finally have an answer—and it all comes down to bloated, floating dinosaur carcasses.
Since the 1930s, paleontologists have suspected something funny about ankylosaurs’ physiology or behavior led to their belly-up preservation. But although several theories have been put forward, none has been proved. So Jordan Mallon, a dinosaur paleontologist at the Canadian Museum of Nature in Ottawa, decided to test each of these theories and try to solve the mystery.
First, Mallon’s team wanted to make sure that the common wisdom on ankylosaur orientation wasn’t just an old wives’ tale. After examining 36 ankylosaurs from the province of Alberta in Canada, along with photos and field notes of the excavations, they found that 26 were discovered upside-down—more than would be expected by chance.
The team then turned its attention to the four theories. One, that ankylosaurs simply fell down hills and ended up on their backs, was easy to discount: “If ankylosaurs were universally clumsy, then you wouldn’t expect them to stick around for 100 million years,” Mallon says. Likewise, the researchers found no support for the theory that predators flipped ankylosaurs over to access their delicious underbellies. Teeth marks were only present on one of the specimens they examined.
The “armadillo roadkill model” proved trickier. Because armadillos found on the side of the road supposedly swell up with gases as they decompose, tipping them onto their backs, the same could be true of ankylosaurs. Mallon enlisted the help of armadillo experts Colleen McDonough and Jim Loughry at Valdosta State University in Georgia who spent 3 months surveying armadillo carcasses by the side of the road. The husband-and-wife team would set off at sunrise, McDonough says, before roadkill had become “pancaked” or had been pecked to pieces by vultures. With help from family and friends who sent tip-offs about the location of fresh carcasses (“we call them our network,” McDonough says), the duo recorded the orientation of 174 armadillos—and found no indication that they regularly ended up on their backs.
On the roads, dead armadillos could be moved into their final resting places by scavengers or vehicles, so the scientists also decided to bring some fresh carcasses home, leaving them in their garden to see what would happen as they decomposed. In the paper, the pair thanks their understanding neighbors. “There was a prevalent stench coming from the corner of our yard where we meet three other yards—and no one said a word,” McDonough says. But none of the armadillos placed on their stomachs rolled over, again suggesting that this model didn’t hold up to scrutiny.
Finally, the researchers examined the “bloat-and-float” model, which proposes that the bodies of ankylosaurs got washed into rivers or the sea, where they bloated and became unstable, flipping upside-down and eventually sinking or being deposited in the river bank. Mallon’s co-author Donald Henderson at the Royal Tyrrell Museum in Drumheller, Canada, created 3D digital models representing the two families of ankylosaur, the ankylosaurids and the nodosaurids. The team fleshed these out with estimations of the dinosaurs’ physical characteristics like lung capacity and bone density. They also created bloated versions of the same dinosaurs, inflating the animals’ stomachs like balloons. They then placed the models in virtual water, and looked at how easily they tipped over.
The nodosaurid model was very unstable: Tipping it merely 1° caused it to turn upside-down, whether it was bloated or not. The ankylosaurid was more resistant, requiring a much larger tilt before it would flip over. Still, says Mallon, a big wave or a predator could easily be enough to force the dinosaur onto its back, especially after bloating. That meant the bloat-and-float model was the only theory that held any water, the team reports this month in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.
Victoria Arbour, a vertebrate paleontologist at the Royal Ontario Museum and the University of Toronto in Canada who was not involved in the research, says that the study “reasonably seals the deal” on the long-standing mystery. She adds that she admires the scientists’ approach to the research. “I always really like it in paleontology when we can use totally independent lines of evidence like that to tell a bigger story,” she says. Mallon agrees: “It’s pretty rare that the scientific method plays out so cleanly in practice, but I think this is a nice case where it does.”