This dinosaur dug its own grave

Dinosaurs are difficult to find among the 99-million-year-old rocks of Utah’s eastern desert. Most are only ever found as fractured bits and pieces eroding in the sun. But a new dinosaur named Fona herzogae presents an exception to the rule. The small herbivore burrowed into the ground, creating dens where unlucky dinosaurs were entombed and preserved more often than their Cretaceous neighbors.

In the broader dinosaur family tree, Fona belonged to a group of medium-sized herbivores called thescelosaurs. These plant-eaters didn’t have ornate spikes, horns, crests, or other bizarre ornaments, making them the dinosaurian equivalents to sheep. But recent evidence suggests that thescelosaurs were among the few known dinosaurs species that burrowed into the ground, a habit that may have helped their bodies enter the fossil record more frequently than the other small dinosaurs they lived alongside. Paleontologists described the new thescelosaur earlier this month in The Anatomical Record.

Amid the early Cretaceous-period rocks of eastern Utah, “skeletons of Fona are among the most abundant dinosaur material we find,” says North Carolina Museum of Natural Sciences paleontologist and study author Haviv Avrahami. In addition to a nearly-complete skeleton, the team found multiple other Fona skeletons and individual bones at different locations in the same rock layer.

Digging in

Paleontologists usually don’t expect to find such a rich accumulation of small dinosaurs. Adult Fona were about seven feet long, or about the size of a large dog when the dinosaur’s long tail is accounted for. Not only were such small dinosaurs often prey for larger dinosaurs, small species had more delicate bones, and their carcasses were more likely to be broken up or destroyed prior to fossilization. Even the waterways that once crisscrossed this ecosystem, Avrahami says, could cause a carcass to rot, pull apart, and scatter.

Burrowing allowed Fona to defy expectations for small dinosaurs. The dinosaur skeletons appear to have been buried and preserved within underground cavities, similar to another burrowing dinosaur named Oryctodromeus described in 2007. Until that discovery, paleontologists had no idea that dinosaurs burrowed at all. Now the ability seems to have evolved more than once among small, beaked herbivores. One fossil site described by the authors contains at least two Fona intertwined with each other, “which is exactly what you’d expect from animals sharing the same burrow,” says Emory University paleontologist Anthony Martin, who was not involved in the new study. 

Like other burrowing dinosaur fossils, the bones of Fona show adaptations associated with digging into the soil. Fona had sturdy hips, with some bones fused together, and expanded shoulder bones for broader arm muscle attachments. These skeletal modifications would have given the dinosaur’s arms greater power for scratching at the ground, the hips helping the dinosaur bear the stress of the activity. Avrahami hypothesizes that the dinosaur might have dug the same way some mammals like springhares do, using their arms to scratch soil loose before using their legs to kick the sediment out of the way. (Dinosaurs weren’t the only ancient burrowers. Read about an ancient burrowing mammal.)

Burrowing benefits

Creating underground refuges could help explain why small, plant-eaters like Fona were able to survive and thrive in landscapes full of large predators and harsh weather. “If you’re wee little dinosaur without many other natural defenses, burrowing makes sense as an adaptation for surviving in environments with big predators, fires, storms, heat, cold, or any other stresses,” Martin says.

Other small, herbivorous dinosaur species might have dug shelters into the ground, as well. Martin notes that similar, small dinosaurs such as Nanosaurus from the earlier Jurassic period are good candidates to further investigate the history of burrowing dinosaurs. The earlier species resemble Fona, and it’s possible that they, too, dug burrows to shelter from sharp teeth and harsh storms. Older herbivores may have independently evolved burrowing abilities just like Fona and Oryctodromeus. 

While perhaps tragic for the living Fona, the burial of so many small dinosaurs is a lucky break for paleontologists. Precisely what happened to the various thescelosaurs found in eastern Utah may never be known. Perhaps a local flood inundated the dinosaurs while they were sheltering underground. Whatever killed them, the dinosaurs were buried rapidly enough that several skeletons have articulated bones instead of just a jumble of pieces. “Their burrows would have acted like a protective royal tomb,” Avrahami says, “preventing the erasure of their life story from the fossil record.”