Dinosaurs got buried in all sorts of ways. Their bodies were covered by lake sediment, engulfed by quicksand, encrusted by marine life, and more. But one of the most spectacular burials of all took place in the Cretaceous high Arctic, where scores of juvenile dinosaurs were buried together.

A diagram of a mass dinosaur burial in Alaska, showing layers of plants and bones that spilled over the prehistoric floodplain. From Flaig et al, 2014.

University of Texas geologist Peter Flaig and colleagues have laid out what happened in a new PALAIOS paper. Around 69 million years ago, on land now preserved on Alaska’s North Slope, dinosaurs roamed along the coast. There were horned dinosaurs, tyrannosaurs, and more, but the most common dinosaur of all was the shovel-beaked herbivore Edmontosaurus.

Life wasn’t easy for the Arctic Edmontosaurus. They likely remained in the same habitats through the cold and the dark of Cretaceous Alaskan winters. Their growth slowed in response to the lack of vegetation, a trend seen in the microscopic details of their bones. And even when spring returned, unwary Edmontosaurus fell victim to a different environmental danger.

The dinosaurs lived in the lowlands near the ancient Brooks Range. When spring temperatures started to melt the snow and ice held in the mountains, floods started to rage down the rivers towards the sea. And as the water rushed downhill, it started to pick up ash and clay that had built up on point bars and riverbanks. This turned the flow into something more dangerous than a regular flood.

A skeleton of Edmontosaurus on display at the Natural History Museum of Los Angeles. Photo by Brian Switek.

Regular floods pick up and deposit sediment as they go, and, under the right conditions, a combination of water, sediment, and rocks can create extra-slick slides called debris flows. What buried the Edmontosaurus bones was something in-between – what geologists call a hyperconcentrated flow.

The slurry of water, clay, ash, and mud picked up dinosaur bones that had already come to rest inside the river channels. These animals died prior to the event, perhaps, Fiorillo and others have speculated, as they broke through thinning ice as spring crept into Cretaceous Alaska. But the fast-moving flow also took living dinosaurs, too, drowning them in the viscous flow. (This idea is based on the microstructure of Edmontosaurus bones that show the dinosaurs were just starting to grow quickly again, a sign that they died as spring was returning to the Arctic.)

As the flood approached the coast, it jumped levees and other barriers. The slurry of dinosaurs, plants, and debris spilled out onto the floodplain and started to slow. The larger bones and dinosaur body parts, which had been rolling at the bottom, stopped. Smaller bones and plants, suspended in the muddy water, also slowed and came to rest on top.

All this mess didn’t stay on the surface for long. The bones don’t show signs of scavenging or long-term exposure, Flaig and colleagues write, indicating that they were covered up as springtime lakes and swamps formed over them, burying and protecting them from disturbance.

Calling these events “dinoslides” might not be quite right. There are certain geologic particulars that distinguish floods, debris flows, mud flows, and other events. But I just can’t resist, if only for the fact that this new paper has given me an idea for a fossil-themed cocktail…

Reference: Flaig, P., Fiorillo, A., McCarthy, P. 2014. Dinosaur-bearing hyperconcentrated flows of Cretaceous Arctic Alaska: Recurring catastrophic event beds on a distal paleopolar coastal plain. PALAIOS. 29: 594-611. doi: 10.2110/palo.2013.133