Not long after these animals died, the rains finally arrived — too heavily — triggering massive mud flows that caught up and quickly buried the animal remains.
"I think they came together in death, perhaps in a drought," said paleo-death specialist Raymond Rogers of Macalester College in St. Paul, Minn.
Teeth marks on the bones indicates that scavengers — sometimes of the same species as the dead — gnawed on the carcasses before they were buried by mud. There are even signs that insects made use of some bones before the floods came, Rogers said.
Rogers published his findings on the creation of the Berivotra fossil beds in the April issue of the journal Geology.
The floods that buried the bones were not an annual event, but something exceptional, not unlike those recently experienced in Southern California, Rogers said. The heavy rains probably caused severe erosion of the clay-rich landscape in the area, leading to muddy slurries that filled the low-lying areas, entombing the corpses of the drought victims there.
All that clay turned out to be ideal for protecting and preserving bones, teeth, skin impressions and even some softer materials, like keratin found in the bones of at least one bird at Berivotra, he said.
Today, the ground containing the fossils is a rolling grassland, Rogers said, tumbling out of which are ample fossils of giant dinosaurs alongside tiny birds. That juxtaposition of sizes is a sign that the mud flows were powerful, moving carcasses together without sorting larger from smaller remains.
Excavations of the grasslands have revealed at least three flood events that created six- to seven-foot-deep fossil-loaded clay layers, interspersed with fossil-poor, non-flooding river deposits. It all points to a climate with occasionally extreme wet and dry cycles, Rogers said.
In fossil beds from the same period elsewhere in the world, the sediments have usually undergone deeper burial, heating and other changes that crush, flatten, re-mineralize and otherwise alter the fossils. Not so at Berivotra, Rogers said.
"Fossil preservation is not the norm," said sedimentologist David Loope of the University of Nebraska, in Lincoln. "It's the unusual event that leads to fossilization."
"It's a pretty unusual window into the fossil record," Rogers said.
