The findings might also help explain how turtles’ ancestors survived a mass extinction 250 million years ago that wiped out most plants and animals on earth, scientists report online July 14 in Current Biology.
Most shelled animals, like armadillos, get their shells by adding bony scales all over their bodies. Turtles, though, form shells by gradually broadening their ribs until the bones fuse together. Fossils from ancient reptiles with partial shells made from thickened ribs suggest that turtles’ ancestors began to suit up in the same way.
It’s an unusual mechanism, says Tyler Lyson, a paleontologist at the Denver Museum of Nature and Science who led the study. Thicker ribs don’t offer much in the way of protection until they’re fully fused, as they are in modern turtles. And the modification makes critical functions like moving and breathing much harder — a steep price for an animal to pay. So Lyson suspected there was some advantage other than protection to the partial shells".