SAN DIEGO (KGTV) — A tweet from a UCSD Postdoc went viral this month. Turtle expert Ceri Weber posted that a turtle's shell is actually not a shell at all, but rather a set of modified ribs.
It's true. Turtles are the only known animal on earth that forms a shell on the outside of its body through a fusion of modified ribs, vertebrae, and shoulder girdle bones.
It's believed that process started around 250 million years ago.
This differs greatly from a crab whose shell is made up of hard bone, and snails whose shells are composed of calcium carbonate.