Skeleton season may be just around the corner, but the skeleton age dawned with the early Cambrian Period, about 538 million to 506 million years ago.
In this time span, most major animal groups independently evolved methods to build mineral skeletons or shells, usually in one of two ways: They either built up mineral tissues using organic scaffolding, like how we grow our bones and teeth, or they gathered materials from their environment and “glued” them together in a protective coating.
Then they stuck with that technique for the next 540 million-plus years. One notable exception can be found in the fossilized remains of Salterella, a tiny creature that thrived in the early Cambrian and is so common in rocks from that time that paleontologists use it as an index fossil to orient themselves in time.