August 14. Permian ammonites

Cephalopods are the group that includes the octopuses, squids, and chambered nautilus. The extinct ammonites were spiral cephalopods that thrived in the Paleozoic and Mesozoic. Ammonites diversified during the Permian Period, but many varieties including the goniatites were killed off in the end Permian extinction.

Most Permian ammonites were relatively small, a few inches across. Many ammonites are associated with another fossil, called aptychus, which was thought for years to be the shells of a clam-like mollusk. Now it is thought to be part of the ammonite animal, but exactly what isn’t settled yet. It might be a trap-door-like mechanism, an operculum, that sealed the animal inside its shell when it was inactive. Snails have things like that. But it’s also possible that it may be some kind of jaw apparatus that helped the animal munch its prey. Or just maybe, it served both purposes. Paleontologists are still working on that little question.

—Richard I. Gibson

Drawing of Permian ammonite from an old text (public domain)