Everybody knows how oysters make pearls — a bit of sand or grit slips through the protective barrier of their outer shell, irritating the mollusk’s body, and the invertebrate encircles the invader with shell material. As it turns out, ammonoids — the extinct, coil-shelled cousins of modern squid and nautilus — made pearls, too, but they did so to stave off the onslaught of prehistoric parasites.

The idea that ammonoids made pearls isn’t new. For years, paleontologists had noticed strange pits on the internal casts of Devonian-age ammonoid shells, and this indicated that the inside of the shell — which had since dissolved away — was pockmarked with