Our skeletons are time capsules. Even though the whole marks us as a distinct species, we’re also a collection of elements that hearken back to our deep history. Teeth are among the oldest of these evolutionary curiosities. The enamel-covered structures were supposed to be the first hard parts our squirmy, swimming ancestors developed, later covering the bodies of pseudo-fish and setting the stage for the evolution of the vertebrate skeleton. As it turns out, however, the story of the first teeth isn’t so much inside-out as outside-in.

Enigmatic creatures called conodonts have been key to ideas about how teeth and skeletons evolved. Found in strata between 500 and 200 million years old, these early animals were initially known only as spike-like