Fossils of thumb-sized fish that lived about 385 million years ago hint that the oldest jawed vertebrates copulated and fertilized their eggs internally, pushing back the origins of this form of mating by at least 10 million years and into more primitive groups of these fish than previously known. A new analysis of Microbrachius fossils, which include the most primitive subgroup of vertebrates with jaws, called antiarch placoderms, reveals that some of these armored fish (presumed to be males) have a deeply grooved bone that extends from each side of their pelvis. That feature resembles the “claspers” used by modern-day sharks, skates, and other cartilaginous fish to channel sperm during mating—and, the researchers report online today in Nature, likely served the same purpose in these ancient swimmers. Microbrachius fossils that don’t sport the grooved bones, presumed to be females, instead have dermal plates beneath the pelvis; ridges and bumps on the inside of those plates probably helped the females firmly hold the males’ organ during side-by-side copulation (artist’s representation above). Today’s paper also suggests that external fertilization, or spawning, in fish evolved later than internal fertilization—precisely the opposite of what most scientists have long proposed. One big question now is whether even more ancient species within the antiarch placoderms bred in the same way as the fossils described here. If so, that would push back the origins of copulation within the group to 430 million years ago.