The adult tuatara reptile (Sphenodon punctatus, seen above) has no penis. But among amniotic animals—reptiles, birds, and mammals—that’s not even that weird. Many snakes have double “hemipenes” and ducks have spiraling phalluses that look like fusilli pasta. This diversity in amniotic genitals has puzzled scientists as they try to map out the evolutionary tree of life. The fact that the tuataras don’t have penises makes them a useful study organism because it allows scientists to ask whether the reptiles’ ancestors had penises and lost them, or never had them to begin with. Now, according to new research published today in Biology Letters, the last common ancestor of all the amniotes did, in fact, have an erectable phallus, and that the modern diversity is the result of evolutionary tweaks over time (not the separate evolution of different phalluses). The findings come from an analysis of Victorian-era tuatara embryos preserved on slides at the Harvard Embryological Collection. Scientists used pictures of the reptile embryos to recreate their 3D structure using computer software. The team discovered that, even though the tuatara has no external phallus, it grows the beginnings of one during development. At some point down the line this “genital swelling” recedes and the reptiles are born sans phallus. The embryonic development suggests that amniotic penises only evolved once, but that some animals have lost theirs over time.

*Correction, 30 October, 12:21 p.m.: An earlier version of this article mistakenly referred to the tuataras as lizards. Although they are scaly reptiles, the animals are technically part of their own distinct lineage.