Sometimes, in science, you just have to digitally reconstruct the genitals of a thinly sliced, hundred-year-old embryo from an obscure New Zealand reptile, because you really want to know if your penis shares a common evolutionary history with those of crocodiles, birds, and snakes.

Spoiler: it does.

Among us back-boned animals—vertebrates—penises vary considerably, not just in their size and shape but in their presence, absence, and number. Male mammals have just the one, as do crocodiles, turtles, and some primitive birds like ostriches and ducks. Snakes and lizards have two ‘hemipenes’ (although they only use one at a time). And the vast majority of birds have none at all.

Despite this variety, all these groups start out in much the same