The ancestor of almost all living animals is the, er, "penis worm"

The modern day penis worm’s mouth apparatus is very similar to that belonging to the Pambdelurion.

“The new study demonstrates very well that the mouth apparatus is consistent with the structure of the penis worm’s mouth,” says Milàn.

“The penis worm consists of a primitive mouth apparatus, but it’s strongly modified among arthropods and there’s no resemblance at all among modern arthropods,” he says.

It gives penis worm researchers a model of the common ancestor of all living arthropods, which seems to have been a kind of penis worm with legs.