Onychophorans are the only phylum without current marine representatives and one of the few invertebrates with placenta. Here we propose that the common ancestor, which first emerged to the mainland in Pangea’s time, kept its eggs inside the female, feeding them continuously through a very thin shell (matrotrophic viviparity). Therefore, this mechanism is still common and widely distributed in onychophorans throughout the world: other mechanisms evolved locally in more recent times.

Keywords: parsimony, Occam's razor, lecithotropic viviparity, matrotrophic viviparity, placental viviparity, evolutionary pressures, reproduction with placenta