There were many genera among the family Protocetidae. There were different degrees of aquatic adaptations in this group, with some able to support their weight on land, and others not. The discovery of the southeastern Pacific "Peregocetus" indicates they crossed the Atlantic and achieved circumstances-equatorial distribution by 40 mya. Their amphibious nature is supported by the discovery of a pregnant "Maiacetus", in which the fossilised fetus was positioned for a head-first delivery, suggesting that "Maiacetus" gave birth on land. If they gave birth in the water, the fetus would be positioned for a tail-first delivery to avoid drowning during birth.