 The protocetids form a diverse and heterogeneous group known from Asia, Europe, Africa, and North America. They lived in the Eocene, approximately 48 to 35 million years ago. The fossil remains of protocetids were uncovered from coastal and lagoonal facies in South Asia; unlike previous cetacean families, their fossils uncovered from Africa and North America also include open marine forms. They were probably amphibious, but more aquatic compared to remingtonocetids. Protocetids were the first cetaceans to leave the Indian subcontinent and disperse to all shallow subtropical oceans of the world. There were many genera among the family Protocetidae. Great variations in aquatic adaptations exist among them, with some probably able to support their weight on land, whereas others could not. Their supposed 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.
