 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 . 
