Fig. 20
From: From Land to Water: the Origin of Whales, Dolphins, and Porpoises
The earliest cetaceans had nasal bones (gray) and a nasal opening (black) near the tip of the snout. As cetaceans became more aquatic, the nasal bones retracted and the nasal opening migrated to the top of the skull and became the blowhole (modified from Thewissen and Bajpai 2001b)