 The first oceanic dolphins such as kentriodonts , evolved in the late Oligocene and diversified greatly during the mid @-@ Miocene . The first fossil cetaceans near shallow seas ( where porpoises inhabit ) were found around the North Pacific ; species like Semirostrum were found along California ( in what was then estuaries ) . These animals spreading to the European coasts and Southern Hemisphere only much later , during the Pliocene . The earliest known ancestor of arctic whales is Denebola brachycephala from the late Miocene around 9 – 10 million years ago . A single fossil from Baja California indicates the family once inhabited warmer waters . 
