Ambulocetus natans

The walking whale 

When much of the macro biotic slate was cleaned at the end of the Cretaceous, the surviving mammal species wasted no time explosively diversifying into the newly available ecological niches.

With the absence of Mosasaurs and other Mesozoic aquatic terrors, the opportunity-rich oceans and bays were open to colonization, and some terrestrial mammals took advantage almost immediately. Current thinking is that whale ancestors were small artiodactyls the same as, or similar to the ancestors of Hippos. In roughly 8 million years the process of terrestrial to aquatic adaptation was basically complete.

Ambulocetus is one of the earliest and best examples we have of this step-wise march back to the sea. Dating from the Eocene 49 million years ago, its remains are known to be associated with coastal and estruine environments in what is now Pakistan. Ambulocetus was 12 feet long and is thought to have lived like a crocodile and swam like an otter. Obviously crocodiles survived the Cretaceous extinctions, so assuming Ambulocetus lived in a manner similar to them, it is not clear if Ambulocetus competed directly with these large saurians or for some reason crocs were not numerous when and where Ambulocetus lived. Perhaps Ambulocetus lost the battle for species survival to the Crocodiles.

Skeletal images courtesy of Dr. Hans Thewissen