The Evolution of Whales: 4. Whales become aquatic




Rodhocetus[1] is again more whale-like than Ambulocetus but was still able to return to land. It had paddle-like feet and a strong tail, which may have acted as a rudder when swimming. Rodhocetus provided the evidence of the evolutionary link between whales and the hoofed artiodactyls, thanks to their distinctive ankle bones, which are unique to artiodactyls and whales in the mammal world[2].

Basilosaurus[3], initially mis-identified as a marine reptile in the 19th century, was fully aquatic. Its hind legs are vestigial and would have been unable to support it on land. Its unusually elongated shape means that it was not necessarily closely related to an ancestor of modern whales, but more likely on a branch that no longer survives today.

Further Details

[1] Rodhocetus

Wikipedia: Rodhocetus

[2] Ankle bones

Artiodactyls and whales have an ankle bone, known as an astralagus, with a "double-pulley" shape that is unique in the mammal world. All other mammals have a single-pulley ("single-spooled") ankle structure.

This was the clincher for the relation between artiodactyls and whales, see previous slides.

[3] Basilosaurus

First discovered in Egypt (under the Tethys sea at the time). Initially thought to be as reptile, hence the "saurus" in the name.

Wikipedia: Basilosaurus

Squidoo on Basilosaurus