These archaeocete families document that the Eocene cetacean evolution is characterized by increasing aquatic adaptations, starting at amphibious early whales (pakicetids, ambulocetids) to more marine protocetids.
A reconstruction of
Pakicetus.
Illustration by Carl Buell, and taken from http://www.neoucom.edu/Depts/Anat/Pakicetid.html
The
skeleton of
Ambulocetus
natans (approximately 12 feet long).
Illustration taken from http://www.neoucom.edu/Depts/Anat/Pakicetid.html
Reconstruction of
the
remingtonocetid Kutchicetus.
It was a small animal, no bigger than a river otter.
Illustration by Carl
Buell and taken from http://www.neoucom.edu/Depts/Anat/Remi.html.
Partial
skeleton of Rodhocetus.
Reconstruction of
Basilosaurus cetoides .
Illustration taken from http://darla.neoucom.edu/DEPTS/ANAT/BasilAndDor.htm.
Reconstruction of
Zygorhiza kochii (not to scale).
Illustration taken from http://darla.neoucom.edu/DEPTS/ANAT/BasilAndDor.htm.
Basilosaurids and dorudontids lived in the late Eocene, approximately 35 and 41 million years ago. They are mainly known from the eastern United States and from Egypt, but were probably worldwide in their distribution. Basilosaurids were enormous (possibly up to 60 feet long) and had snake-like bodies. They had a tail fluke. Dorudontids were proportionally more like dolphins. Both basilosaurids and dorudontids had complete hind limbs that included a mobile knee and several toes. However these extremities were tiny, so small that they were certainly not important in aquatic propulsion.