As many readers will likely already know, the ancestors of today's whales started out as land-based animals that walked on four legs. New research now suggests that previously-discovered fossils represent an evolutionarily-important point in prehistory, at which the creatures were moving from swimming with those legs to swimming with their tails.
In 2007, the fossilized remains of a creature known as Aegicetus gehennae were discovered in Egypt's Western Desert. It lived about 35 million years ago, and was a member of the protocetid family of early semi-aquatic whales.
Led by paleontologist Philip Gingerich, a team from the University of Michigan recently took a fresh a look at the bones. Among other things, the scientists determined that Aegicetus likely swam with an undulatory motion, similar to that of a crocodile. This was unlike the swimming styles of earlier protocetids, and of later fully-aquatic whales.
"Protocetid whales living 47 to 41 million years ago were foot-powered swimmers. Later, starting about 37 million years ago, whales became tail-powered swimmers," says Prof. Gingerich. "Aegicetus was intermediate in time and form, and was transitional functionally in having the larger and more powerful vertebral column of a tail-powered swimmer."
The individual whale to which the mostly-complete skeleton belonged was likely a male, weighing almost 2,000 pounds (907 kg) and measuring about 12 feet long (3.7 m).
As compared to earlier protocetids, Aegicetus whales in general had longer bodies and tails, smaller back legs, and lacked a firm connection between those rear legs and the spinal column. These features suggest that it was more aquatic than its ancestors, and less geared toward foot-swimming.
A paper on the research was recently published in the journal PLOS ONE.
Source: University of Michigan