#aegicetus

The protocetids were some of the first oceanic cetaceans, occupying a transitional position in the evolution of whales, with four paddle-like limbs and nostrils only partway up their snouts.

Early members of this group swam like otters, using a combination of undulating their bodies and paddling with large hind limbs, but somewhere in the Late Eocene they switched over to propelling themselves entirely with their tails and gave rise to even more whale-like forms like the basilosaurids.

And Aegicetus gehennae was right in the middle of that switch.

Discovered in the Wadi Al-Hitan ("Valley of the Whales") fossil site in Egypt, Aegicetus lived around 37-35 million years ago. It was similarly-sized to earlier protocetids like Georgiacetus, measuring about 3.5m long (11'6"), but its hind limbs were proportionally smaller. Its hips were also completely disconnected from its vertebrae, giving it much more flexibility to undulate its body and tail – and preventing it from supporting its weight on land, suggesting that it spent its entire life in the water.

It wasn't a direct ancestor to more "advanced" cetaceans, since it lived alongside several species of basilosaurids. Instead it seems to represent a late-surviving example of what the earlier protocetid-basilosaurid transitional forms would have looked like.

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Newly described fossil whale represents intermediate stage between foot-powered and tail-powered swimming

A newly described fossil whale represents a new species and an important step in the evolution of whale locomotion, according to a University of Michigan paleontologist and his colleagues.

The fossilized remains of Aegicetus gehennae were recovered in the Egyptian desert in 2007 and were dated to around 35 million years ago. The creature appears to have been well-adapted for swimming through undulation of the mid-body and tail, somewhat as crocodiles swim today, according to U-M's Philip Gingerich.

The discovery is detailed in a paper scheduled for publication Dec. 11 in the journal PLOS ONE.

The fossil record of whale evolution tracks the transition from land-dwelling ancestors to ocean-dwelling cetaceans. Protocetids are a group of early, semi-aquatic whales known from the middle of the Eocene, a geological epoch that began 56 million years ago and ended 33.9 million years ago. Protocetid remains have been found in Africa, Asia and the Americas.

While modern whales are fully aquatic and use their tails to propel themselves through the water, most protocetids are thought to have been semi-aquatic and swam mainly with their limbs.

In their PLOS ONE paper, Gingerich and his colleagues describe a new genus and species, Aegicetus gehennae, the first late-Eocene protocetid. Its body shape is similar to that of other ancient whales of its time, such as the famous Basilosaurus.

The researchers suggest that an undulatory swimming style might represent a transitional stage between the foot-powered swimming of early whales and the tail-powered swimming of modern whales.

"Early 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," said Gingerich, a professor emeritus in the U-M Department of Earth and Environmental Sciences and curator emeritus at the U-M Museum of Paleontology.

"This newly discovered fossil whale, 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," said Gingerich, who is also a professor emeritus of ecology and evolutionary biology and of anthropology.