Paleontologists have announced the discovery of a new genus and species of extinct protocetid whale, based on the fossilized remains found in the Western Desert of Egypt. Named Aegicetus gehennae, the ancient animal represents an important step in the evolution of whale locomotion.

Protocetidae (protocetids) are a group of semi-aquatic whales known from the middle Eocene epoch of Africa, Asia, North America, and South America.

While living whales use their tails to propel themselves through the water, most protocetids were foot-powered swimmers.

The newly-discovered protocetid, Aegicetus gehennae, was more fully aquatic and less specialized as a foot-powered swimmer than earlier protocetids.

It represents a transitional stage between the foot-powered swimming of early whales and the tail-powered swimming of living whales.

“It is the youngest-known protocetid, dating to around 35 million years ago, and is known from one exceptionally complete skeleton and a partial second specimen, making it among the best-preserved ancient whales,” explained University of Michigan’s Professor Philip Gingerich and colleagues.

Aegicetus gehennae had an estimated body mass of about 900 kg, according to the team.

“Compared with earlier whales, it has a more elongated body and tail, smaller back legs, and lacks a firm connection between the hind legs and the spinal column,” the researchers said.

“These adaptations indicate an animal that was more fully aquatic and less of a foot-powered swimmer than its ancestors.”

The fossilized bones of two Aegicetus gehennae individuals were collected in 2007 from the Gehannam Formation of Wadi Al Hitan (Valley of Whales), a UNESCO World Heritage Site in the Western Desert of Egypt.

“The body shape of Aegicetus gehennae is similar to that of other ancient whales of its time, such as the famous Basilosaurus,” Professor Gingerich and co-authors said.

“These animals appear to be well-adapted for swimming through undulation of the mid-body and the tail, somewhat as crocodiles swim today.”

They published a paper describing Aegicetus gehennae this week in the journal PLoS ONE.

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P.D. Gingerich et al. 2019. Aegicetus gehennae, a new late Eocene protocetid (Cetacea, Archaeoceti) from Wadi Al Hitan, Egypt, and the transition to tail-powered swimming in whales. PLoS ONE 14 (12): e0225391; doi: 10.1371/journal.pone.0225391