An article published in the journal “PLOS ONE” reports the identification of an ancient species of whale that was named Aegicetus gehennae. It lived about 35 million years ago, in the Eocene period, in an area that is now occupied by the Egyptian desert. A team of paleontologists examined a skeleton that’s exceptionally complete for this type of animal and a second partial skeleton concluding that it was a species belonging to the protocetid family, cetaceans that were probably amphibians. This new species represents an intermediate stage between the most primitive whales, which used using their feet to swim, and the modern ones that swim using their tail, providing information on their evolution.
Protocetids (Protocetidae) are a family of primitive cetaceans widespread in most of the oceans during the Eocene period. They were probably amphibians and still had legs they could use to move on land but also to swim. The two specimens belonging to the species later named Aegicetus gehennae were discovered in 2007 in the Gehannam Formation in the paleontological site of Wadi al-Hitan, in Egypt, already famous for over a century for the discovery of fossils of ancient cetaceans useful to understand their evolution into marine mammals starting from land progenitors.
Professor Philip Gingerich of the University of Michigan examined the two specimens of Aegicetus gehennae together with Mohammed Sameh M. Antar of the Egyptian Environmental Affairs Agency and Iyad S. Zalmout of the Saudi Geological Survey. The top image (Courtesy Philip D. Gingerich, Mohammed Sameh M. Antar, Iyad S. Zalmout) shows the hind leg of the specimen classified as CGM 60584. The bottom image (Courtesy Philip D. Gingerich, Mohammed Sameh M Antar, Iyad S. Zalmout) shows the spot in the Gahannam Formation where that specimen was found. The study also led to the creation of three-dimensional photogrammetric models of the fossils.
The age of the two specimens of Aegicetus gehennae has been estimated at around 35 million years and this makes them the most recent protocetids discovered so far. Compared to older whales, this species had a more elongated body and tail, smaller hind legs and no strong connection between the hind legs and the spine. Its anatomical features suggest that it was a whale well adapted to swimming through the undulation of the mid-body and tail, similar to crocodiles. It could be a transitional style between the one based on the use of the legs by the earliest whales and that based on the use of the tail of today’s whales.
Even in a paleontological site as rich as the Gahannam Formation, paleontologists need some luck to find skeletons in good condition like the first specimen of Aegicetus gehennae. When this happens, researchers can get new information on whale evolution.
