Aegicetus gehennae: Fossil Whale Shows Transition Stage to Tail-Powered Swimming

Thursday, December 12, 2019

Aegicetus gehennae is believed to have looked similar to the Basilosaurus, ‘King of the Lizards’ (pictured). DOTTEDHIPPO/ISTOCK

Researchers from the University of Michigan have described a new fossilized whale that represents not only a new species, but also identifies an important step in the evolution of whale mechanics.

In 2007, paleontologists discovered a fossilized creature in the Egyptian desert they called Aegicetus gehennae, dating back to roughly 35 million years ago. The creature was aquatic and able to swim by undulating its mid-body and tail similar to how crocodiles move through water today, according to Philip Gingerich, professor emeritus in the U-M Department of Earth and Environmental Sciences and curator emeritus at the U-M Museum of Paleontology.

Details of the discovery were published in the journal PLOS ONE.

Whale evolution, according to fossil records, transformed land-dwelling ancestors walking on all fours to the ocean-dwelling cetaceans we know today. During the transition came an early, semi-aquatic whale known as the protocetid, present during mid-Eocene epoch 56 million years ago to 33.9 million years ago. Remains of this ancestor have been uncovered in parts of Africa, Asia and the Americas.

The fully aquatic modern whales use their tails to maneuver through the water, but protocetids were only semi-aquatic and had limbs that helped them swim. In their paper, Gingerich and his colleagues describe Aegicetus gehennae, the first late-Eocene protocetid.

The Aegicetus gehennae has a body shape similar to that of ancient whales from its time period, including the famous prehistoric predator, the Basilosaurus or “king lizard.” The researchers believe that the side-to-side undulatory swimming style could represent the evolutionary transition from the foot-powered swimming of early whale ancestors to the tail-powered swimming we see in 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, also a professor emeritus of ecology and evolutionary biology and of anthropology.

These are cervical and thoracic vertebrae of Aegicetus gehennae type specimen, an early ancestor of the modern whale. (Gingerich et al., 2019)

“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,” Gingerich added.

The fossilized remains of Aegicetus gehennae were found in the Wadi Al Hitan World Heritage Site in the Western Desert of Egypt. The dating of the bones makes this extraordinary creature the youngest known protocetid. Roughly two-thirds of its bones were recovered, along with another partial skeleton of a second specimen – making Aegicetus among the best-preserved whales from this time period.

The specimen recovered that was mostly complete is believed to have been male, about 12 feet long and weighing in at nearly 2,000 pounds. When compared to earlier ancient whales, Aegicetus has a much more elongated body and tail with smaller hind legs that are very loosely connected to the spinal column. These traits point to a much more fully aquatic animal that was less of a foot-powered swimmer than its semi-aquatic ancestors.

The original fossils discovered at the dig site in Egypt have been held at the U-M Museum of Paleontology to be studied but will soon be returned to the Egyptian Geological Museum in Cairo. Molds and casts are currently being taken of nearly all of the specimens which will remain in Ann Arbor, Michigan.

Wadi Al Hitan is also known as the “Valley of the Whales.” It is a UNESCO World Heritage Site world renowned for yielding a collection of complete and nearly complete skeletons of prehistoric whales. During the dig of 2007, a partial skeleton of Aegicetus was found eroding from the harmful sediments in the eastern part of the World Heritage Site, but later that year the much more intact and complete skeleton was discovered.

Gingerich’s co-authors include Mohammed Sameh M. Antar of the Egyptian Environmental Affairs Agency and Iyad S. Zalmout of the Saudi Geological Survey. Their research in Egypt was sponsored by the Egyptian Geological Museum, the Egyptian Environmental Affairs Agency, the Egyptian Mineral Resources Authority, the University of Michigan Museum of Paleontology, The U.S. National Science Foundation, and the National Geographic Society.

The authors say the specimens described in their paper were uncovered, collected, and analyzed following the protocols outlined in a three-way memorandum of understanding between the Egyptian Geological Survey and Mining Authority, the Egyptian Environmental Affairs Agency, and the University of Michigan.