Ancient whale species sheds light on shift from limb- to tail-powered swimming

Dec. 20 (UPI) -- Modern whales use their tails to swim, but their earliest ancestors, a group of semi-aquatic species known as protocetids, swam with their limbs.

Due to gaps in the fossil record, the transition from limb- to tail-powered locomotion among whales isn't well-understood. But a newly discovered species of ancient whale, unearthed in Egypt, has offered scientists some clarity on the matter.

"The biggest gap is that we lack associated skeletons of whale intermediates between land mammals and the earliest of early known whales to document the transition to foot-powered swimming," researcher Philip Gingerich, professor of earth sciences, evolutionary biology and anthropology at the University of Michigan, told UPI in an email. "We also need more late middle Eocene skeletons to fully document the transition to tail-powered swimming."

The protocetid species Aegicetus gehennae -- newly named and described this month in the journal PLOS One -- has helped scientists begin the fill in the latter of the two gaps.

The fossil skeleton is the most well-preserved protocetid whale specimen from the late middle Eocene. Gingerich and his colleagues used 3D photogrammetry to image the skeleton and took standard measurements of all of the whale's bones. Their data suggests later protocetids were different from their earlier relatives in important ways.

"It has the first 39 vertebrae preserved in sequence, and the mid-body and tail vertebrae are longer than expected for a protocetid of its size," Gingerich said. "The pelvis shows that it and the hind limbs no longer articulated directly with the vertebral column. The feet are as small as the hands, and smaller than expected for an earlier protocetid of its size."

"All of these features -- elongated vertebrae, loss of a connection of the pelvis to the backbone, and relatively small feet -- all point to a whale swimming, more or less eel-like, with its body rather than its feet or just its tail," he said.

Scientists first found the skeleton in 2007, but it took over a decade for the whale to make it into the scientific literature. During the first few years after the initial discovery, researchers made several trips to the dig site to ensure they had indeed gotten the entirety of the skeleton.

"Then it took time to remove the hard rock matrix from the fossil, time to study and measure it, time to compare it to fossils known previously, time to illustrate it, time to write a full report, and time to have the report reviewed by peers and published," Gingerich said. "This is just the nature of the process."

Researchers hope future protocetid discoveries will help them fill in the other gap in the record of whale locomotion, the transition from land-dwelling ancestors to the earliest whale-like species.