The evolutionary history of whales is strange, to say the least. After the first tetrapods (animals with arms and legs) climbed out of the oceans and began walking on land, millions of years ago, a group of four-legged mammals then transitioned back to the seas to become the sleek cetaceans we know and love today.
That early transition from “limb to fin” is not very well understood, though, as limb bones of early cetaceans in the fossil record are scarce. Now, the remains of a new protocetid – “legged whale” – found in Senegal is offering an insight into how these proto-whales propelled themselves across the seas. Apparently, they doggy-paddled.
It had been assumed that protocetids were “foot-powered” swimmers, and that the forelimbs were used more like a modern pectoral flipper, for steering, like Basilosaurus – extinct early cetaceans that are thought to be the first fully aquatic cetaceans. However, 3D models of the nearly complete articulated forelimb of the new specimen suggest this one used its “hands” actively during swimming, the researchers note.
"The morphology of the carpus and the ligament insertions at the finger level indicate that this whale could still bend its fingers during swimming, and therefore did not yet have a real "fin" like in modern whales," first author Quentin Vautrin told IFLScience.
The fossil remains of the protocetid found in western Senegal include two vertebrae, two ribs, partial fragments of the feet and tail and, incredibly, an almost complete right forelimb. Dated to between 43 and 41 million years old, the specimen sits in the middle of the transition from land to aquatic creature, which occurred between 50 and 35 million years ago. Analyzing its morphological characteristics, the team puts it close to Carolinacetus, another extinct early whale discovered in South Carolina in the 1990s.
The forelimb, however, offers unprecedented insight into how early cetaceans swam. The bones clearly show an ability to bend at the "elbow", and the arm would have had powerful muscles that it likely used (along with its legs) to propel itself through water, not dissimilar to a dog.
"We think that this protocetid used its anterior limb as a propellant because the bones have significant insertion area for the muscles responsible for the flexion of the limb," Vautrin told IFLScience. "The flexor muscles must therefore have been very developed and probably provided a high propulsion force during the locomotion. In addition, the elbow joint allowed flexion, which is no longer possible in today's cetaceans."
Although without further skeletal evidence, like shoulder bones, it’s hard to know whether the arms could fully rotate or just move forwards or backward, it does suggest that early cetaceans tried a few different swimming styles before settling on the undulating mechanism of modern whales, using flippers for steering.
The study, published in Palaeontology, also confirms that despite not quite being the smooth, perfectly adapted swimming machines later cetaceans became, this didn’t stop protocetids from doggedly swimming great distances. Most early whale-like animals have been found in and around India, and this one was found in Africa. It was only earlier this year the first skeleton of an early "otter-like" legged whale was discovered to have lived in the Pacific. This new discovery, the researchers say, also confirms that the dispersal of these early cetaceans to the New World did not occur in a single colonization event.