A new fossil study has pinpointed the moment when whales lost their distinct legs and tail and developed flukes, sometimes called tail fins, instead:
Flukes are the two wide, flat triangular lobes on a whale's back end and are made of skin and connective tissue, with bones in the middle [National Geographic News].
Researchers say that the Georgiacetus vogtlensis, whose fossil was found in Alabama, was one of the last whales to have powerful back legs and a tail like a dog's, and that whales evolved flukes between 40 and 38 million years ago. Paleontologists already knew that
the ancestors of whales once strode on land on four legs, just as other mammals do. Over time, as they evolved to dwell in water, their front legs became flippers while they lost their back legs and hips, although modern whales all still retain traces of pelvises, and occasionally throwbacks are born with vestiges of hind limbs [LiveScience].
In the new study, published in the Journal of Vertebrate Paleontology [not yet online], researcher Mark Uhen examined the vertebrae of the Georgiacetus, a sharp-toothed whale that cruised the Gulf of Mexico about 40 million years ago.
After analyzing the fossils for almost three years, Uhen concluded the individual had a tail, but no fluke, and that Georgiacetus wiggled its hips and moved its entire trunk up and down through the water to move forward—a swim stroke whales no longer use [National Geographic News].
The first whales known to possess flukes are close relatives of Georgiacetus that date back to 38 million years ago
This suggests that in those two million years, whales developed flukes and took the final step in their transition from land to sea. Check out pictures of the fossils that link whales to their closest living relatives--hippos--in the DISCOVER article, "All in the Whale Family." Image: Mary Parrish/Smithsonian Institution