Oldest-Known Dolphin Skull Sheds Light on Evolution and Origins of Marine Mammals

Dolphins include species that range from the bottlenose dolphin to the killer whale. Yet scientists have long wondered exactly how all of these species evolved and developed. Now, a fossil may shed some light on that. Scientists have taken a closer look at the oldest species of dolphin and its partial skull, learning a bit more about the evolution of these marine mammals.

The species in question is called Eodelphis kabatensis. Although the partial skull of this creature was first described in the 1970s, it's been largely ignored since then. It was originally collected from a small tributary of a river in Japan, and scientists estimate that the dolphin existed around the late Miocene, which occurred about 13 to 8.5 million years ago.

The fossil itself is a partial skull, and is an important part of understanding dolphin evolution. Before now, there has been inconsistency in the fossil record of dolphins; the oldest true dolphin fossils discovered were less than 6 million years old. In contrast, molecular studies indicated that dolphins originated and started to diversity between 9 to 12 million years ago.

Using this skull, the scientists  created the most comprehensive analysis to date of the relationships within the Delphinoidea, the group that encompasses toothed whales. More specifically, they received a much clearer picture of the evolution of toothed whales.

"Eodelphis kabetensis, being discovered from sediments that were deposited 8-13 million years ago, has largely resolved this discrepancy and provides the best glimpse yet of what the skull of the first dolphins may have looked like," said Jonathan Geisler, one of the researchers, in a news release.

That's not all that they discovered, though. The researchers also noted that the fact that this ancient specimen occurred in the Pacific Ocean during the late Miocene means that dolphins might have had their origins in the Pacific. This has implications for the geographic history of dolphins.

The findings are published in the Journal of Vertebrate Paleontology.