Understanding the origin and history of true dolphins has been complicated by the fact that scientists were presented with an incomplete and unreliable fossil record. But a recent re-description of the oldest delphinid fossil, now Eodelphis kabatensis, may finally shed light on the cloudy evolution of early dolphins.
Delphinidae is the most diverse family of marine mammals, encompassing 36 extant species such as spinner dolphins and killer whales. A huge number of delphinid fossils have been described, however, many are thought to have been wrongly assigned to this family, resulting in an unreliable fossil record. It’s thought that the oldest specimens were from the late Miocene, but the majority of these fossils were fragmentary and thus difficult to confidently assign.
Back in 1977 the partial skull of a delphinid was found in the Mashike Formation in Hokkaido, Japan. The specimen was the oldest known delphinid fossil, dating back to the late Miocene (8.5-13.0 million years ago), making it an important piece of the evolutionary puzzle of delphinidae. It was originally assigned to the genus Stenella; however, many scientists were dubious about this decision because of the limited availability of species for comparison. Consequently, a 2005 review of Japanese cetacean fossils highlighted the need for re-examination of this species.
In this latest study, published in the Journal of Vertebrate Paleontology, the scientists reviewing the fossil propose that the specimen in fact belonged to the Eodelphis genus. “The early evolution of true dolphins is still covered in mystery. Eodelphis kabatensis informs us about the morphology of early dolphins,” said lead author of the study Mizuki Murakami in a press-release.
Before this particular study, molecular-based studies contradicted the fossil record of dolphins. The former suggested that dolphins originated and diversified around 9-12 million years ago, whereas the fossil record implied that the oldest true dolphins appeared slightly later, around 6 million years ago. “Eodelphis kabatensis, 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 marine mammal palaeontologist Jonathan Geisler in a press-release.
Furthermore, this new analysis suggests the origins and early diversification of Delphinidae may have occurred in the middle Miocene Pacific Ocean, although more samples would be required to confirm this hypothesis.