Diagram of the ancient dolphin's asymmetrical skull Credit: Copyright : Mizuki Murakami

Waseda researchers have described the earliest example of a true dolphin in the known fossil record.

In an article published in the Journal of Vertebrate Paleontology, Waseda Department of Earth Sciences researchers (lead author Mizuki Murakami) conclude that the specimen, which was originally categorized in 1977 as Stenella kabatensis, actually belongs to the Eodelphis kabatensis species. This means that true dolphins existed 2 to 7 million years earlier than had been previously suggested by fossil records, and aligns with the results of molecular studies.

This work also includes the most comprehensive analysis yet of the relationships within the group of marine mammals that encompasses toothed whales such as orcas and sperm whales.

In another study, Murakami and a team of researchers also described the oldest known dolphin fossil with an asymmetrical skull, and created a new simulation of their evolution.

It has been accepted theory that dolphin ancestors earlier than 10 million years ago all had symmetrical skulls, and that asymmetry was the result of subsequent evolution. This study suggests a different evolutionary path for the marine mammals.

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