Dolphins, Delphinidae, are the most diverse group of Whales (or indeed any form of Marine Mammals) alive today, with 36 described species including charismatic animals such as Bottlenose Dolphins and Killer Whales. Historically a large number of fossils have also been assigned to this group, but the vast majority of these are no longer considered to be True Dolphins. Twelve fossil Dolphin species are currently widely accepted, all from the Pliocene or later and the majority from Italy. The oldest known True Dolphin is thought to be an undescribed specimen from the Late Miocene of California (LACM 52147), though this specimen lacks a head so its precise classification is hard to determine.
In 1977 palaeonotologist Hideo Horikawa described a fossil Dolphin from the Late Miocene Mashike Formation of Hokkaido Island, Japan, however this description was published in Japanese, and has received little attention outside Japan. Horikawa placed the specimen in the extant genus Stenella, giving it the specific name kabatensis. However the original description apparently contains a number of inaccuracies, and the genus Stenella has recently been shown to be polyphyletic (i.e. the five living species placed within the genus are not one-another’s closest relatives) by a genetic analysis, which is likely to lead to the reassignment of a number of Dolphin species in the near future.
In a paper published in the Journal of Vertebrate Palaeontology on 6 May 2014, Mizuki Marakami of the Graduate School of Creative Science & Engineering at Waseda University, Chieko Shimada of the Mineral Industry Museum at Akita University and the Geological Survey of Japan, Yoshinori Hikida of the Nakagawa Museum of Natural History, Yuhji Soeda of the Historical Museum of Hokkaido and Hiromichi Hirano of the Department of Earth Science at Waseda University redescribe Stenella kabatensisas Eodelphis kabatensis (Eodelphis meaning ‘Dawn-Dolphin’).
The specimen comprises a partial skull lacking the rostrum (snout) but having both mandibles and a number of teeth. Marakami et al. feel confident assigning this specimen to the Delphinidae, making it the oldest known member of the group. The specimen is at least 7.6 million years old, and probably between 8.5 and 13 million years old, though Marakami et al. suggest dates older than 9 million years are less likely.
The skull of Eodelphis kabatensis. Photograph of the skull whitened with ammonium chloride. (Top left) Dorsal view, (bottom left) left lateral view, (top right) anterior view, (bottom right) antereoventral view. Marakami et al. (2014).
A phylogenetic analysis suggested that Eodelphis kabatensisis more closely related to the living Orcinas orca (Killer Whale) than any other extant Dolphin, rather than being close to the common ancestor of all Dolphins. This supports previous molecular studies, which suggests that the group diversified earlier in the Miocene.
The Squalodelphinidae are a small group of small to medium-sized Toothed Whales known from the Miocene of Europe and North and South America. They are thought to be related to the modern Asian River Dolphin, Platanista gangetica, which lacks any close living relatives. The group is not well understood, with most described specimens being fragmentary in nature.
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