A fossil of an ancient dolphin found in Japan may serve as key to the mystery behind the most diverse species of the cetacean mammals, a new study revealed.
Around 40 species of dolphins are currently known to man but scientists remained mystified about their evolution and development because of the dearth in potential specimens.
Dolphins, such as the bottlenose dolphin and killer whale (or orca), are often considered as one of the Earth's most intelligent creatures. They are found almost anywhere in the world, particularly in the shallower seas of the continental shelves, preying in schools on fish and squid.
From the Mashike Formation outcrop, the partial skull of an ancient dolphin species was unearthed from a narrow stream of the Oshirarika River in Hokkaido. Its age is estimated to have been around 13 to 8.5 million years ago in the late Miocene epoch, prompting scientists to conclude that they have been thriving the world's oceans six million years earlier than estimated.
Following the discovery, a team of researchers came up with a new study, which appeared in the publication Journal of Vertebrate Paleontology of the Society of Vetebrate Paleontology, that filled the gap in the evolutionary line of the dolphins by renaming the oldest ancient dolphin as Eodelphis kabatensis.
"The early evolution of true dolphins is still covered in mystery. Eodelphis kabatensis informs us about the morphology of early dolphins," said Mizuki Murakami, lead author of the study.
Previously thought of as part of the genus Stenella, the Eodelphis kabatensis found from the sediments deposited eight to 13 million years ago showed dolphins largely inhabited the Pacific in the Miocene epoch and it also gave a clearer image of how toothed whales evolved.
They were the "best glimpse yet of what the skull of the first dolphins may have looked like," according to Jonathan Geisler, a marine mammal paleontologist at the New York Institute of Technology College of Osteopathic Medicine.
The study cleared the inconsistencies that previously surrounded the fossil record of the dolphins and the conducted molecular-based studies. Records showed that the oldest true dolphins were younger in age but molecular studies stated that dolphins began to diversify around 9 to 12 million years ago.
"Eodelphis is significant for understanding the origin, early evolution, and paleobiogeography of Delphinidae, as well as calibration of the molecular divergence estimates: Eodelphis kabatensis is the oldest and only valid Miocene delphinids species yet described," the researchers concluded in their paper.
