The most ancient platanistid: An insight into the early evolutionary history of the dolphin group

The fossil record of Platanistidae (Indian river dolphins and extinct partially marine relatives, Early Miocene to today) is suggestive of an early radiation of the group reflected in the diversification of the skull morphology, and a wide distribution in the Old World and New World during the Miocene that dropped coincidentally with the appearance of delphinoids (pelagic dolphins and closest relatives) in the Late Miocene. River dolphin radiation is correlated with the rise of shallow epicontinental seas, as a result of high sea levels during the Middle Miocene. Their stem group is interpreted to have remained in marine areas, serving as ancestor to species entering and adapting to freshwater environments. Here, a new Platanistidae skull is described from deep-marine sediments from the lower Miocene of Austria with an age of ca. 22 Ma. Morphological comparison with other dolphins provides insights into ecology and evolution through dentition, and cranial asymmetry related to hunting and navigating in aquatic environments. The skull belongs to a single-rooted homodont dolphin, known as a characteristic of the Platanistidae, in contrast to heterodont dentition in their platanistoid ancestors. According to our preliminary phylogenetic analysis, the skull represents the most ancient member known to date of the Platanistidae, predating all previous early records by 2 to 5 million years, and documenting a pelagic habitat adaptation.