Late Miocene Survival of a Hyper-Longirostrine Dolphin and the Neogene to Recent Evolution of Rostrum Proportions Among Odontocetes

Hyper-longirostry, the character of having extremely elongated rostra, emerged in the early and middle Miocene among several different clades of echolocating toothed whales (odontocetes) followed by a rapid decline near the end of the middle Miocene, and postdated by a much lower number of occurrences in the late Miocene and Pliocene and a complete absence among extant odontocetes. New finds of unreworked fossils of Xiphiacetus cristatus (Eurhinodelphinidae) in the middle Tortonian Diest Formation in Belgium (9.54-8.8 Ma) allow for the documentation of the survival of a hyper-longirostrine dolphin into the early late Miocene. An extensive dataset of the rostral index (calculated as the ratio between rostral length and condylobasal length) of Neogene and extant odontocetes is compiled and presented here, which facilitates discussion of evolutionary trends of rostrum proportions during a time period spanning 23 million years. Of interest, the iterative survival into the late Miocene of a single different species of hyper-longirostrine dolphins in a number of paleogeographic regions (North Sea Basin, Atlantic Coastal Plain, and probably the southeastern Pacific) is noted, whereas hyper-longirostrine morphologies only seem to re-appear by the late Messinian in the Northeastern Pacific. A correlation between this pattern and a decrease in habitat size for coastal to estuarine dolphins linked to a major sea level drop is tentatively proposed; such a process may also have played a role in the ecological shift in several dolphin families to freshwater habitats.

Automated summary

The characteristics of hyper-longirostry, referring to extremely elongated rostra, emerged in several clades of echolocating toothed whales during the early to middle Miocene but rapidly declined towards the end of the middle Miocene and are now absent among extant odontocetes. Fossil finds in Belgium indicate the survival of a hyper-longirostrine dolphin into the early late Miocene. Evolutionary trends of rostrum proportions over 23 million years suggest a correlation between habitat size changes and the ecological shift of dolphin families to freshwater habitats.