First of the Fossil Dolphins

Hemisyntrechalus

The dolphin family is the largest family of aquatic mammals, by number of species. As I've mentioned a few times before, exactly how we're supposed to divide it up, and what scientific names we should give its various members is a matter of some debate. Genetic analyses designed to show which species are related to which just show a tangled mess. So, while we do know that the scheme we use at the present is wrong, we don't really know what we should replace it with, and, for the time being, we're stuck with what we've got.

The reason for this is likely a very rapid burst of evolutionary change, with numerous new species arising at more or less the same time. By examining the genetics of different species, and using what we know of mutation rates, we can make a guess as to when this happened. It turns out to be shortly before, or perhaps during the beginning of, the Ice Ages, in the late Pliocene to early Pleistocene. That at least one species seems to have arisen as a hybrid between two of the others - with the arms of the family tree effectively crossing back over again - doesn't really help matters.