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.
However, this muddle, frustrating though it is, doesn't apply to all dolphin species. It applies mainly to the bottlenose, common, and spotted dolphins, probably with a few others thrown in, for a total of about a dozen or so species - about a third of the total. The white-beaked dolphin of the north Atlantic, for instance, seems to belong to a group that diverged from the ancestors of the bottlenose/common/spotted cluster about three million years before things got messy for the latter.
To really figure out how old the group as a whole is, though, we want to try and date the very first branch within the family. That means looking at the dolphins that are least like the others. When we do that, by including killer whales and their relatives, we get a date somewhere around ten million years ago, perhaps a little more. So, the dolphin family likely arose around that time, when it diverged from what would later become the porpoise and white whale families. Then, around four or five million years ago, dolphins became much more common, and the immediate ancestors of modern species started to appear.
At least, that's what the genetics seem to say. But do we have evidence to back it up? Most fossil dolphins come from either the Mediterranean, and most of them are less than five million years old, dating from either the Pliocene or Pleistocene epochs. We don't have very many from the earlier, Miocene, epoch, during which we expect the group to have first appeared.
|Numbers indicate estimated date in millions of years ago|
Still, some early fossils have been described on the basis of enough material that we at least have some idea what we're talking about. One such is Stenella kabatensis, first described in 1977... and almost completely ignored since, presumably because the only description that exists is written in Japanese. (Oh, the perils of international science...) Fortunately, we now have a new, updated description, in English, and it looks like we can now say that this is the oldest undisputed fossil of a true dolphin.
The fossil comes from a tributary of the Oshirarika River in Hokkaido, the northernmost main island of Japan. As is often the case, precise dating is difficult, but judging from what we know of the local geology, it's probably around nine million years old. Although, since it was the 1970s, the scientist who originally described it wouldn't have known this, that makes it far too old to belong to the genus Stenella, which is part of that tangled mess that arose no more than three million years ago at the most.
The new description, following up on earlier suspicions that this didn't really make sense, also shows that the exact shape of the skull bones is different from that of any other dolphin, Stenella or otherwise. With that, and the age of the fossil, in mind, the researchers assign it the new name Eodelphis kabatensis.
What do we know about this early dolphin? It isn't particularly big, and had a short snout, so that it may have looked somewhat like a large porpoise. In the absence of DNA from such an old fossil, we can't see how its genetics compare with those of modern species, but we can look at features of the skeleton, to see what it most resembles. Doing so implies that its closest living relative would be the killer whale - although it's much smaller than that animal is - although it's also related to an extinct dolphin named Hemisyntrachalus, which lived in the Mediterranean and North Atlantic around four to five million years ago.
Quite how much that means is difficult to say, because of how old the fossil is. Since it would, presumably, be quite similar in appearance to the original dolphins, it may just be that, in some respects at least, killer whales have changed less from that early form than other modern species have. The fact that it lived near what is now Japan may also be significant, perhaps indicating that dolphins originated in the Pacific, before swimming eastward into the Atlantic. (Since this would be before Central America existed, it would hardly have been difficult).
While the authors of the new description are quite keen on this Pacific origin theory, it has to be said that it's based on not much more than one data point. Although Eodelphis is by far the oldest undisputed dolphin fossil, there are contenders from elsewhere of similar age that we can't say definitively aren't dolphins. So this may yet be overturned.
But, for the time being, Eodelphis is the oldest known dolphin, and further study may tell us more about how dolphins came to be distinct from porpoises and whales.
[Picture by "Ghedoghedo" from Wikimedia Commons. Cladogram adapted from Murakami et al 2014.]