|The common dolphin (Delphinus delphis) still lives|
in the Mediterranean
This may partly be due to something of a gap in the fossil record - a time for which we don't have many deposits of the right age from the right place - but there are other possible reasons as well. Many mammal fossils are identified from quite small parts, and often just the teeth. Mammalian teeth, especially those at the back of the mouth that grind up food rather than biting into it, have complex shapes that vary from species to species, enabling an expert to tell quite a lot from looking at only one. At least enough, in most cases, to tell what family it belongs to, even if you can't precisely finger the species in question.
But the majority of dolphin teeth look more or less the same. They have no cheek teeth at all, only stabbing teeth for grabbing onto fish, and those are usually quite simple in shape. So, while we may have plenty of dolphin teeth, and other scattered bits of their anatomy, they tell us rather less than they might for land-based mammals, making them useful than we might like for building up a picture.
Most of the really good dolphin fossils that we have come from either Italy or South Carolina, although there are a few exceptions. There are just about enough to tell us that, while dolphins may first have appeared about 10 million years ago, they really only started to become common during the Pliocene, around 4 million years ago, when a number of new species seem to have appeared more or less at the same time.
Many of the Italian fossils come from Piedmont, where seven major specimens were discovered in the late nineteenth century. Since that time, however, only one more has been uncovered, rather stalling out knowledge of this era of dolphin evolutionary history. That fossil was given a brief write-up (in Italian) by its discoverer in 1980, but, unfortunately, he died before being able to describe it properly. Worse, he also neglected to say what he'd done with it, and a search for it in the mid '90s failed to turn it up.
Well, it has now been found again, at a museum in Asti, just a few miles from the site where it was originally excavated. This has allowed Giovanni Bianucci, a researcher at the University of Pisa, to give it it's first full scientific description, and a name: Septidelphis morii. It is, in fact, quite a complete fossil, missing the flippers, tail, and part of the area around the ears, but otherwise largely intact.
The fossil does not appear to be from a fully-grown individual, since a number of bones that we'd expect to be fused together in an adult are distinct here. The original, brief, description, said that it had been killed by a shark, but the new analysis finds no evidence of this. Instead, Bianucci argues that the broken bones that led to this idea may actually have been caused long after the animal died - in places, the skeleton has fossilised sea-shells growing on it, implying that it had laid around on the sea floor for a long time before becoming buried.
Given how little we understand of how living dolphins are related to one another, it's hardly surprising that it's difficult to say quite where in the dolphin evolutionary tree this 'new' fossil fits. It's similar enough to say that it quite clearly is a dolphin - rather than a porpoise, say - and it's closest known relative appears to be another extinct Italian species, Etruridelphis. Beyond that, there's some indication that it evolved some time after the living humpback dolphins branched off from the ancestors of all the other living species, and that it's closer to that latter, larger group than to the humpbacks themselves.
If that's right, then, according to other studies, this species must have evolved some time between 3.8 and 3.5 million years ago. The sediments it was found in have been dated to between 3.8 and 3.2 million years ago, which fits that prediction perfectly, and lends strength to the models we already had as to how dolphins evolved.
That picture includes the presence of at least thirteen species of dolphin found across the seas of the Pliocene world, 6 million years or so after they first evolved. They were already widespread by this time, with fossils having been found as far apart as Peru, North Carolina, and, of course, Italy. Unsurprisingly, none of the thirteen survive today, although some, including the two American examples, were very similar to living species. One Italian specimen, Orcinus citoneniensis, for instance, was very close to modern killer whales, demonstrating that they, too, were around at the time (indeed, they're thought to be much older, with the first ones perhaps 4 million years before this time).
Since many of these species are known from just one fossil, it's a fair bet that there are plenty more we haven't found yet, since we clearly don't have any sort of wide sample. The fact that so many species are found at around the same time, and shortly after other lines of evidence suggests that they first evolved implies that there were a lot of new kinds of dolphin popping up around 4 million years ago. A second major burst of evolution seems to have occurred during the Pleistocene, probably as the result of rapid changes in climate as the first of the Ice Ages began.
But if the Ice Ages are responsible for that second burst of new species, it's less obvious what might have responsible for the earlier one. It's been suggested that, since many of the fossils are from Italy, that it might have to do with major changes in the composition of the Mediterranean that occurred around that time. On the other hand, given how few decent fossils we have, it could just be a fluke that we've found so many in one place, and the explanation lies elsewhere.
Clearing up the mess that is current dolphin classification, and fitting the fossil forms clearly into whatever pattern that reveals, may help to give us more of an idea of which it is.
[Picture by Scott Hill of the National Oceanic and Atmospheric Administration, in the public domain]