Sunday 9 July 2017

First and Last of the South Asian River Dolphins

South Asian river dolphin skeleton -
note the strange shape of the skull
Although the whales are undeniably spectacular, the majority of cetacean species are much smaller; the sort of animals we generally refer to as "dolphins". The great majority of these belong to the family Dephinidae, variously termed the "oceanic dolphins", "pelagic dolphins", or even "true dolphins". This is a large family, with nearly forty species, including killer whales and pilot whales alongside their smaller kin.

Whatever we call this family, it, in turn, belongs to the larger group of the "delphinoid cetaceans", which also includes the six species of the porpoise family and two other whales - the narwhal and beluga. Taken together, these animals and their extinct relatives have dominated the count of cetacean species across the world for millions of years, forming a key part of the ocean ecology. Of all the other cetacean groups, only the mysterious deep-sea beaked whales come close in terms of the number of species, and even they don't appear to be as varied.

But it wasn't always so. Even today, a handful of species are sufficiently odd that they are considered to fall outside this group, their earliest ancestors having appeared long before the dolphins separated from the porpoises. But, if we go back further in time, the balance was quite different. The dolphins, as we know them today, have really only been the dominant form of cetacean since around 16 million years ago, during the early Miocene epoch, although their earliest forms date back much further than this. Before this, a different group of dolphin-like animals, the platanistoids, swam across many of the world's oceans, and a considerable number of fossil species are known, apparently having become diverse surprisingly early on in cetacean evolution.

With the rise of the "modern" dolphins this early group steadily declined in both number and variety of species. Today, just one species survives, and it has had to retreat from the sea. This is the endangered South Asian river dolphin (Platanistes gangetica), which survives only in the Ganges and Indus Rivers of India and Pakistan. Given that it's not possible to swim from one of those rivers to the other without entering salt water, which they hate to do, and that this has been the case for many millions of years, for much of the late twentieth century it was thought that the two populations were distinct species. We're now pretty confident that that's not true, but either way, it's a huge decline from where they were so many millions of years ago.

Fossil platanistoids have been found from across the world. One such site lies just outside the town of Chesapeake Beach in Maryland, which has also been the source of a number of other fossil cetaceans. Now a new fossil from the site has been described, and gives us some more insight into the diversity and evolution of the group. It consists of most of a skull, on the basis of which it was possible to determine that it belonged to a species of dolphin-like animal first described back in the 1950s.

Back then, all that we had were some bits of the jaw. These were enough to demonstrate that the animal was something previously unknown to science, and thus to give it a name, Araeodelphis natator, but not enough to determine what other cetaceans it might have been related to. What we do have is a skull about 47 cm (18.5") long, most of which consists of a long and thin snout, with over 40 teeth on each side. This suggests an animal roughly the size of the living South Asian river dolphin, which is about 2 to 2.5 metres (6 to 8 feet) in length - although, without any parts other than the skull, it's hard to know for sure.

The fossil has been estimated to be around 16 to 17 million years old, which places it at an interesting time within the history of the group. As noted above, it's about the time that more "modern" types of dolphin and porpoise began to oust the platanistoids, and the latter group began its long decline into obscurity. More significantly, perhaps, it's also around the time that the family to which the living species of river dolphin belongs first appeared, supplanting the earlier (and now long extinct) sister families within the larger group.

An analysis of the skull's features, and comparison with what we know of those various other platanistoid groups places Araeodelphis within the living family, but it has a number of odd features. In particular, the living family is itself divided into two subfamilies, one of which, distinguished by, among other things, a different shape to the snout, is, of course extinct. But this animal has a snout that doesn't fit either description; you're supposed to tell them apart by whether a portion of it is flattened horizontally or vertically, and in this case it's not flattened at all.

Another difference between the two is that the extinct subfamily had longer snouts than the living sort (which are hardly short themselves), apparently so that they could fit in a larger number of teeth. Araeodelphis, however, has just as many teeth, but the snout is, compared with its relatives, unusually short. The most reasonable explanation, especially given the fossil's age, is that it doesn't belong to either of the known subfamilies, and probably first arose before they separated from one another.

In other words, this is not only one of the earliest members of the living family, but also the most "primitive". Now, of course, "primitive" is a rather loaded term, perhaps implying an animal that wasn't yet well adapted to whatever it did. Whereas, in reality, it was presumably very well adapted to its own lifestyle, and it's perhaps more accurate to say that it had not yet changed so much from the ancestral form.

Indeed, living South Asian river dolphins are strange animals indeed. Spending their lives in the murky waters of the Ganges and Indus, rather than clear blue ocean waters, they are virtually blind, able to sense light, but not to see the shapes of objects. Instead, their echo-location abilities seem to be highly developed, allowing them to find food along the river bed, probably helped by that remarkably long snout, which should be good at stirring up mud. The long snout also allows them to have lots of sharp teeth, which would be good for catching slippery fish, but this fossil seems to show that they had the extra teeth before they had the full snout, perhaps because, in those days, they still lived in coastal or estuarine waters and had a slightly different diet.

Araeodelphis stood on the cusp, as a once diverse group of small cetaceans that, perhaps, lived much as typical dolphins do today, were being supplanted by a different evolutionary branch. Their descendants, or, more likely, the close relatives that followed them would find themselves increasingly pushed to the margins, adapting to an unusual world of murky water in two of the most heavily populated river systems on Earth.

There were many, many species of platanistoid dolphin 20 million years ago. Now there is just one. And it is endangered.

[Photo by "Notafly", from Wikimedia Commons.]


  1. What about the other river dolphins - are they too remains of previously more widespread and pelagic groups?

  2. Aside from the tucuxi, which is a member of the Delphinidae, I believe hat's true, although not to the extent of the South Asian sort. Certainly, they all descend from pelagic species, and have their own lineages that predate the dolphin/porpoise split. But the platanistoids are the oldest, and, so far as I'm aware, most fossil-rich of the groups.