Sunday, 10 July 2022

The Dolphins of Switzerland

The Mediterranean is very nearly an inland sea. Its only natural connection to the world's wider oceans is through the Strait of Gibraltar between Spain and Morocco, a passage just 13 km (8 miles) wide at its narrowest point and in places just 300 metres (1,000 feet) deep; pretty shallow as such things go. It's probably because of this that some of the larger whales that inhabit the Atlantic (blue whales, humpback whales, etc.) are rarely if ever seen venturing into its waters.

Dolphins are a different matter, with the majority of North Atlantic dolphin species also being commonly seen in the Mediterranean, albeit in some cases only in its most westerly waters. This includes some of the really big dolphins that we'd normally call "whales", such as the killer whale, and there are three species of genuine whale that live there, too. The connection between the Mediterranean and the Atlantic has not always been there, however; for a long time during the Late Miocene, the two bodies of water were separated by a land bridge between Spain and Morocco, entirely cutting the Mediterranean off until it ended in the cataclysmic Zanclean Flood

Going back to a time before the rise of that land bridge, into the Middle Miocene, we find a time when the Mediterranean existed, but the Strait was much wider than today, and the shape of the sea was noticeably different. We have quite a few fossils of dolphins and whales from this time in the Mediterranean's history, with fossil sites in France, Italy, Spain, Malta, and Egypt. Those may not be surprising, especially Malta, which is a relatively small island. What may be slightly more surprising is that we also have fossil cetaceans from the north coast of Switzerland and the south coast of Germany.

Because, of course, those coastlines don't exist any more.

But, back in the Middle Miocene, they did. This was due to the existence of a body of water called the Paratethys Sea. This was a very ancient body of water, lying north of what is now the Mediterranean and at various times either connected to or separate from it. It's so-called because, prior to the Mediterranean becoming closed off from the Indian Ocean by the creation of the Arabian land bridge, that body of water - essentially the wide sea separating Europe from the island continent of Africa - is referred to as the "Tethys" and this has an even older history.

By 17 million years ago, in the Middle Miocene, the Mediterranean was definitely in existence and the Paratethys had become split into two, with the eastern part becoming the Black Sea. In the west, the much shrunken Paratethys stretched northwards from the Adriatic, connecting to the Mediterranean through channels roughly around modern-day Venice and Genoa, cutting southern and central Italy off as an island and covering approximately what would now be the Po Valley and the Hungarian Plain.

Those, of course, are south of the Alps, but the northward march of that mountain range, as Africa slowly pushed up from the south, forced the neighbouring parts of the European landmass downwards, buckling under the multi-million-year pressure. Seawater rushed into this dip on at least two occasions during the Age of Mammals, and the Middle Miocene lines up with the second one. This time around, it wasn't quite so deep as it had been previously (during the Oligocene), but at least some of the time, it was deep enough for truly marine organisms to inhabit it.

This meant that there was a long sea-filled channel running from the Paratethys in the east, through southern Germany and northern Swizerland, before turning south through what's now the Rhone Valley in southeastern France. In the Late Miocene, starting around 10 million years, it would become filled in, partly by the ongoing rise of the Alps to the south and the Jura Mountains to the north, but the traces of it can still be seen today in the Swiss Plateau that separates those two mountain ranges, with Lake Geneva and Lake Constance at either end. Present-day cities such as Zurich, Berne, and Geneva are all located on what was then the sea floor.

So what sorts of cetaceans lived in this channel? The Middle Miocene was a time of increasing diversity amongst toothed whales in general and the fact that the eastern end of the Mediterranean had been recently closed off, blocking access to the warm water species of the Indian Ocean, seems to have done nothing to alter this on a local scale. From across the Mediterranean, and what remained of the  western Paratethys, we know of beaked and sperm whales, and several dolphins, many belonging to extinct groups. Others include the platanistids, which are now restricted to two endangered freshwater species, but back then included many more, mostly in saltwater.

The last thorough survey of the cetaceans of northern Switzerland was conducted back in the 1980s and the third of a century since that time has seen a refinement in our knowledge of this group of animals, and subsequent changes in our understanding of how they interrelate. Last month, a new survey, examining the fossilised periotic ear bones of over 300 fossils updated that knowledge for the 21st century.

Fossil periotics, incidentally, are a good choice for this sort of study since they are reasonably common and have a highly detailed structure, being the bones that wrap around the complex shape of the inner ear canals. For most mammals, we would use cheek teeth, which have a shape that similarly varies between species and, being even harder than bone, are often preserved without any major breakages. While cetacean teeth are, as we would expect, far more often found as fossils than ear bones are, the problem is that most of them look exactly the same; simple points that are identical across the length of the jaw. 

What works for a horse won't work for a dolphin. Ear bones are, at least for cetaceans, the next best thing.

Note: fossil groups not mentioned in the
post are omitted

One of the questions it had to answer was whether or not this area - and the Mediterranean and Paratethys more widely - were home to true dolphins at the time. There's no question that they existed back then, and the 1980s survey had confirmed them as present, but by 2022, this was no longer widely accepted. The new study confirms this, identifying the fossils previously thought to belong to true dolphins as actually belonging to an animal called Kentriodon, although, without more of the animal than the ears, it's impossible to state which particular species it might be. 

Whether or not Kentriodon counts as a "dolphin" is a matter of definition. If we were able to see them in the flesh, they would absolutely have looked like dolphins, with a similar size and shape and they must have had a very similar lifestyle. They don't count as true dolphins only because they (probably) aren't descended from the last common ancestor of all living oceanic dolphins. Instead, they form a side-group that likely branched off from regular dolphins even before the porpoises or narwhals did, making them equally related to all three of those groups. They're significant because they appear to be the first such animals to develop narrowband biosonar, making them harder for larger predatory sonar-hunting whales to detect.

A couple of other specimens proved harder to pin down, but the authors of the study suspect that they probably belonged to sea-going relatives of the modern platanistid river dolphins. These would be part of a much larger range of such "dolphin" species at the time, forming a fairly distant sister group to the living sort, although closer to them than to anything else. Technically referred to as "squalodelphinids", other examples include Medocinia from southern France, but, further afield we also know of fossils from the Atlantic coast of North America, and even from Peru, showing that they were a successful and widespread group. These particular examples were, so far as one can judge based on ear bones alone, fairly small by the standards of dolphins, and likely fed on small fish. 

Out of the over 200 bones analysed in the study one stood out as different, in part due to its larger size. This was identified as belonging to a sperm whale, albeit an extinct relative of the living animal. The shallow waters of the channel would not seem an ideal place for such animals to live, but, in fact, several species of sperm whale are already known from the area at the time, and they had a much wider range of habits, and presumably hunting strategies, than they do today when only three species still exist. Where this one fits into that larger picture is, again, impossible to say, but the range of possibilities is surprisingly broad.

While the appearance of squalodelphinids in this particular area is a new discovery, they did live elsewhere in the Mediterranean and Paratethys at the time, so it isn't a huge surprise. Overall, the picture presented by this new study matches with what we've been able to put together from elsewhere, suggesting that many of the cetaceans of the time lived across as much of the available waterways as they could, as one might expect given the lack of obvious geographic barriers. Indeed, there is a strong similarity between the types of dolphin and whale found here and those of the same age from Maryland and Virginia. Exceptions include baleen whales, which don't seem to have entered the Mediterranean  until later, and porpoises, which are still absent today, despite living along the Atlantic coasts of both Europe and Africa and, more surprisingly, in the Black Sea.

If anything, this confirms that what we currently know is likely to prove fairly accurate in the long run, although are always some surprises somewhere along the road. But this is a refinement of our knowledge of a body of water that simply doesn't exist today; a time when dolphins swam off the north coast of Switzerland.

[Photo from the Smithsonian Museum, in the public domain. Cladogram adapted from Aguirre-Fernandez, et al. 2022 and Guo and Kohno 2021.]

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