Sunday, 8 November 2020

Last Gasp of the Australian Seals

Today, true seals only live much further south
In common English parlance, the word "seal" refers to a range of animals that, scientifically speaking, constitute two different, but related, mammal families. (A third related family, which today includes only the walrus, is distinctive enough that most people probably don't think of them as "seals" in the regular sense). I've previously written a post that goes into detail about what the difference between these two families is, but the technical names are phocids (for "true" or "earless" seals) and otariids (for the sea lions and fur seals, both of which have visible ears).

For simplicity, I'm going to refer to these two groups as "true seals" and "sea lions" in this post, although you should be aware that the so-called fur seals fall into the latter group. We've long known that "fur seals" aren't really a specific type of animal, but are at least two different kinds of sea lion that coincidentally look a little different from their relatives.

When we look at the true seals today, we see a clear pattern as to how they are distributed. Most species live in the Northern Hemisphere where they're found right around the world, and even in freshwater. Typically, they live in cold waters, and the number of species dwindles as we head south. By the time we reach the equator, there's nothing, and then there's a huge gap before we reach Antarctic waters, where we suddenly find the remaining five species.

The sea lions, however, are a different matter. While they don't quite reach the Antarctic coast proper, most, but not all, of their species do live in the Southern Hemisphere. Moreover, there's no vast geographic gap between the southern and Arctic species since there are some species living right up the western coast of the Americas (most obviously, perhaps, the California sea lion), including some that live pretty much exactly on the equator.

Thus, while both kinds of animal are found in subantarctic waters, and in the North Pacific, the sea lions have the warmer waters of the Southern Hemisphere largely to themselves. But surely that cannot always have been the case, or how did the true seals of the Antarctic ever get there?

In fact, for most of the evolutionary history of the two families, the situation was very different from today. For millions of years during the Late Miocene, true seals were common across the waters of the Southern Hemisphere, and sea lions were found only in the north, having first evolved, so far as we can tell, somewhere in the North Pacific.

This situation remained the case for at least 10 million years, with sea lions first crossing the equator during the Pliocene, perhaps no more than 5 million years ago, about two-thirds of the way through their existence. There then seems to have been a relatively rapid turnover, with the true seals of the southern temperate waters dying out in a comparatively short period of time. By the time the Pleistocene epoch dawned, around 2.6 million years ago, the current arrangement was in place, with the only true seals in the Southern Hemisphere being those sheltering south of the Antarctic Circle, where even the hardiest sea lions were unwilling to go.

Genetic analysis confirms the pattern shown by the fossils; the few species of sea lion that remain in the Northern Hemisphere form a number of different, ancient, lineages, while the species that live in the south are all descended from a single ancestor that lived fairly recently as such things go. 

The last known true seal to live in the temperate waters of the Southern Hemisphere seems to have been Hemiphoca capensis. This lived off the coast of what is now South Africa, with the few remains known for the animal having been dated to no more recently than 3 million years ago, in the Late Pliocene. Although its exact relationships remain unclear, it does seem to be more closely related to the living Antarctic seals than to anything else around today.

At least that's the picture from South Africa, which broadly matches that from South America, from which we have plenty of true seal fossils through the Miocene and early Pliocene before they suddenly get replaced by sea lions about 3 million years ago. But you don't need to be an expert in geography to recall that there's another continent in the temperate/tropical zone of the Southern Hemisphere. So did the same thing happen there?

The problem is that we don't have many fossil seals from Australia. Most of the true seal fossils we do have are, as we'd expect, from the late Miocene and early Pliocene, while most of the Pleistocene fossils are sea lions, similar to the species that live there now... but it's harder to judge when the switch-over occurred, or whether it was as rapid and complete as elsewhere in the world. 

A couple of fossil true seals from New Zealand do, in fact, date from the early Pleistocene, well after we'd expect the changeover to have finished, but one is indistinguishable from living Ross seals and the other is a near-modern elephant seal - so it could just be that, when the world was much colder, some Antarctic species lived further north than they do now. (For that matter, elephant seals lived along the south coast of Tasmania recently enough to be hunted by aborigines, and leopard seals do occasionally breed around New Zealand even today). 

But what we need to answer any questions about the changeover are fossils from the gap in between these earlier and later fossils... and it now seems we have one

This is not, it should be stressed, some magnificent fossil of a mostly-complete skeleton. It is, in fact, a tooth. Literally one tooth, from a late Pliocene site in western Victoria. This is not enough to give the animal it came from a scientific name, although it does have enough distinctive features to indicate that whatever it was was related to today's Antarctic seals without actually being one.

But the mere fact of its existence does tell us something. It's at least of a similar age to Homiphoca from South Africa, and may well be younger. Either way, it represents part of the last hurrah of the true seals in temperate waters of the Southern Hemisphere, and it may even show that this took place later in Australia than it did elsewhere in the world. Which, since sea lions seem to have reached the south by travelling down the Pacific coast of the Americas, might make sense - it simply taking a long time for them to cross the cold open waters of the southern Pacific to reach Australia.

Of course, what we really want to know is why this happened at all, and that, unfortunately, remains a mystery. It's possible, for instance, that true seals died out for some reason and the sea lions simply moved into the vacant rookeries. There are, after all, no fossil sites in the Southern Hemisphere that feature both kinds of animal (there are in California, though, so such things wouldn't otherwise be impossible). But, equally, the sea lions could have out-competed the true seals rapidly, except for those living in the frozen south where they remain today.

There does appear to have been a slew of marine extinctions in general around this time though, and whatever happened may be part of that. The world was cooling in the prelude to the Ice Ages, and this would have meant that coastal environments were changing as the sea level dropped and the original coastlines ended up inland. The world was no longer what it had been, and seals may have been more vulnerable to these climatic changes than we might expect.

Which could be a fact that's relevant even today...

[Photo by Jerzy Strzelecki from Wikimedia Commons.]

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