Saturday 26 November 2022

Miocene (Pt 36): Dawn of the Seals

In this series so far, I have covered the mammalian wildlife of the Miocene continent by continent, showing how the diversity of animals then was just as great as it is now. But there is a group of mammals that a time traveller to the period would have been able to observe along the coasts of those continents - especially during their breeding season - that I have not yet mentioned.

Today, the seals are divided into two subfamilies (I have reviewed all living species of seal here): the phocine or "northern" seals, which live in the Northern Hemisphere, and the monachine or "southern" seals, which are primarily found in the Southern Hemisphere, but do include three living species in the north. 

Although both groups existed by the Middle Miocene, their distribution was not so clear-cut then as it is now. The phocine seals were almost entirely restricted to the Northern Hemisphere, with several fossils known from Europe, from Belgium to Ukraine. The one possible exception is Kawas (the name means 'seal' in a local native American language), which lived along the southern coasts of Argentina during the Middle Miocene. While it seems to resemble the "northern" seals more than it does its more modern neighbours, this has been questioned, and, even assuming it really is phocine, it's clear that such seals can't have remained south of the equator for long, since there are no other examples.

Most of the undoubted phocine seals come from Europe, although Leptophoca did live on the Maryland coast during the Middle Miocene; it is also known from slightly older deposits from Belgium and so must have crossed the Atlantic in one direction or the other. It seems to have been a close relative of Prophoca, which is perhaps the most primitive phocine seal known, and still seems to have been able to use its fore-flippers to aid with propulsion, rather than merely for steering as such seals do now. Afrophoca was another non-European example, living on the coast of Libya - although this wasn't exactly far from Europe, even then.

Perhaps the best-known of the many Miocene seals that are known from European deposits is the diminutive Nanophoca, which was probably no more than a metre (3 feet) in length. We know that it lived in the North Sea and that its bones were unusually dense. This would presumably have made the seal heavy in the water, which doesn't seem a good idea on the face of it; perhaps it fed along the bottom of very shallow seas, rather than spending most of its swimming time at the surface. A similar phenomenon is seen in seals from the Paratethys Sea (of which the Caspian Sea is the largest modern remnant) such as Pachyphoca, thought to be a relative of the living hooded seal. This is probably, in their case, because the sea in question was very salty at the time, and they needed to counteract the added buoyancy that would create.

The "southern" seals were more common in the Northern Hemisphere during the Miocene than they are today, with several species known from Europe, with Callophoca being known both from Belgium and North Carolina during the Late Miocene. The fossil record of such seals in the Southern Hemisphere is comparatively sparse, although that's probably for lack of looking rather than because they didn't exist. Indeed, Neomonachus, from Late Miocene New Zealand actually appears to have been a monk seal - the one group of "southern" seals that is currently restricted to the Northern Hemisphere, showing that, contrary to all previous expectations, this group once crossed the equator, too. 

Hadrokirus is another Late Miocene example of "southern" seals. It lived in Peru, but it closely related to Piscophoca, which is known to have reached Chile. Hadrokirus is significant for its heavy teeth, suggesting that it fed on either hard-shelled animals (such as lobsters) or on something relatively large with thick bones (such as other seals). Acrophoca, possibly another close relative, was unusual because of its long neck. Although it had previously been suggested that it might have been a filter feeder, more recent analysis seems to show that it fed by gripping and tearing at prey, like a modern leopard seal, in which case the long neck probably made it easier to lunge at its victims.

One of the ways of distinguishing the "northern" and "southern" seals is the number of incisor teeth at the front of their jaws. Devinophoca, which lived on the Slovakian coast 15 million years ago when there still was such a thing, is unusual in that it had three pairs of incisors in the upper jaw, like a "northern" seal, but only one in the lower jaw, like a monk seal. The consensus is that it probably belongs to neither group, representing an early third branch that died out, at the latest, towards the end of the Middle Miocene.

Genetic evidence suggests that the two (or three) branches of the seal family diverged around 18 to 16 million years ago, during or shortly before the very hottest time of the Miocene. The oldest fossil seal known, however, is Noriphoca, which dates to at least 22 million years, the only example currently known from the Early Miocene - indeed, it's probably a holdover from the previous epoch, the Oligocene. It had more teeth than any living (or other fossil) seal, suggesting that it was particularly primitive and, while originally described as an early "southern" seal, it seems more likely that it predates the split.

While, as indicated above, the fossil record of Miocene seals is, especially in the Northern Hemisphere, reasonably good, the same cannot be said of the sea lions. Although they must logically have lived right through the Miocene, we have almost no examples of them. The oldest currently known is Eotaria, from around 15 million years ago, the only known example to predate the Late Miocene. It lived in California, as did the only other Miocene sea lions, Pithanotaria (known from only scant remains) and Thalassoleon. The latter, which lived at the very end of the Miocene, was still primitive enough to have likely predated the main split within the sea lion family - between the sea lions proper, and the "fur seals" - although it likely already fed, and swam, in much the same manner that all living members of the family do, and probably looked quite similar. There is, for instance, evidence that the males of the species were much bigger than the females, as is true of modern sea lions.

It seems probable that we have so few Miocene sea lion fossils because only a very few species existed at any one time, perhaps all of them restricted to California. But, when we come to the walruses, the exact opposite is true: there is only species alive today, but there used to be many more. The walruses originated in the North Pacific, and did not reach the Atlantic until much later, presumably by using the northern route around Canada. They initially seem to have lived in relatively warm waters, unlike their modern descendants (even allowing for the fact that the Miocene was warmer than today), with most Miocene fossils known from California and Japan - although there are also some from Oregon and from Kamchatka in Russia.

The most distinctive feature of modern walruses, the giant tusks, had yet to evolve, with the result that they would probably have looked more like sea lions than their modern kin in life. Nonetheless, their canine teeth were often enlarged and, in many cases, could reasonably be described as "tusks" - if more like a wild boar than the sort we would think of. One group, the dusignathines, such as Gomphotaria,  even had four tusks, rather than two, with the canines in the lower jaw being almost as large as those in the upper. But the dividing line between "tusks" and "large teeth" is a vague one, and by some definitions, the 6 million-year-old Osodobenus, living on the south California coast right at the end of the Miocene, was the first truly tusked walrus.

The most primitive known fossil walrus is Prototaria, which lived in Japan around 16 million years ago, not longer after the group are first thought to have split from the sea lions. It was much smaller than modern walruses, although still in the size range of many seals, and its teeth were far less specialised than those of later examples. It probably fed on fish in the same way that seals and sea lions do today, rather than specialising on clams and other invertebrates like the modern walrus.

From these humble beginnings, walruses grew steadily larger as the epoch unfolded. Pelagioarctos, another Californian walrus which lived not much later than Prototaria, had already reached the size of a large sea lion. It has been hypothesised that it might have been a powerful predatory hunter feeding on large prey, although more recent analyses suggest that it probably just ate fish

Titanotaria is named for a local university sports team, rather than for its size, but it was still impressive, probably comparable in size to a modern walrus at over 3 metres (10 feet) in length. It and its close relative Imagotaria lived along the California coast around 9 million years ago, early on in the Late Miocene and its not much later that the tuskless walruses went extinct, perhaps driven by the cooling climate as the Pliocene approached and an accompanying shift to a more mollusc-based diet among those walruses that did survive. Another notable late survivor is Pontolis from Oregon, thought to have been the largest walrus to have lived; at around 4 metres (13 feet) in length, it would have been about the size of an adult male elephant seal, its only known contender for the title of "largest carnivoran species ever".

The desmatophocids represent a fourth group of pinnipeds, distinct from the seals, sea lions, and walruses, that existed only during the Miocene. Desmatophoca, for which the group is named, is known only from limited remains, and it is difficult to say too much about it. Allodesmus, living around 10 million years ago on both sides of the Pacific, is fortunately better preserved. It may have been the last surviving desmatophocid, and therefore potentially more specialised than its earlier relatives, but it does nonetheless have some unusual features. For instance, it had particularly large eyes, suggesting that it hunted in conditions of poor light - presumably deep water, in its case, rather than at night - and had a skull shape that has led some to suggest that it might have had a proboscis like an elephant seal. The relationship to other pinnipeds is obscure; the limbs suggest that it swam like a sea lion or walrus, rather than a seal, but this might not mean much, given that very early seals did much the same.

Almost all the creatures I have described so far lived during the Middle or Late Miocene, since this is as far back as we can trace the existing families of pinniped. That Noriphoca is already identifiable as a seal suggests that the ultimate origin of the group is even further back but some of its contemporaries are also contenders for the title of "oldest known pinniped".

Enaliarctos lived around 23 million years ago, right at the dawn of the Miocene, and is known from both California and Oregon. It doesn't fit with any of the living families, partly because the bones of its limbs have points for muscle attachment that suggest it could still walk - although possibly not very well - and it could also still chew its food, rather than gulping it down, suggesting that it may have carried its prey onto land to finish its meal. Interestingly, analysis of some fossils shows that males were already larger than females, so that the extreme harem-based mating system of modern pinnipeds may already have existed, even this early.

Enaliarctos is undoubtedly a pinniped, and if we could see one in the flesh, it would probably still look much like one, even if its flippers were at least partially leg-like. But, as always, there is the question of where exactly we draw the line between pinnipeds and not-yet-pinnipeds, and some other Early Miocene animals could fall on either side of that line, depending on where we place it. While they lived too late to be the actual ancestors of pinnipeds, which presumably first arose towards the end of the Oligocene, they may well be close descendants of the creatures that were, changing rather less than the likes of Enaliarctos had.

The clearest example here is probably Puijila, a 23 million-year-old fossil from northern Canada. While it had a number of features that seem to place it close to pinnipeds, it still had legs and feet, and a long tail. While the shape of the toe bones suggest that its feet may have been webbed, and there is ample evidence that it was suited to the water, it would have looked much more like an otter than a seal. Indeed, it may be related to Potamotherium, now thought to be another early pinniped relative, but for a long time assumed to be a mustelid. It most likely lived in freshwater, and its anatomy strongly implies that it was semi-aquatic, as otters are, spending most of its time on riverbanks and only diving in to catch food.

But, while the pinnipeds of the Early Miocene were probably relative newcomers to the oceans there, was of course, another group of mammals that had already been there for a long time. The final post in this series on the Miocene will take a dip further into the sea, and look at the cetaceans of the day...

[Photo by Jonathan Chen, from Wikimedia Commons.]

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