Sunday, 17 March 2019

The Beaches of Bakersfield

Modern walrus skull
Seals have a moderately long fossil history, with some of the oldest examples dating from Europe, around 15 million years ago. Although they are now found worldwide, back in those early times, they seem only to have lived in the North Atlantic, reaching the Pacific only around 11 million years ago (probably by the simple expedient of swimming through the seas that then covered what is now Central America).

However, one of the world's best fossil sites for recovering fossil aquatic mammals is at Sharktooth Hill, just outside of Bakersfield, California. Part of the larger Temblor Formation, the deposits here consist of silts and sands, laid down just over 15 million years ago, when California was sweltering in the Middle Miocene Climatic Optimum, a time of unusually high worldwide temperatures - and correspondingly high sea levels. This was before seals had reached so far west, but that does not mean that a number of similar animals did not already live there at the time.

In fact, even today, there are three living families of seal-like animal (technically called pinnipeds), of which the seals themselves are merely one. The other main group, the otariids, includes both the sea lions and the so-called "fur seals" - animals quite distinct from the true or "earless" seals. The great majority of otariid fossils date from no more than 10 million years ago (which is likely just shortly after the sea lions and fur seals diverged from another), but the solitary exception is Eotaria, which did, indeed, live in California, probably around 16 million years ago.

As it happens, though, the fossils of Eotaria didn't come from Sharktooth Hill specifically, so the many pinniped fossils we have from the site are not, in fact, otariids any more than they are true seals. So what are they?

There are at least four different species of pinniped known from the site, some with a number of fossils. The most common is Allodesmus, which has been quite extensively studied and turns out to belong to a group that is now entirely extinct. The second most common, however, is something slightly more familiar... yet we know rather less about it.

When it was first described by Remington Kellogg in 1931, he gave it the rather unimaginative name of Neotherium... which literally translates as "new beast". As was apparent from the beginning, it belongs to the third of the living families of pinnipeds: the walruses.

There is only one species of walrus living today, but this is merely the last survivor of a lineage almost as old as the seals themselves. While, at 15 million years or so of age, Neotherium is not the oldest known fossil walrus, it certainly isn't too far off, so that a recent re-evaluation of some of the existing fossils casts some light on what the earliest examples of walrus-kind may have been like.

It's hard to determine the exact size of an animal from its skeleton alone, especially since these are usually incomplete, but our best guess is that Neotherium was about two metres (six feet) in length. This is much smaller than a modern walrus, and a little on the small side even for a seal, but hardly insignificant. (It's larger, for example, than a modern harbour seal, but smaller than, say, a leopard seal).

Intriguingly, the fossils we have cluster in two different size ranges, with few, if any examples, in between. This is strong evidence that, just like their modern relatives, Neotherium was sexually dimorphic, with males about 25% larger than females. This may, in turn, imply a similar mating pattern, with large males fighting for control of a harem of females. Even so, it's less of a size difference than in living walruses, closer to that of some of the less dramatic species of sea lion, and may mean that this feature had only recently developed. (It's hard to know for sure, though, since we don't have enough samples of even earlier walruses to make the same kind of estimate).

One of the most obvious features of the living walrus is, of course, the presence of tusks. These, however, are a relatively modern innovation in walrus evolution, being present in only the most modern of species. Indeed, the teeth of Neotherium are relatively normal, in some respects resembling those of early sea lions. For example, it has considerably more teeth than a typical modern walrus, still having molars and a full set of premolars.

Even in later tuskless walruses, the premolars tend to be flattened and to have less enamel (often none at all) compared with the teeth of most other mammals. These features are missing in Neotherium, which had narrow, relatively sharp, teeth in the back of its mouth, suggesting a rather more meaty diet than the shellfish-guzzling modern species.

All of this means that Neotherium is descended from a very early group of walruses, sufficiently derived to distinguish them from animals such as early sea lions, but outside either of the two main groups of walrus that appeared later on. Its closest relative is probably Kamtschatarctos, a species known from roughly 12 million-year-old deposits in the Russian Far East.

Although that's still in the North Pacific, it is still quite a distance from California, implying that early walruses may have been more widely spread than seals were at the time. Nor is it unique, since Allodesmus, the other pinniped known from Sharktooth Hill, also had close relatives in Japan.

If anything, it's curious that otariids, known from just a little further south at around the same time, weren't also found sharing the beaches of Bakersfield during the Middle Miocene. Maybe there wasn't room, with at least four other species of pinniped in the same area leaving little food to spare. Or perhaps there was something that early sea lions wanted that just wasn't available further north.

Over the millions of years that followed, both seals and sea lions diversified and became more common, with the walruses forced into the remote Arctic to feed on a specialised diet that few other animals wanted to rely on. Now only one species remains, but there were once many more, at least some of which basked on the shores of a swelteringly hot Miocene California.

They had their time in the sun.

[Photo by Mike Peel, from Wikimedia Commons.]

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