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| Hawaiian monk seals |
Now, monk seals don't have have the same sort of problems when it comes to dispersal as lemurs and the like do. Given time, they can just swim across vast stretches of ocean, feeding on fish as they do so. The apparent puzzle of their wide separation is also reduced by the fact that there used to be a third species, living in the Caribbean, which sadly went extinct in the late 20th century. The Caribbean is about half way between the other two locations, and that helps rather more than you might think at first glance.
Monk seals like warm to tropical waters, and, indeed, they are among the few seals that really do - there are two or three others that quite like temperate waters, but most of them want it to be freezing cold. Today, if you want to stick entirely to warm waters, swimming between Jamaica and Honolulu may pose a bit of a problem. But, so long as they did it before the dawn of the Pleistocene, then, really, the sheer distance was the only issue they would have faced. That's because, prior to the Pleistocene, Central America was just a chain of islands, easy for any seal to simply swim between and head out west.
Hawaiian monk seals don't have any significant fossil history that we know of, so presumably, they're the newcomers. We also know, from genetic evidence, that their ancestors probably parted company from those of the Mediterranean sort a little over 6 million years ago. That's long before the Panamanian Isthmus closed off the western side of the Caribbean, which suggests that the scenario in the previous paragraph is pretty much exactly what happened. (Indeed, this 6 million year gap is long enough that it was recently proposed that the Hawaiian monk seal should be placed in its own genus, Neomonachus, to emphasise its long separation from its Mediterranean cousin).
At some point, of course, monk seals must also have swum across the Atlantic. That, in itself, is fairly obvious, and there's nothing that would have particularly prevented them from doing so. But it does leave one question if we really want to understand the evolutionary history of the genus: which direction were they going? Did they start out in the Mediterranean, head west, and then, later on, go even further west, or did they start out in the Caribbean, and then head out in both directions?
We do have Pleistocene fossils of both Mediterranean and Caribbean monk seals, but we need to push back further in time than that to untangle the riddle. Back in 1900, a fossil monk seal was unearthed from a deposit in Tuscany, and identified as being essentially indistinguishable from the living Mediterranean species. More modern analysis has determined that the rock stratum in question is about 3 million years old, placing it in the late Pliocene, the epoch immediately prior to the Pleistocene. So far as we can tell, at that time Tuscany consisted of a broad series of shallow bays dotted with islands - exactly the sort of place one would expect to find seals.
In 1941, the fossil was examined again, and, this time, enough differences were spotted to assign it, not just to a new species, but to an entirely new genus. It became Pliophoca etrusca - the "Etruscan Pliocene seal". It remains perhaps the most complete fossil of the species that we have, consisting as it does of the skull, flippers, and bits of the backbone, but, since then, some other specimens have been found. Some are from Italy, but there are also examples from southern France, eastern Spain, and, most intriguingly, from Virginia, North Carolina, and Florida.
Despite the different genus name, it's clear that Pliophoca was sufficiently close to the living species that we can reasonably call it a "monk seal". But exactly how close was it, and how does it fit within the wider seal family tree?
The seal family is often divided into two sub-families, sometimes termed the "northern" and "southern" seals. These are, to be honest, not the best names ever; while the "northern" seals are, indeed, all from the Northern Hemisphere, so are three out of the eight living species of "southern" seal. Yes, they do live further south than most of the "northern" seals, but even then, there's a bit of an overlap.
At any rate, monk seals belong to that "southern" group, and are thought to be related to the elephant seals and to the various kinds of Antarctic seal. The reality of this traditional grouping was questioned for a while, but modern genetic analysis has confirmed that it is genuine, and that monk seals are more closely related to the species off Antarctica than they are to, say, the harbour seals found off the coasts of Britain.Such analyses, however, are of no use when it comes to species known only from fossils. There, we have to rely on anatomical features of the skeleton, trying to estimate which are the most similar to others. When we attempt to combine skeletal studies with what we know of the genetics, a picture starts to emerge.
As we basically already knew, Caribbean and Hawaiian monk seals turn out to be more closely related to each other than to the Mediterranean species (and, hence, the former also belongs in Neomonachus, if we accept that genus). But that, in itself, doesn't prove that they descend from a single stock that left the Mediterranean 6 million years ago. It could just be that the Hawaiian species left the Caribbean later than the ancestors of the Mediterranean one did - most likely around 3 million years ago, shortly before the rise of Panama would have blocked them off.
What is significant, though, is two facts that the analysis appears to show about Pliophoca. First, it's an early branch, representing an animal that diverged from the living monk seals before those species diverged from one another. Secondly, at least according to this study, those fossils from the southern Atlantic seaboard of the US do not represent the same creature: they are something else entirely, albeit not entirely unrelated.
This, then, answers the question. Monk seals first appeared in the Mediterranean, where they gave rise to the local Pliophoca and to the living European species, before some of them headed west to the Caribbean, and, ultimately, Hawaii.
We can't, on the other hand, say how the ancestors of the monk seals got to the Mediterranean themselves. Much older "southern" seal fossils are known from the area, dating back to at least the middle Miocene, so they may have been there for a very long time. But, equally, those American fossils could be the last survivors of some even earlier stock there. That, however, is a different question, and one it will likely be harder to answer, whatever clues there may be lying much further back in time.
One species of monk seal has gone extinct in the last hundred years. The remaining two are, as of today, on the verge of extinction themselves, with only a few hundred adults of each alive, and the population shrinking with each new generation. Let us hope that we have not discovered the origin of the group shortly before we discover how it is to end.
[Photo by Paulo Maurin for the National Oceanic and Atmospheric Administration; in the public domain. Cladogram adapted from Berta et al, 2015.]
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