Saturday 24 November 2012

Weasels in Stone: Mustelid Evolution

Sthenictis, a fossil mustelid
Last September, I began a survey of the weasel family, the largest, and arguably the most varied, family of carnivorous mammals. Since then, I have managed to give at least a brief description of every living species in the family, from stoats and badgers to otters and wolverines. But how do all these animals relate to one another, and where did they come from?

In the past, scientists would work this out by examining the physical features of animals, reasoning that those with the greatest similarity were the most likely to be related. By adding in information gleaned from fossils. such as their age and location, and any evolutionary changes they may have undergone, it's possible to get quite a lot of information this way. However, today, it is more common to use molecular and genetic information, seeing how genes and the proteins they produce have changed in subtle, often invisible ways, over the course of time. Taken together with the fossil information, this can reveal many relationships that might otherwise remain obscure.

In the very first attempt at scientific classification of animals, way back in 1735, weasels were grouped together with martens, polecats, and civets as a sort of collective group of small slender mammalian carnivores, although otters and badgers were placed separately.  However, it was not until 1817 that the weasel family was formally named by German naturalist Johann Gotthelf Fischer von Waldeheim. That, of course, was still before we understood evolution, and it's hardly surprising that we've moved on since then.

For much of the twentieth century, it was agreed that otters were distinct from all other members of the weasel family, given their considerable adaptations to their aquatic environment. Badgers, and in particular, American badgers, were sometimes also considered to represent a separate evolutionary line within the weasel family. Everything else was a "musteline" - a "typical weasel", if you will. Once we began to use molecular techniques, however, the picture became more complicated.

Mustelid cladogram according to Koepfli, et al. 2008
The study I have been using as the basis for these posts was published by Klaus-Peter Koepfli and co-workers in 2008. Analysing and comparing a range of gene segments from almost every species in the weasel family, they produced the tree seen here to the right, and the various other trees that I have been using throughout this series. This seems to be the most widely used classification scheme used today, and is consistent with some earlier, preliminary studies by other workers.

The researchers didn't stop with simply working out what was related to what. By assuming that closely related groups of animals tend to evolve at about the same speed, and using the age of fossils to calibrate that "molecular clock", they went further and constructed a chronogram - a chart of what they think happened when.

The resulting picture showed that the evolution of the weasel family could be roughly divided into five phases. After the initial origin of the family, the second phase consists of a rapid diversification of different kinds of mustelid. This takes place during the mid to late Miocene, 11 million years or so ago. It is that time that most of the main groups within the weasel family make their appearance: it is the time of the first Old World badgers, the first marten-like animals, the first otters, and so on. Clearly this was an important stage in the evolution of the family, and is responsible for the wide range of different kinds of animals it currently contains.

The rate of formation of new groups slows down in the third phase, during the late Miocene. Even so, we should see some new forms appearing, often caused by species being isolated on different continents. For example, it's this time that the researchers suggest the American and Asian otters separated from one another, and that the South American grisons parted company with their Old World kin, such as zorillas.

Things speed up again during the Pliocene, a time of prolonged climate change that altered the environment for many animals. According to the analysis, while individual modern species may not have originated at this time, the evolutionary lines that led to them often did. We'd expect to see the the first American mink, the first clawless otters, the first hog badgers, and maybe the first wolverines appearing at around this time, among others.

The Ice Ages represent the final phase, and the general types of mustelid had all already appeared by that time, leaving only some "fine tuning" as individual populations became isolated by sheets of ice and formed new species. This may have happened, for example, with the closely related pine martens and sables. Perhaps the most significant newcomer they predict at this time would be the appearance of the first polecats.

Although this presents a useful story, we have to remember that, on its own, it's really only educated guesswork. How much of it can we back up with actual fossils? Being generally small animals, often with fragile bones, mustelids don't always make the best of fossils. They have also been somewhat neglected compared with more glamorous carnivores, such as sabretooth cats. Even so, they're pretty numerous, and, in fact, we do have a number of fossils to look at.

Otters appear to have originated in the Old World, and spread to the Americas in two waves, with relatives of giant otters, such as Satherium, crossing over well before the smaller species. While giant otters are today only found in the south, Satherium lived across North America too, with fossils being found from Washington state to Florida, and dating to 2 or 3 million years ago.

Today, the giant otter is the largest of all mustelids, but it isn't the largest ever to have lived. That honour may go to Enhydriodon, a gigantic species of otter from Africa. Although we only have the skull to go on, if it's in proportion to the rest of the animal, it may have weighed 100 kg or more when it was alive, making it the size of a small bear. It seems unlikely that it was particularly aquatic at that size, although it may have used rivers as bears do today; it's also not very clear what it ate. Despite its land-based lifestyle, it's teeth suggest (although not conclusively) that it's closest surviving relative may be the sea otter.

Another particularly large prehistoric "weasel" may be the fossil wolverine Plesiogulo.  Probably not a direct ancestor of the modern forms, it seems to have first evolved in Asia, and crossed over into North America around 6 or 7 million years ago, which would place it very close in time to the purported origin of the modern species from more arboreal marten-like animals.

I've mentioned the possible origin of "true" badgers before, with the Pliocene species Meles thorali being the possible ancestor for the living Eurasian forms. The similarly old Arctomeles may represent a link between it and the related hog badgers of southern Asia, although, as with M. thorali, the variation in detailed skeletal form of badgers makes it difficult to say for sure.

American badgers are a different matter. According to genetic evidence, they were the first group of mustelids to diverge from their kin, and should have the longest independent evolutionary history. According to the study by Koepfli et al. they originated well before the great burst of evolution that gave rise to the other groups of mustelid, during the early Miocene, something like 20 million years ago. However, we don't have any fossils approaching that age, with close relatives of the living sort and the extinct relative Chamitataxus both appearing around 6 million years ago.

Even so, the American badgers seem to be the only living group of mustelid that originated in North America; all the others found there now, such as otters, martens and the like, seem to have first appeared in Eurasia and only subsequently crossed over to the New World. However, the American badgers probably weren't the first mustelids on the continent. While South American species appear much later, after the formation of the Central American land bridge, the oldest definite weasel fossils on the Northern continent date back over 20 million years, possibly before American badgers even existed (and well before the oldest clear evidence for them).

These fossils belong to a group of purely American animals called the oligobunines. They are long since extinct, wiped out something like 10 million years ago, possibly by the arrival of their more modern relatives from Asia. The oligobunines somewhat resembled badgers, and, like them, may have been effective diggers. A fossil of one species, Zodiolestes, was even found in what appeared to be the burrow of a prehistoric beaver, although whether it ate the original inhabitant, or simply took over an abandoned tunnel, is less clear. Given the size of the muscle attachments on its jaw, it probably had a powerful bite, and may have had similar feeding habits to wolverines.

Several different species of oligobunine have been reported, so that, even before the arrival of modern forms, there may have been a number of weasel-like animals living across North America. Nor were they alone. Even ignoring the presumed American badgers of the day, there was also another group, including the rather raccoon-like Leptarctus, that lived on the continent at the same time. Unlike the oligobunines, this group also included some species in eastern Asia, such as Kinometaxia from Tibet, and a single European species, Trocharion. They were evidently widespread and successful in their day, despite being quite primitive in comparison with later forms; unfortunately there are not enough fossils for it to be at all clear on which side of the Atlantic they first appeared.

When the oligobunines were first described, they were said to include the five-foot-long semi-aquatic Potamotherium, which, unlike the regular oligobunines, was also found in Europe. It's less clear now whether it really was a member of the weasel family at all; it has also been suggested that it's a very early seal, dating from a time when they still had legs instead of flippers. Another early, vaguely otter-like animal, Metanomictis from India, probably is a mustelid, but dating back around 23 million years, is so early that it almost certainly doesn't belong to any group we'd recognise today.

So far as we can tell, much of this broadly supports the story of mustelid evolution provided in the 2008 genetic analysis. However, there are enough gaps and uncertainties that it's difficult to tell. In particular, the apparently very rapid appearance of so many different kinds of mustelid - weasels, badgers, otters, and all the rest - can mean that it's difficult to tell the exact order that these events occurred. This means that later studies, comparing different genetic patterns, haven't always reached the same conclusions.

A 2011 study by Li Yu and co-workers showed a very similar pattern, although it argued that ferret-badgers, rather than Eurasian badgers, might be the closest relatives of martens. However, just this February - after I started this series of posts - a new study by Katrin Nyakatura and Olaf Bininda-Edmonds came to a very different conclusion. They examined a large number of genes, as part of a study into the evolutionary history and relationships of all carnivorans, not just the weasel family, and the result can be seen here.

Mustelid cladogram according to Sato, et al. 2012
It's a pretty weird pattern. In summary, if this is right, the galictines aren't a group. Some, including the South American grisons, are related to otters, while others, such as the marbled polecat, represent a separate evolutionary line. Even stranger, the ferret-badgers, which all appear remarkably similar, and live in the same area, may not be a single group, either, and it's the honey badgers, not the American sort, that represent the first branching within the mustelid evolutionary tree. The last point, in particular, seems to disagree with every other study I've seen.

I find it a bit unlikely, and note that an even more recent study, published in June by Jun Sato and co-workers, agrees rather more with previous work. Still, it is different, and I present it here to the left. The summary here would be that galictines (once again a single group) diverged from otters after the weasels, rather than the other way around, and, again, that badgers and martens may not be as closely related as we thought. It's nothing too dramatic, bearing in mind how closely related all these animals are.

A couple of points are more significant, though. For one, honey badgers are here just another kind of Old World badger - "true" badgers once again, rather than something a little different. The other concerns the placement of the Patagonian weasel. As I mentioned when I discussed it, this is a rather odd little animal, somewhat different from all the other weasels, and it was also one of the few species that Koepfli et al. couldn't analyse in their study, leaving its relationships uncertain. Well, Sato et al. did manage to acquire a sample to analyse, and the reasons for its oddity suddenly make a lot more sense. If they're right, it's not really a true weasel at all: it's a very small grison. Since it lives on the same continent as they do, and has a very similar coat, that all starts to fall into place.

What about the very origin of the weasel family? The earliest fossils clearly belonging to the family date back around 30 million years, to the late Oligocene. There might be some older ones, but, the further we go back, the more the lines blur between the weasels and their closest relatives. Apart from the oligobunines, all the major kinds of weasel seem to have originated in Eurasia, before spreading out to the rest of the world. That's also where the oldest fossils come from, and it's likely where they first appeared. The very first weasels were probably close relatives of the raccoon family, evolving to pursue early rodents, long before their descendants would pick on everything from fish to earthworms.

[Picture by "Ghedoghedo" from Wikimedia Commons]

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