Fossil Martens... or Not?

When talking about fossil animals on this blog I often mention the earliest known example of a particular group. But this often hides a degree of uncertainty, or even controversy, because such the exact identity of such fossils can be difficult to pinpoint. That's partly because, being, by definition, older than other fossil examples of the group, they are also the most likely to be incomplete or poorly preserved. Often, since we're talking about mammals here, the "oldest known fossil" may consist of little more than a distinctive tooth. 

The second, and perhaps even bigger, problem is that the further you go back to the origin of a group the more it blurs into whatever preceded it. Even if we had perfect remains, or if we could travel back in time and see the animals in life, or take blood samples from them for genetic analysis, there would always be a question of what exactly we were looking at. Where do you draw the line when, in reality, one group will have slowly and perhaps imperceptibly, evolved into a newer one?

Weasels in the Tundra and the Jungle: Wolverines and Tayras

The largest terrestrial member of the weasel family is the wolverine (Gulo gulo). Indeed, by the standards of weasels, it's exceptionally large, around twice the weight of badgers, and about three feet long, not counting the tail. Within the weasel family, it belongs to the same evolutionary branch as the pine martens and their kin, and, size aside, that is apparent in its appearance.

Compared with martens, however, the wolverine is not only larger, but stockier, and much of its weight - three times that of the next largest marten-like animal, the fisher - is muscle. Indeed, they may well be the most physically powerful of all mammals in their size range. Other martens vary in the degree to which they live in the trees, but wolverines are the most terrestrial of them all, although they remain capable of climbing trees when the need arises. Compared with other marten-like animals, they have flat-soled feet, which are large and furry, making them ideal for running on snow. They also lack the pale 'bib' commonly seen on the throat of martens, although there is generally a band of paler fur running across their flanks to the base of the tail. Their fur is also exceptionally thick, which makes them look even larger than they are.

More Weasels up Trees: Sables and Other Martens

Sable

Today, true pine martens venture no further east than the Ural Mountains. But it was not always so, for, at one time, their immediate ancestors ranged across a wider swathe of forest. During the Ice Ages, those in the east became separated from their western cousins, and became a distinct species. Today, beyond the Urals we find, not pine martens, but their close relatives, the sables (Martes zibellina).

Indeed, sables and pine martens still look very similar, although the former have shorter tails, and the 'bib' on their chests is usually less distinct, varying from pale brown to yellowish. Their fur is rich and luxurious, especially during the winter months, and can be anything from pale brown to the near-black colour for which they are most famous. Largely on the basis of this variation in colour, up to thirty different subspecies have been identified, although, personally, I suspect that the animal is just rather variable in appearance, and many of these subspecies won't turn out to be valid under more thorough investigation.

Like pine martens, sables inhabit both deciduous and coniferous forest, although they prefer the latter. They were once found across the vast forests of Siberia, into Mongolia and northern China, and, even today, they inhabit the islands of Hokkaido and Sakhalin either side of the Japanese/Russian border. They can, of course, climb trees, but they seem to do so less than pine martens do, and they make their homes in underground burrows, sometimes of considerable length, with entrances partially concealed beneath tree roots or similar cover.

Weasels up Trees: Pine and Beech Martens

Pine marten

In my survey of the weasel family, I have so far looked at the mustelines; those members of the family that are most closely related to the weasels themselves. Aside from the numerous kinds of "true" weasel, this has included stoats, polecats, and mink, but, of course, the mustelines are not the only members of the weasel family. It is now time to turn to a second grouping within the family, the marten-like animals.

The oldest fossil martens date from the Pliocene; the epoch immediately before the great Pleistocene Ice Ages. By this time, they had already begun to develop some of their distinctive features. Martens are much larger than true weasels, and within, or slightly above, the size range of the largest mustelines, the European polecats. Where mustelines have evolved narrow bodies for chasing prey down into their burrows, the martens have never needed to do so, and while they have the short legs typical of most members of the family, their bodies are noticeably more compact. They are generally brown in colour, and most species have a highly visible 'bib' of paler - usually yellowish - fur on their chest.

The most distinctive feature of the martens is that they spend much of their lives in the trees, quite a different habitat from other members of the family. By heading up into the trees, they have been able to find a source of food quite away from their ground-dwelling kin, and over much of their range (although by no means all), they are the only small carnivores that hunt among the branches.