Sunday, 20 February 2011

The long-nosed herbivores of Argentina

Macrauchenia patagonica, the last member of its family
Today, there are two main groups of large, herbivorous hoofed mammals; one that includes those with cloven hooves, and one that includes the horses. But that hasn't always been the case.

Millions of years ago, South America was an island continent, as separated from the rest of the world as Australia is today. It separated from the other continents before either of the living groups of hoofed mammals had evolved, but the creatures that would give rise to those groups - a rather vaguely defined group of primitive herbivores called the condylarths - did exist, and were already present. In their isolation, these animals evolved into a whole array of large herbivores, creating a rich, and to modern eyes, rather strange-looking, mammalian fauna.

When North America finally hit its southern neighbour, it brought with it large mammalian carnivores, which had previously been absent. Every single one of these strange creatures has since died out - most of them not long after the arrival of sabretooths and jaguars, although at least one species probably survived long enough to be wiped out by humans. Yet, for millions of years, far longer than the paltry period of time since their disappearance, these were among the dominant mammals of their day.

There were many families, varying from each other at least as much as horses do from antelopes, but one of the more successful ones was that of the macraucheniids. A look at even this highly simplified version of their family tree reveals just how distant they were from living groups, for everything at all close to them is also extinct:

Macraucheniids    Adianthids     Prototheres
      ^               ^              ^
      |               |              |
      |               |              |
      -----------------              |
              |                      |            Notoungulates
              |                      |                 etc.
              ------------------------                  ^
                          |                             |
                          |                             |

The macrauchaeniids looked much like llamas in their bodily proportions, which makes sense, as they lived in a similar environment. (The llamas themselves, of course, arrived only when North America did). Like the living hoofed mammals, they had a reduced number of toes. In their case, they had three, with the central one being the largest and bearing the most weight. This is somewhat similar to modern rhinoceroses, although the feet of most macrauchaeniids would have been more slender, closer to those three-toed animals from which horses evolved. On the other hand, while all the living species of large herbivore have lost many of their teeth, keeping only those best suited to chewing plants, some of the macrauchaeniids had the full set of 44, although these were clearly adapted for a herbivorous diet.

One of the more distinctive features of the group is the position of their nostrils, which are far up on their skull, not at the tip of the snout, where one would expect to find them. This position became more extreme as the family evolved, so that the bones that would normally form the top of the snout (and in humans form the bridge of the nose) are remarkably short. It has generally been assumed that this means they had a short, flexible trunk, running forward from the nostrils over the face. A very similar pattern is, after all, seen in the skulls of living tapirs, which do have a proboscis of exactly this sort.

Because such a trunk, not being made of bone, has never been found in any fossils, its hard to know for sure that it really existed. It seems likely, but there are at least some other possibilities. For example, nostrils placed far back on the head make it easier for an animal to swim without drowning, which would be useful if they lived in a particularly wet environment.

Although the oldest fossil macrauchaeniids go back further still, it seems that the family really began to take off and diversify between about 29 and 21 million years ago. Yet most of what we know about them comes from more recent specimens, typically less than about 20 million years old, and in particular, from Macrauchaenia itself, for which the group is named. Macrauchaenia is undoubtedly a fascinating animal, and, at around five feet high at the shoulder, one of the larger members of its family. But, having died out just 20,000 years ago, one thing it doesn't tell us is how the family came to be in the first place.

The earlier forms, from the little we know of them, were less specialised than their descendants later became. For example, the nostrils were in a less extreme position, which suggests that the trunk, if present, would have been very short, and resembling more of an enlarged nose than anything else. As is often the case, however, the fossils that we do have are frustratingly incomplete. Its actually quite unusual to have anything approaching a complete skeleton in any fossil except the most recent ones, and the macrauchaeniids are no exception.

In this context, last November an Argentinian fossil was described that, for the first time, includes both a complete skull and some other parts of the skeleton belonging to a macrauchaeniid that dates from the time that we think the family was really beginning to diversify. Skulls are good for identifying fossil mammals, since they tend to be much more distinctive than other bits of the skeleton, although at least one early macrauchaeniid species has been described based on the discovery of part of a foot and the associated ankle. (This may not sound like much, but its frequently all there is to work with, and, at least in this case, the peculiar three-toed feet are fairly identifiable).

This new fossil belongs to a species that's already known, Cramauchenia normalis, although it hadn't been previously known to have existed so early. When I say that there are some other parts of the skeleton that can be matched with the skull, its still not a lot - the right humerus, a small bit of the left foot, and one toe. But that's actually quite a lot more than you often get, and its worth noting that when the species was originally described by Florentino Ameghino back in 1902, all he had to work on was half a crushed skull.

This skull, however, is relatively well preserved. We can tell, for example, that there is nowhere for sizeable muscles to attach to the area around the nostrils. Which means that, whatever the case might be for later species, this one probably didn't have a trunk. The shape of the skull and teeth also tell us that the animal was a browser, feeding on relatively soft leaves and shrubs, rather than tough grasses. While we may not have a complete limb, the shape of the bones we do have indicates that the animal would probably have been good at running, enabling it to escape quickly from predators.

What predators, you might wonder, given that I've already mentioned this animal lived long before the arrival of large mammalian carnivores. Its true that it would never have had to face large cats, wolves, bears, or any of the other creatures that we normally think of as preying on, for example, deer or antelopes - and, with a skull 27cm long, that's roughly the size range we're thinking of. But, strange as some of the mammals of this time might have been, the dominant South American predators were perhaps stranger still.

For it was not lions or sabretooths that Cramauchenia would have needed to flee from, but giant, flightless, terror birds. South America has changed a lot since those days.

[Picture from Wikimedia Commons]

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