Sunday 3 October 2010

Ground Sloths and the Size of Fossils

The sloths are among the stranger groups of mammals, belonging to a lineage that split off from all other placental mammals probably even before the dinosaurs went extinct. They originated in South America, a continent that was, for a long time, as isolated as Australia is today. Just as marsupials survived in Australia, so a number of strange and early placentals survived in South America (as did some marsupials, come to that). Of course, South America eventually joined itself up to North America, and a great number of odd animals went extinct as more familiar forms crossed the Panama land bridge heading south. The sloths, however, survived - or, at least, some of them did.

Today, there are only six living species of sloth, all of which are tree-dwelling animals. Their ancestors lived not in trees, but on the ground, and many of them were much, much, bigger than those that live today. The last of these giant ground sloths died out remarkably recently, perhaps around 9000 BC, meaning that they must have lived alongside humans for thousands of years.

Perhaps the best known ground sloth is the largest one, Megatherium, which reached the size of an elephant. But there were a number of others, many of which were still quite substantial. In zoological terms, the ground sloths aren't even a real group; this is because they just represent those sloths that didn't happen to head up into the trees, and they aren't all directly related. Instead, the ground sloths form a number of different families, with at least some being more closely related to one or other of the families of living tree sloth than to each other.

2-toed  Megatherium      Mylodon       3-toed
sloths      etc.           etc.        sloths
  ^          ^              ^             ^
  |          |              |             |         Anteaters
  |          |              |             |
  ------------              |             |             ^
       |                    |             |             |
       |                    |             |             |
       ----------------------             |             |
                   |                      |             |
                   |                      |             |
                   ------------------------             |
                                    |                   |
                                    |                   |

Mylodon is a genus of ground sloth, whose members were more closely related to the 2-toed than to the 3-toed sloths. Exactly how many species belong to the genus isn't really clear, and only one, Mylodon darwini, seems to be undisputed. As tree sloths go, its not particularly well known - even the image accompanying this article is of an animal now placed in entirely different genus, Glossotherium. Mylodon lived from central Argentina and Chile as far north as Bolivia, first appearing around one million years ago, and dying out roughly with the end of the last Ice Age around 10,000 BC. In life, it probably weighed around 200 kg or so (over 400 lbs), around the size of a grizzly bear.

A recent report of a fossil Mylodon discovered near the city of Concordia in Argentina adds further to our knowledge of this animal. Interestingly, the specimen belongs to a smaller, slimmer, individual than those seen before, especially in this region. Assuming, as seems plausible, that the fossil really does belong to the same species as the other, larger ones, why might it be smaller?

Perhaps the most obvious reason is that it isn't an adult. There is no reason why young animals shouldn't sometimes fossilise, and failing to notice this fact could easily lead one to make the wrong conclusions. (There is, for example, an ongoing debate of this sort about Nanotyrannus, which is either a midget relative of Tyrannosaurus rex, or just a very young one). In this case, a the fossil of a juvenile Mylodon is already known. We know that that fossil is a juvenile because the bones of its limbs had not yet fully fused when it died, indicating that they were still growing. The limbs of the new specimen were not preserved, but the skull does appear significantly more solid and well developed than in the known juvenile, implying that it probably was an adult.

A second possibility is that it isn't the same sex as the other fossils. In living sloths, the females tend to be larger than the males, so it could just be that this one is a male, and the others happen to be female. This is possible, but is fairly difficult to determine from just looking at the bones. The authors who described the fossil consider it unlikely, largely because known Mylodon fossils don't fall easily into two different sizes, although that wouldn't necessarily be the case if the biggest males were larger than the smallest females. They also dismiss quite perfunctorily the possibility that the animal in question just hadn't eaten very well during its life, which would seem (to me, at least) an entirely plausible explanation.

We must also remember that animals do change over geological time. There is some evidence that, at least until the ground sloths died out, sloths as a whole were getting larger over the course of their evolution. This could well happen even within a single species, as they slowly evolve, without actually losing the ability to breed with one another. Unfortunately, there aren't really enough specimens overall to rule this possibility in or out.

So, even aside from "well, maybe it was a just a small one" (because, surely, they weren't all the same size even as adults), there are a great number of reasons why a fossil might differ from others of its own species. All of these sorts of things have to be taken into consideration when trying to analyse a particular specimen and determine what it tells us. But there is something else that the fossil can tell us about its species, and that's due, not to the fossil itself, but to the geologic environment around it.

The animal that left this particular fossil died around 80,000 years ago (give or take a few millennia), at a time when the Argentinian climate was different than it is today. A number of plant fossils were found in the same bed, including chunks of fossilised wood. Taken together, these indicate that the terrain at the time was forested, with cashew and acacia dominating, and also a significant number of palm trees. This would indicate that the climate must have been fairly warm and damp, and the structure of the rocks also suggests that the specific area where the animal died had a number of rivers or lagoons - probably tributaries of the Uruguay River. (This sort of thing, as you might imagine, is rather easier to tell with deposits just 80,000 years old than with those that are several million).

Furthermore, there were fossils of South American tapirs discovered nearby. This species is still alive today, and we know that it likes tropical and semitropical forests, and wouldn't venture out onto the open pampas that marks the region today. All of this makes sense, since we know from other evidence that the Earth was in one its warm periods at the time, in between the Ice Ages.

So far so good... except that other fossils from this species have been found in sites that imply a cold, relatively dry environment. They appear, on the whole to have grazed on open grasslands, rather than foraging among palm trees. A sample of fossilised dung from Chile that had (apparently) been left by a Mylodon showed that its last dinner consisted of sedges and grasses, supporting the theory of the sorts of terrain they usually lived in.

So, presumably, they were pretty adaptable creatures, certainly more so than had previously been thought. Normally, as the Ice Ages came and went, animals moved to warmer habitats, descending from mountains or migrating towards the equator, and then moving back again when things warmed up. From what we know of ground sloths, they don't seem to have done this much - which might suggest that they simply adapted to eat whatever was around them rather than taking the trouble to wander elsewhere.

Oh, and there is a twist: Bergmann's Rule. This piece of nineteenth century zoological lore states that closely related animals will tend to be smaller the warmer the environment they live in. For instance, polar bears are, on average, bigger than grizzlies, which are quite a lot larger than tropical bears. It suffers somewhat from the fact that nobody can really explain why this should be so - and perhaps more seriously from the fact that it may well be complete bollocks anyway. But, well, you never know...

[Picture from Cladogram adapted from Gaudin 2004.]

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