Saturday 29 March 2014

The Antlers of Early Deer

Dromomeryx, a palaeomerycid

Probably the most distinctive thing about deer is that they have antlers (and not horns). However, especially when we're looking at fossil species, it's important to remember that not all deer have antlers. Granted, the enormous majority do, and those that don't have lost them during their evolutionary history, rather than being holdovers from some ancient form that never had antlers in the first place. (This isn't true of musk deer, but that's partly why they aren't considered to be true members of the deer family).

And even that's assuming your fossil belongs to a male. (Or a female reindeer, oddly enough).

Antlers first appear on deer fossils in the early Miocene, on the general order of 20 million years ago. Still, in the grand scheme of the Age of Mammals, which has so far lasted 66 million years, that isn't all that long. It's a lot closer to us than it is to the dinosaurs, at any rate. So we ought to have a reasonable idea of what some of these first antlered forms look like.

And, indeed, we do, although we immediately run into the problem of where we'd like to draw the line between deer and not-deer. The immediate ancestors of both deer and musk deer are most likely a group of animals called the palaeomerycids. As in so many other cases, these animals didn't all die out as soon as true deer appeared - there's always going to be some close relatives that hang around for a while before being replaced. So we know that both deer and paleomerycids lived at the same time, and the early Miocene is slap in the middle of that. So sometimes its hard to tell which group a 'primitive' deer-like animal from the time comes from.

Lagomeryx and its close relatives are a perfect example of this: they're sometimes said to be palaeomerycids, sometimes deer, and sometimes neither. For the purposes of this post, I'm going to assume that they're deer, if only because it's a shorter word, but it's worth bearing in mind that that's not necessarily the case.

Although the exact relationship is unclear, this particular group of animals seems to possess three known genera, with at least five known species. Although Lagomeryx itself was the first to be described, back in 1904, we know rather less about it than we do some of the others, such as Stephanocemas. Now, however, a new species of Lagomeryx has been described, and the fossil has rather better preserved antlers than the previously known ones, making it easier to trace the evolution of those appendages.

Antlers consist of two parts. The "pedicle" is the base part, and it is permanently attached to the skull. The remainder, or "deciduous antler" is shed and regrown every year, and we assume that this was generally true of extinct species as well. The shape of this part of the antler varies significantly between living species, but there are generally three broad patterns. The most familiar is the branching antler that we see in red deer, elk, fallow deer, and so on. Some of the smaller species, such as muntjacs, have unbranched antlers, that more closely resemble a true horn in shape. Finally, there's the "palmate" form, where the initial stalk flares out into a flattened sheet, with multiple prongs around the edge. This is, among living species, typical of moose.

Antler of a European species of
The antlers of Lagomeryx don't quite fit any of these patterns, although they're perhaps closest to moose. The base of the antler rises as a long stalk, at the tip of which sprout a cluster of very short upward-pointing spikes. There were about six spikes on the most complete antler, but that only includes about two-thirds of the cluster, so we can infer that there might have been as many as nine. Since antlers basically are just bone, this must have been what the animal looked like in life - there's no outer, non-bony sheath that might have extended the structure, as there might be in, say, an antelope.

With this information, it's possible to construct a family tree showing how this animal and it's relatives evolved. With the large number of spikes on the tips of its antlers, we'd expect this to be a relatively late form, having evolved from earlier types with a less complex structure. Which is exactly what we do find.

This new species comes from Vietnam, and appears to be about 13 million years old. The other species known all have simpler antlers, with the simplest belonging to species with just two or three spikes on their antlers (and, to modern eyes, a less odd shape). Crucially, these species with the simplest antlers are older, up to 20 million years in age, and physically smaller than the new species, L. manai. Moreover, they're European - mainly from France and Spain.

So what this shows us is that the group first appeared in Europe, and then grew larger, and developed more complex antlers as it moved eastward, firstly through China, where intermediate forms are found, and then down into Southeast Asia, where we find this relatively late species with the odd antlers, like miniature moose antlers on a long stalk.

But it's not just the antlers that can give us information about this animal. By analysing its teeth, both in the way that they're worn down, and by the precise distribution of oxygen and carbon isotopes within them, we can also get some clues as to how the animals actually lived. For example, the great majority of plants use a type of photosynthesis called the C3 pathway. But a few, most notably grasses, use the more complex C4 pathway. This means that it's possible to tell whether an animal has eaten a lot of grass, even if all you have left are the bones. (It's not perfect, of course; there are other C4 plants, such as cabbages, and some grasses are C3, but it's a pretty good indicator).

From this, and other indicators, the researchers studying these fossils concluded that this primitive deer lived and browsed in relatively open woodland, probably in a climate subject to periodic droughts. In contrast, older fossils from Europe have indicated that they, like many modern deer, lived in denser forests. As time went on, later deer in the area began to adapt to grasslands, especially during the Pleistocene Ice Ages.

Their ability to do so may have been a continuation of the changes they, or animals like them, had already undergone since the move out of Europe.

[Photo by "ghedoghedo" from Wikimedia Commons. Drawing by Rober Bruce Horsfall, copyright expired]

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