Miocene (Pt 30): Horned Armadillos and Marsupial Dogs

Borhyaena

At the dawn of the Miocene, South America had been an island continent for many millions of years, allowing many strange forms of mammal to evolve that were not found elsewhere. Few of these survived the creation of the Central American land bridge, allowing more familiar creatures from the north to replace them. Some of those that did, such as the ground sloths, failed to make it through the Ice Ages, but there are exceptions still around today - descendants of ancient and peculiar animals that first originated in South America.

Among these are the armadillos, a group that was already well-established by the Early Miocene and with many living species, including one that has successfully colonised parts of North America. Miocene armadillos were perhaps more varied than those alive today, largely due to the existence of the glyptodonts. 

Better known from much later examples, these were often giant animals with solid domed shells rather than the flexible banding seen in living armadillos. The group seems to have first evolved all the way back in the Eocene, but the evidence for that largely comes from discoveries of the distinctive hexagonal plates that made up their shells, and the first really good skeletons that we have come from the Early Miocene. One of the earliest is the 18-million-year-old tongue-twister Parapropalaehoplophorus from northern Chile. (The name, incidentally, translates loosely as "the animal that's a bit like the other animal that came before the early example of an armoured creature"). 

This was much smaller than the giant glyptodonts of later times, but still somewhere around 70 cm (2' 3") in length and weighing an estimated 67 kg (147 lbs), similar to some of the smaller giant tortoises of the Galapagos. It already had the solid domed shell, three-lobed teeth, and short jaws that later glyptodonts were known for although the shell, for example, was not quite so complete around the edges as it would later be. Since it lived on what were then high altitude grasslands, it likely fed on low-lying vegetation.

Another armadillo living alongside Parapropalaehoplophorus was, in some respects, even stranger. Peltephilus was about the size of a large armadillo today, weighing around 11 kg (24 lbs). It also had a similar shape, although the shape of its bony scales suggest that the whole body was flexible, rather than having the solid plates that cover the shoulders and hips of most living species. But this isn't what was strange about it; rather it's the fact that it had horns. 

Technically, these weren't horns in the sense that antelopes or cows have them. Instead, they were a pair of bony scales embedded in the skin just like the other armoured plates that armadillos have but, in this case, high and conical and rising just in front of the eyes on the top of the snout. Everything else about the animal's form suggests that, like modern armadillos, it would have been good at digging and the idea that a burrowing animal would have horns is, to be honest, really peculiar. 

They don't seem suited to helping it dig, so presumably they must have been used in defence of some kind, but the details are unclear. For this and other reasons, Peltephilus is no longer considered a close relative of modern armadillos (although it's closer to them than to anything else alive today). Originally thought to have been carnivorous, more recent evidence shows that it most likely fed on tough plant roots, probably along with any worms or insects it happened to come across.

Stegotherium was another peculiar armadillo, living slightly later, during the Middle Miocene about 14 million years ago. More closely related to modern armadillos than either glyptodonts or Peltephilus, it was similar in size to its modern relatives, and the body would have looked fairly similar. However, it had an unusually long snout with far fewer teeth in it than any armadillo would normally have - and the few it did have were near the back of the mouth and were so small that they must have been virtually useless. The only likely explanation for this is that it fed almost entirely on ants, an odd thing for an armadillo to do.

The anteaters proper are another group of peculiar animals that first appeared in South America, although their fossil history is fairly limited. Nonetheless, we do know of Neotamandua, which first appeared at around the same time as Stegotherium but seems to have survived rather longer, perhaps into the Pliocene. It was very similar to the giant anteater of today, but noticeably smaller, somewhere between it and the modern tamandua (which, despite the name, does not seem to be such a close relative). In fact, it's so similar that it may well be a direct ancestor of the giant anteater, and some researchers even place it in the same genus.

Necrolestes was not the sort of animal that a time traveller to Early Miocene South America would have immediately noticed. It was a small, insect-eating animal, highly specialised for burrowing and that probably spent most of its time underground. It probably looked a lot like a mole, although it clearly isn't closely related to true moles which don't live in South America even today. In fact, it's so specialised - having, for example, a third bone in the forearm formed from an ossified tendon alongside the radius and ulna - that, for a long time, it wasn't apparent where it belonged in the mammalian family tree.

As in, it wasn't even clear whether it was a placental mammal or a marsupial relative of opossums. Which, since there were no monotremes left in South America by that time, is about the most basic taxonomic question one could possibly ask. It's only in the last decade that the answer has become clear: it wasn't either. Instead, it was a dryolestoid, a member of a group of mammals that split off from the placental mammals and marsupials before they parted company, and that may well resemble their common ancestor. 

That's significant, not just because it's so distant from any mammal that's alive today, but because prior to this analysis, the dryolestoids were thought to have died out not long after the dinosaurs. Granted, the previous last known survivor was Peligrotherium, which did in fact, live in South America. But that was way back in the Paleocene, a whopping 40 million years before Necrolestes - which is to say, twice as far back from it than it is from us. 

Marsupials have a long history in South America since it was originally from there that they travelled to Australia. A number of early opossums are known from the Early Miocene, along with another strange burrowing animal, Patagonia, traditionally thought to be related to the living shrew opossums but possibly, like Necrolestes, the last survivor of some other group of primitive mammals from the time of the dinosaurs (although, in its case, without such a long gap in the fossil record, since relatives are known from the Eocene).

The sparassodonts, however, were noticeably more dramatic than these small insectivores and gnawing animals. Technically, they are not marsupials, in that they are unlikely to be descended from the last common ancestor of the living forms, but they were members of the same general branch of mammalian evolution - the metatherians - and so are closer to 'true' marsupials than to anything else. They're notable because they were the only large mammalian predators on the continent at the time, long before the ancestors of jaguars and the like would arrive.

The sparassodonts serve as an interesting example of parallel evolution, since they were found in many forms that closely resemble more familiar placental predators. These include a family called the hathliacynids, which may have been analogous to dogs because of their long snouts and rather doglike teeth, although the rest of their body had a rather different shape. 

Acyon is the largest known hathliacynid, perhaps around 70 cm (2' 4") in length, not counting the tail. Most were smaller, with a typical example being Cladosictis, which lived 16 million years ago in Argentina. Both species had comparatively short legs and a build closer to that of a modern pine marten than a dog. They seem to have been active predators, although probably not very fast running, given their flat feet. Cladosictis was not only able to climb trees but had a partially opposable thumb, perhaps to help grip onto branches. The shape of their teeth suggests that they were not pure carnivores although meat would have been a significant part of their diet.

If these animals loosely resembled dogs, the thylacosmilids are often said to have resembled cats. Anachlysictis lived during the Middle Miocene around 12 million years ago and was about the same size as a large bobcat at 18 kg (40 lbs). It had greatly enlarged canine teeth, if not quite to the extent of the most dramatic of true sabretooth cats and was, like cats, likely a hypercarnivorous predator.

Borhyaena was among the largest of the sparassodonts in the Early to Middle Miocene, being about the size of a wolf. Although belonging to a different family, like the hathliacynids it had a somewhat dog-like body with short limbs. However, it would have been much less flexible, with feet adapted for walking along the ground, not climbing, and a heavier build. The jaws resembled those of hyenas, hence the name, although without the heavy bone-cracking molars and it was almost certainly another hypercarnivore. Strong neck and jaw muscles nonetheless likely gave it a powerful bite and, since it doesn't seem to have been able to run fast, it probably ambushed its prey in a cat-like manner, rapidly striking from hiding.

As the climate began to cool again after the sweltering of the Middle Miocene, the diversity of these carnivorous marsupials began to decline. While most of the main groups did survive in at least some form, they seem to have been less numerous, perhaps even starting to cede their predatory niche to other animals, such as crocodiles and birds. At the same time, of course, the herbivorous fauna also changed as the continent headed towards the cooler times of the Pliocene. So next time, I will begin taking a look at what happened during the Late Miocene.

[Photo by "ghedoghedo" from Wikimedia Commons.]