Sunday, 20 February 2022

Miocene (Pt 31): Terror Mice and the First Howler Monkeys

As the heat of the Middle Miocene gave way to the more moderate temperatures of the Late Miocene, grasslands grew across much of the Northern Hemisphere. In South America, however, the changes were, perhaps, less significant due, in part to its more equatorial position. Nonetheless, even aside from the general worldwide cooling trend, the South American climate was changing as the Andes continued to rise, affecting weather patterns across the continent. It was also continuing to edge closer to North America, so that, towards the end of the epoch, it became possible for a few animals to make the crossing using the islands of what is now Central America as stepping stones.

Otherwise, the story of Late Miocene South America was more one of gradual change than sudden upheaval, as the animals already living on the continent continued to evolve. It seems to have been a particularly good time for rodents, with a number of rodent groups making their first appearance in the fossil record at this time. Among the most notable of these were the tuco-tucos. These are burrowing rodents somewhat resembling the gophers of North America, but that are, instead, more closely related to guinea pigs. The oldest known tuco-tuco is probably Chasichomys, which lived in Argentina around 9 million years ago, although there is some evidence that the group itself is a good deal older than this, but didn't happen to live in good fossil-forming sites until the rise of the Andes changed the river patterns in the area.

The capybaras are the largest rodents alive today, with adults weighing a whopping 65 kg (140 lbs) and standing up to 60 cm (2 feet) at the shoulder. Although only two species remain, a number of fossil ones also existed, and they too, first appeared in the Late Miocene; the oldest known species is Cardiatherium, which lived in Argentina alongside the early tuco-tucos and perhaps not long after its particular group split from that of the guinea pigs. Capybaras soon spread north, with a slightly later example, Caviodon, being known from across the continent, including Brazil and even Venezuela.

Telicomys, however, would have been noticeably larger than even a modern capybara. It was a member of a large group of rodents of which only one example survives today, the 15 kg (33 lb) pacarana. It may have weighed as much as 100 kg (220 lbs) and had teeth and jaws that suggest a surprisingly strong bite, perhaps to fight off predators. 

But even it was not the largest rodent of its day, because that honour (so far as we know) goes to the mighty Phoberomys, the name of which roughly translates as "terrifying mouse". Sometimes classified in the same family as Telicomys, it's more usually placed in a separate but closely-related family, the fully-extinct neoepiblemids. Both are thought to be branches of the chinchilla-like rodents although the exact relationships between them are not yet fully clear. 

Whatever it was, Phoberomys was gigantic. Exactly how much it weighed is difficult to judge, since it was so far outside the range of any living rodent that the usual formulae for deriving rodent weight from the size of fossil bones won't work. But it was somewhere around the size of a bear, and estimates range from 220 to 700 kg (485 to 1,500 lbs). Its strong, grinding teeth, suggest that it may have fed on grasses or similarly tough vegetation and it is thought, like the capybara, to have spent a lot of time swimming in rivers and lakes.

Primates also seem to have been diversifying at the time, although the fossil record from this particular time period is quite poor for South American monkeys. The oldest definitively identified fossil monkey from Brazil is Solimoea (there are some older teeth, but it's not clear what they belonged to). This was about the size of a modern spider monkey, and was originally thought to be a close relative. More recent analyses, however, suggest that it might be a very early howler monkey. The shape of its teeth suggests that, like its living relatives, it fed mainly on soft food.

Apart from giant rodents, the herbivorous mammals of Late Miocene South America continued to include two of the main groups of mostly three-toed hoofed animals, the litopterns and the notoungulates, both of which would eventually make it through to the end of the Ice Ages before finally going extinct. 

One of the more famous litopterns, the rhino-sized Macrauchenia, is first identified from the Late Miocene, although it's better known from the following, Pliocene epoch. Huayqueriana is a more distinctly Late Miocene form, and was a close relative, with many similar features. Probably about the size of a small horse, with an estimated body weight of around 250 kg (550 lbs), it also had a similar build. A 2016 study of a particularly well-preserved skull showed that the teeth were unusually worn down, considering that other features showed that the animal was not especially elderly; this would suggest that it ate a diet of tough vegetation.

Like its earlier relative Theosodon, however, a more distinctive feature of Huayqueriana is the unusual position of the nostrils, which were even further back on its head than they were on that animal. This has traditionally been interpreted as indicating that both it and Macrauchenia had a proboscis of some kind, a small trunk at least as well developed as that of modern tapirs, and possibly more so. Although the same might not be true of its more famous relative, however, some of the features of Huayqueriana's skull don't seem to fit with the idea of a fully mobile proboscis. The bony nostrils may instead have opened up into an inflated chamber lined by soft tissue on top of the nose, more closely resembling the snout of a saiga antelope, or even a moose, than it would have that of a tapir.

While the litopterns had once been more diverse, just two subgroups continued to survive into the Late Miocene. The last surviving of the other groups, the adianthids, had died out in the Middle Miocene, and we know very little about them, beyond the fact that they seem to have been fast-running animals. While the macrauchenids were more heavily built, the other surviving group were the prototheres, which were, like the departed adianthids, slender animals that seem to have been some adaptations for running (presumably from predators).

Neobrachytherium and Diadiaphorus were typical Late Miocene examples, and would have had a vague resemblance to medium-sized deer, although obviously without the antlers. Unlike some earlier prototheres, they retained the usual three-hoofed foot, although the two side toes were so small that they would not have reached the ground, rendering them functionally single-toed like a horse. Like deer, they lived in forests and likely had a similar diet, feeding on leaves and twigs

Like other prototheres, but quite unlike deer, they had two pairs of tusks, one each in the upper and lower jaw, and placed at the front of the mouth, separated from the other teeth by a long gap. (Some deer do have tusks, but these are canine teeth, not incisors as they were in prototheres, and they are only present in the upper jaw). These tusks were kept sharp by the two pairs being constantly honed against each other and its possible that, in at least some species, they were larger in males suggesting that they may have been used in competing for mates as well as for biting off twigs or the like.

Some of the notoungulates of the time were, however, at least as impressive as the likes of Macrauchenia and Phoberomys, and it is to them, and some of the other large herbivores of the time that I will turn next...

[Picture by A.C. Tatarinov, from Wikimedia Commons.]

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