Sunday 4 August 2019

Miocene (Pt 15): The Coming of the Rain Shadow

As the world passed into the Late Miocene, worldwide temperatures dipped from the high summer of the Middle Miocene towards... well, if not autumn, perhaps a late summer, since the world was still overall, probably warmer than it is today. For North America, however, perhaps a more significant effect was caused by the slow shuffling of the continents. A large chunk of rock that corresponds, broadly speaking, to present-day California began to tilt, causing the eastern edge to rise up.

We're not talking a huge tilt, of the sort that you'd notice if you were standing on it - it's been estimated that the eastern end was rising by somewhere around 0.25 mm (1/1000 of an inch) per year relative to the western one. Give it a few million years though, and that starts to be noticeable. While this process really only accelerated during the subsequent, Pliocene, epoch, even at this time, the rise of the eastern end of that block of crust was slowly, but surely, creating what we now call the Sierra Nevada Mountains.

It may not be a coincidence that, as this happening, there was a change in the sort of animals that lived on the continent. In particular, there is a shift towards herbivores that appear to have survived on tougher food than their predecessors. During the Middle Miocene, many herbivorous mammals in North America show every sign of living in a fairly lush, forested environment, with lots of soft, juicy leaves to browse on. Not so much thereafter.

To be fair, the Sierra Nevada were nowhere near as impressive as today even by the end of the epoch (they mostly formed during the Pliocene) and, the Rockies, for example, already existed. But an increase in the rain shadow is one possible reason for a drier climate that would, in due course, lead to the open prairies of the Great Plains. The true grasslands may not have appeared until the Late Miocene was nearly over, about 7 million years ago, but there was already plenty of tough grass around, and it seems to have become more common in the Late Miocene, perhaps mixed in with thick scrub.

A number of the many rodents that had prospered during warmer times now died off, being replaced by animals more closely resembling modern mice. These new arrivals included animals such as Pliotomodon, which looked rather like today's pack rats, and seems to have had a similar diet. If that's what it is, it would be the earliest known pack rat, having lived around 7 million years ago, although there is some suggestion that it's merely a close relative, with true pack rats appearing closer to the dawn of the Pliocene a couple of million years later.

The success of these animals inevitably came at the expense of older forms. The primitive, squirrel-like eomyids all but died out at this time, although they survived rather longer in Europe. Aplodontids, which are also related to squirrels, also suffered a considerable decline, although one narrow lineage did survive, and is the ancestor of the living "mountain beaver" or sewellel.

A third group of rodents, the mylagaulids, had also been numerous during the Middle Miocene, and were much less so thereafter. In their case, however, the survivors evolved into something rather more specialised. Most of the skeletons we have of these animals are incomplete, and those that do survive show that they had an unremarkable body, but a solid-looking head, of the sort seen in modern rodent species that spend a lot of time digging. In the Late Miocene, the digging adaptations become more apparent, and it's likely that these few late mylagaulids, such as Ceratogaulus, looked rather like gophers.

Well, apart from the fact that they had horns.

Gophers are not known for their horns, and neither, quite frankly, is anything else alive today that spends most of its life down a hole. You would, after all, rather think that the horns would get in the way. But, for a long time, it was thought that the opposite might have been the case. Ceratogaulus had two horns, which were short and blunt, and located, not on its forehead, but on the end of its snout. The idea was that perhaps these were used for digging, shovelling earth out of the way as the animal burrowed with its head.

An analysis of the shape of the skull and how muscles would have been attached to it, however, makes this seem unlikely, not least because later species have the horns positioned further back on the nose, suggesting that they would have become less useful for digging as the animal evolved. Burrows thought to have been dug by mylagaulids, which do resemble gopher burrows in their overall shape, also have scratch marks that suggest that the animals dug with their claws, rather like moles. While it has to be said that the likely culprit isn't (so far as we know from the remains we have) one of the horned species, it's unlikely that close would evolve an entirely different way of doing the same thing.

Which rather implies that they were there for something else. It's not entirely clear what this might be, although a number of possibilities present themselves. The most current theory seems to be that they were defensive weapons, presumably used on those occasions when the animal had to head above ground, exposing itself to danger.

Of course, larger animals were affected by the change in the climate and vegetation, too. The three-horned dromomerycids, thought to be related to modern deer, also suffered a dramatic decline. Only a small number managed to survive through the Late Miocene, since the great majority seem to have been adapted to verdant forests, rather than scrubby grassland. While there were probably others, only two seem to have been particularly numerous, and both show wear on their teeth that suggests a tougher diet than their ancestors; of the two Pediomeryx was much larger, with a particularly impressive 'third' horn, rising from the back of its skull. It was virtually a grazing animal, although its close relative Cranioceras, with a more mixed diet, may have survived just as long as it did, dying out at the dawn of the Pliocene.

The drier climate also saw the end of the North American musk deer, which had never been very numerous. Another group that did survive, however, were the protoceratids. Like the dromomerycids, these were vaguely antelope or deer-like animals, with three horns on their heads - although, in their case, the 'extra' horn was on the snout, rather than at the back. While they had never been terribly numerous, they became much less so after the end of the Middle Miocene, with just genera surviving.

The more famous of these is probably Synthetoceras, which was not only unusually large (the size of a big deer) but had particularly dramatic horns. The two horns on the forehead were curved, pointing slightly inwards, but the horn on the snout was much longer, projecting straight up before dividing into a sharp fork at the tip. The last protoceratid, however, was Kyptoceras, a slightly smaller animal with a pair of tall semi-circular horns on the forehead that point forward and a nose-horn that divided in two right at the base. It was the only protoceratid we know of that seemed to have any ability to eat tough vegetation; even so, the fact that it was found only around the Gulf of Mexico suggests that it sought refuge in the semi-tropical forests there, away from the harsher plains of the interior.

When the climate cooled further in the Pliocene, even that was not enough, and it died out, the last of its kind.

Where these browsing animals declined, however, others prospered. The antilocaprids, a group of antelope-like animals native to North America, but related more to giraffes rather than to true antelopes, were also relatively numerous during the Miocene. The drier climate of the continental interior seems to have presented them less of a problem although, in later epochs, they would almost entirely die out leaving just one species still alive today - the pronghorn.

The living pronghorn is known for its unusual horns, which have a distinct spur of a kind never seen on any genuine antelopes. The extinct forms, however, were rather more dramatic. Among the Late Miocene forms, Osbornoceros was perhaps the least impressive, if only in comparison to its kin, It had unbranching horns that nevertheless twisted around on themselves by 180 degrees as they swept back from the head; the living nyala antelope is perhaps the closest analogy today.

Ramoceros had perhaps the most branched horns of any antilocaprid, with two long spurs projecting forward from the main core. The result was that the animal must have looked rather like a miniature deer, although its cranial adornments would likely have had a horny sheath of some kind, rather than being bare bone, as antlers are. Arguably most bizarre of all was Ilingoceros, with 30cm (1 foot) horns sticking straight up and twisting round in a tight corkscrew spiral before terminating, possibly in a tiny double-spike.

In all of these animals, the females either lacked horns altogether or had ones that were far less impressive; presumably, the males used them to fight one another, although at least some must have also been useful in defence.

Of course, these were not the only herbivores to benefit from the change in climate, and next time, I will look at some of the others...

[Photo by Ryan Somma.]

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