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

Ramoceros

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.

The Time of Horned Rodents

Ceratogaulus hatcheri

The rodents are the most successful order of mammals, with literally thousands of species worldwide. There are a number of reasons for this success, but one of them concerns the arrangement of the jaw muscles, which are modified to improve their ability to gnaw with their powerful, chisel-like teeth.

The muscle in question is the masseter, a muscle that, in humans, runs from the forward part of the cheekbone down to the back of the jaw, and which is important in chewing - it is often particularly large in herbivores, for example. It has two parts, one deep and one superficial, and in rodents, one or both of these parts extend much further forward than normal, reaching up onto the side of the snout. Generally speaking, in squirrel-like rodents, its the superficial part of the muscle that does this, in guinea pigs and their relatives, its the deep part, and in mouse-like rodents, it's both. Because the shape of the skull has to be modified to allow for these attachments, it is possible to tell which of the three possible arrangements even a fossil species had.

But there is one rodent species alive today that doesn't have any of these features. (Actually, mole rats don't have the full muscle arrangement, either, but we can tell from the shape of their skulls that their ancestors once did). That animal is the mountain beaver (Aplodontia rufa), and it's only found along the Pacific Coast of North America. Despite the name, it is not particularly closely related to real beavers, and it doesn't look much like them, either, looking more like some kind of giant vole. Indeed, its a burrowing animal that doesn't like water, and usually only ventures above ground at night.