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| Leptomeryx |
Deposits across the continent show a sudden change in the climate at around the dawn of the Oligocene. By 'sudden' in this context, we mean over the course of hundreds of thousands of years, so it's hardly something you would have noticed had you been there at the time, but it's still rapid as such things go. The exact nature of the changes, and the speed at which they appear to have happened, depend on which part of the continent we're talking about, but nowhere was unaffected.
Take Oregon, for example. At the end of the previous epoch, this was covered in damp, subtropical woodlands, with annual temperatures around 21°C (70°F) and rainfall around 180 cm (70 inches). So, basically, Seattle would have had a climate like the one Tallahassee does today. A million years later, annual mean temperatures had dropped to 12°C (54°F) and the rainfall had halved, both of which are in the ballpark of what we'd expect in the state today. Notably, however, this temperature drop wasn't even, with winters cooling by far more than summer did, so that plants, and the animals that fed on them, not only had to cope with cooler temperatures overall, but a much bigger variation over the course of the year.
Similar patterns are seen elsewhere, with the continent becoming drier as well as colder (compared to the sweltering conditions it had been in before, at any rate). Some of the evidence for this comes from fossil leaf impressions, showing how denser, moist, forests gave way to drier open woodland across much of the continent, with a shift in the species of tree found within them. A decline in the number of fossil freshwater turtles, crocodiles, and salamanders also reflects the drier conditions... but, surprisingly, the changes among mammal species seem to have been comparatively mild.
There was, for example, less change among rodents than we might expect, and certainly less change than occurred in Europe. Agnotocastor, for example, an early beaver, seems to have made it through the transition just fine. It's both the oldest and most primitive beaver known and may represent the ancestral stock from which all later beavers evolved - although it's sufficiently similar to the Oligocene Eurasian Propaleocastor that it's not immediately clear on which of the two continents beavers first appeared.
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| Devil's corkscrew - note the fossil of its excavator |
By the late Oligocene, Agnotocastor had been replaced by Palaeocastor and related "burrowing beavers". Smaller than modern beavers, these likely spent little, if any, time in rivers, instead being adapted for digging deep burrows in which to shelter, and may have physically resembled a prairie dog. Initially scratch-diggers, they later evolved towards a more efficient use of chisel-like teeth to excavate soil and left no modern descendants, dying out in the early Miocene. They are best known for the fossil remains of the burrows, tight helical tunnels descending into the earth for up to 3 metres (10 feet) and now formed into bizarre rocky shapes known as "devil's corkscrews". Since the remains of potential predators have occasionally been found inside the burrows, they can't have been very effective as a means of defence and were more likely a protection against cold weather.
Palaeolagus was another small mammal that made it through the transition, and one that lasted until at least the end of the Oligocene. Originally thought to be an early rabbit, judging from the number of fossils found this must have been a common and widespread animal. More recent analysis has shown that, while it is a lagomorph, its skeleton has a mix of features of the two living lagomorph families, placing it as neither a rabbit nor a pika, but a member of a now-extinct family that had, perhaps, changed little from their last common ancestor. The internal shape of the skull reveals that the parts of the brain related to the sense of smell were proportionately larger than in modern lagomorphs, while those of its close relative Megalagus suggest that, while we'll likely never know whether it had rabbit-like ears, it had already evolved keen hearing. At least some of its other close relatives were adapted for running across open ground, rather than hopping.
Camels likely began to diversify towards the end of the previous epoch, but they seem to have become a more significant part of North American wildlife during the Oligocene. The best-known Oligocene camel is Poebrotherium, although usually considered distinct from both true camels and the llamas and their relatives. Indeed, those two groups did not appear until much later, perhaps not long before one left their original home continent for Asia, and the other for South America.
Although Poebrotherium was not an especially close relative of the modern species, it had taken a significant step in their direction by developing the two-toed cloven foot, as well as becoming much larger than its ancestors, reaching about the size of a sheep or a gazelle. While it probably looked quite llama-like in other respects, a gazelle is a good analogy, since Poebrotherium's long legs would have made it a fast runner - possibly the fastest animal on the continent at the time. Although the scientific name literally translates as "grass-eating beast", studies of the shape of the teeth suggest that grass was a relatively minor component of its diet, although whether it was a dedicated browser on leafy shrubs or more of a mixed feeder is uncertain.
While Poebrotherium survived throughout the whole of the Oligocene, it was joined by other species of 'camel' later on. Paratylopus is perhaps the most significant example, notable for being the first to have the long neck seen in modern species. Others include Miotylopus, another gazelle-sized animal, but one that probably did feed on grass. None of these, at this point, inhabited the desert or mountain habitats of the modern species.
As in Europe, the only true ruminants of the time in North America were small hornless animals loosely resembling musk deer. The exact relationship of these animals to other ruminants is unclear; they may well represent a side branch with no close modern relatives. Hypertragulus is a typical example, with most other primitive North American ruminants being placed in its family. They were small creatures, only weighing around 3-4 kg (7-9 lbs) and had several features that place them amongst the most primitive of all ruminant animals.
Many of these features are highly technical, but they include a relatively short snout with a tusklike premolar tooth in the lower jaw separated from the teeth to either side by a distinct gap. They still had the full set of five toes on the front feet, although those at the sides were reduced so that most of the weight would have been taken on those forming the modern "cloven hoof"; the hind feet had just four toes each and closer to what we would see today in living ruminants such as deer.
Once included in the same family, Leptomeryx is now split off into its own group but would, in life, have looked superficially quite similar. Distinguishing features include the lack of tusks - indeed, the equivalent tooth is unusually small and vestigial - and the presence of only four toes on the front feet. It's another animal that survived from earlier times, although the species do become smaller after the climatic shift of the Early Miocene. Although less diverse than the hypertragulids, the leptomerycids as a whole survived longer, making it into the Miocene before Leptomeryx's last descendant died out.
Leptomeryx species also developed more complex teeth, with a larger grinding surface, in the Oligocene compared with those of their earliest forms, perhaps because the drier climate favoured tougher plants. Nonetheless, compared with modern ruminants, the teeth of both hypertragulids and lepotmerycids were primitive and do not seem as suited to a purely herbivorous diet as we might expect. They likely fed on soft leaves, some fruit, and perhaps even insects. One study based on isotope analysis suggests that, despite their skeletal affinities to modern ruminants, they had not yet developed the full ruminating digestive system and would likely have been less efficient at processing tough plant matter than their later relatives.
These animals were unique to North America, although Leptomeryx is similar enough to an animal living in India at the time that the latter was once considered synonymous, suggesting that its ancestors may have been Asian. However, they were not the only cloven-footed mammals in North America at the time, since the continent was also home to others that were either unique or that had close relatives elsewhere. Next time, I will be looking at some of those.
[Photos by "ghedoghedo" and "inazakira", from Wikimedia Commons.]
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