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| Arsinoitherium |
Monkeys first appeared in Africa in the latter half of the preceding epoch, but their early record is patchy. This is likely because many of the earliest monkeys lived in areas that simply weren't conducive to forming fossils - although the fact that many parts of Africa have not had the same detailed paleontological surveys that other continents have may also be a factor. Much of the history of the primates during the epoch is a blank... but not entirely.
As it happens, the best-known fossil monkeys from Oligocene Africa date right from the beginning of the epoch. Propliopithecus and its close relative Aegyptopithecus lived in what is now Egypt. They were fairly large for tree-dwelling monkeys; at around 6 or 7 kg (13 to 15 lbs) they were about the size of a modern gibbon although they likely walked on all fours, rather than swinging through the trees. Indeed, some features of their skeleton led early researchers to conclude that they might be ancestors of apes specifically, rather than of the other kinds of African monkey.
It turns out, however, that these features are almost certainly primitive for monkeys, and simply were not lost when some of them later evolved into apes. Indeed, it's most likely that Propliopithecus and its kin represent an early branch that includes the ancestors of both apes and Old World monkeys. Since we have some relatively intact skulls of Aegyptopithecus, we can see that its brain was smaller in comparison with its body than is typical of monkeys, more closely resembling that of a lemur. The snout and accompanying olfactory lobes were smaller than in lemurs, however, indicating that the animal would have looked, and probably acted, more like a monkey. Significantly, there is evidence that males were significantly larger than females, and had larger canine teeth; these may suggest that they competed for mates, maintaining a harem-like social structure.
That these were not the only kinds of monkey living in the area at the time suggests that there had already been considerable diversification within the larger group during the preceding epoch. Apidium seems to have been even more common, at least judging from the number of fossils we have found, and is the best-known member of a different family, the parapithecids. The placement of that family is, perhaps, even less clear than that of the propliopithecids.
This is because Apidium has some skeletal features in common, not just with Old World monkeys but with the South American sort. Various proposals have been made to explain this. They might, for example, genuinely be related to New World monkeys, representing a group that stayed behind in Africa. Or it could be parallel evolution, with Apidium adopting a lifestyle similar to that of modern squirrel monkeys without necessarily being a close relative. Perhaps the most likely explanation, however, is that they are descended from the common ancestor of both Old World and New World species, a distinct evolutionary line that continued on into the early Oligocene before petering out.
Analysis of the teeth of these monkeys suggests that all were primarily fruit-eaters. There are, however differences, perhaps explaining how they could live alongside one another. Apidium, for example, seems to have fed primarily on softer fruit like berries, while Aegyptopithecus favoured harder seeds. Parapithecus, a close relative of Apidium, while still primarily fruit-eating, likely ate a higher proportion of leaves.
It's after this, however, that the gap begins. While many of the above fossils were discovered early in the 20th century, it would be almost a hundred years before we discovered any fossil monkeys dating from any later in the Oligocene. However, while the record remains sparse, it's no longer entirely empty. In 2010, a fossil monkey from Saudi Arabia was named as Saadanius. Dating to around 29 million years ago, it easily predates the creation of the Red Sea or the closure of the straits between the Mediterranean and the Persian Gulf, so what is now Arabia was very much a part of Africa at the time. It was about the size of a gibbon, but, like its predecessors, had a longer face and may have relied more on its sense of smell than modern apes and monkeys do.
Saadanius is significant because, like all living Old World monkeys and apes, but unlike the earlier Aegyptopithecus, the bony structure that would have held the eardrum in place had a tubular, rather than ring-like, structure. Because it lacked any other features that would have definitively confirmed it as either a member of the modern family of Old World monkeys or as an ape, it is likely related to their common ancestor. (The original description inferred that it lived before that ancestor, but, while plausible, that doesn't necessarily follow).
In 2013, we were able to add two monkey species from the late Oligocene of Africa, about 25 million years ago. Nsungwepithecus and Rukwapithecus are both from Tanzania, but their remains are too fragmentary to say much about them. Even so, the former has been interpreted as a close relative of the common ancestor of all living Old World monkeys, while the latter is thought to be closer to apes (although outside either of the living ape families). Assuming this is correct, it still doesn't fill in the blanks in this part of simian fossil history - but at least it's a start.
Monkeys and apes are not, of course, the only primates. However, there is essentially no fossil record of lemurs, due to the paucity of geologic deposits of the right age on Madagascar. The closest we can get perhaps are a couple of primates from Oman that date to about the same age as the Egyptian monkeys. Omanodon and Shizarodon lacked the distinctive tooth-comb that both lemurs and the Asian lorises possess, so they can't belong to any group that is alive today. Nonetheless, despite some similarities to early monkeys, they are generally regarded as more closely related to lemurs and lorises than anything else alive today - they may belong to a group called the adapiforms that is better known from later times in Asia, although even this isn't certain. Whatever they were, they were small, weighing no more than 200g (7 oz.) at most.
At the other end of the scale, however, the same Egyptian deposits that revealed the early monkeys also host the remains of a gigantic herbivore so distinctive that it cannot be fitted into any order of mammals alive today. Arsinoitherium was the last and largest of the embrithopods, a group of herbivorous animals that had been more diverse in the preceding epoch, and whose wider affinities are unclear.
The general assumption has been that they must be related, if distantly, to the elephants, hyraxes, and other mammalian groups originally native to Africa. While even this has been questioned, the debate has tended to focus more narrowly on where they fit within this large array, with the modern consensus coming down in favour of them being more closely related to elephants and manatees than to hyraxes. Even so, they probably diverged even before the elephants and manatees had separated... which, as you might imagine, means very early indeed.
Physically, Arsinoitherium would have looked something like a rhinoceros to modern eyes, being a large heavily-built animal with conical horns on its head. In fact, it was a similar size to the largest rhinos of today, standing about 1.75 metres (5' 8") at the shoulder, and weighing perhaps as much as 2.5 tons. Rhinoceroses, are however, very distant relatives indeed. For example, a closer examination of the legs shows that arsinoitheres had stocky five-toed feet that resembled those of an elephant, not a rhino. Furthermore, the huge nasal horns are not only arranged side-by-side but they are made of bone, which those of a rhino are not. It's even possible that, in life, they were covered by a sheath of true horn, just like those of cows, goats, and antelopes.
The teeth also have several peculiarities in their microstructure, and the animal had a full set of them, which certainly can't be said of elephants (or rhinos). Taken together with an unusual skeletal arrangement of the jaws, this is thought to indicate that they were highly specialised browsers with a narrow diet - although comprised of what is unclear. For a long time known only from Egypt, we now know of others from Tunisia and Ethiopia, among other places, as well as having earlier relatives elsewhere, casting doubt as to where the embrithopods as a whole first originated. The last of them vanished, leaving no descendants, well before the end of the Oligocene.
These were not, however, the only mammalian survivors from the previous epoch that cannot be placed within any of the living orders. Significantly, the carnivorans, the group that includes cats, dogs, hyenas, and so forth, had yet to enter the continent, still being restricted to the northern landmasses. So what meat-eating mammals were there in Africa during the Oligocene? Next time, I will take a look...
[Photo by Aram Dulyan, from Wikimedia Commons.]
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