Sunday, 26 August 2018

Miocene (Pt 9): The First Sabretooths in Europe

When hyenas first evolved, they were small, tree-dwelling animals, searching for rodents, birds, and the occasional insect among the branches. By the Late Miocene, they had already begun to come down from the trees, but many were still small, and resembled a mongoose more than a modern hyena. A few, such as Thalissictis, were larger, an early step on the path to the more familiar forms of today.

During the early part of the Late Miocene in Europe, however, the older forms still dominated, evidently showing that their lifestyle was still a valid one, perhaps in part because they weren't directly competing with their more "modern" looking relatives. Nonetheless, as the Late Miocene dawned, newer species of hyena began to appear, and these had a more dog-like appearance than their relatives. As a result, it has been estimated that as many as 20 different species of hyena may have lived in Eurasia at the time, with a greater diversity than today.

The earliest of these new hyenas, such as Ictitherium, resembled a jackal more than they did the muscular hyenas of today. This was a widespread and successful species, with multiple fossils being known from Greece to China - it probably first appeared somewhere around the Middle East. A moderately-sized, relatively slender animal, it seems to have been omnivorous, the shape of its teeth suggesting that it ate less meat than its closest relatives. Having said which, it did show the first signs of modifications to the skull found in all of its later relatives, and which may be related to supporting powerful jaw muscles. The contemporary European species Hyaenotherium was similar, but noticeably larger, and probably ate more meat.

Even the climate becoming drier towards the end of the Miocene didn't really eliminate the older forms of hyena, which struggled on right up to the end of the epoch. However, it did see some newer species arriving in Europe, probably from further across in Asia. Some, such as Hyaenictis, resembled Ictitherium, but became faster and more predatory, with long limbs suited to chasing down prey. Their teeth suggest a switch back to a more meat-dominated diet, able to strip the flesh from bones, yet with little ability to chew up the bones themselves as a scavenger might. This and related forms, such as Hyaenictitherium and Lycyaena, are known from China to Tunisia.

But yet another form of hyena was also beginning to appear around this time. Adcrocuta was much more like the kinds of hyena we know today, a heavy, stocky animal that lacked the running ability of Hyaenicitis and its ilk. It was about the same size as a modern spotted hyena (the largest living species) and was probably quite a close relative, or even a direct ancestor. More significantly, it was the first hyena to have teeth that could truly crush bone, indicating that it had abandoned both the fast-running and omnivorous modes of life employed by its various relatives of the day in favour of one more reliant on scavenging. It proved very successful, with, so far as we know, just one species found across the whole of southern Eurasia, and even into north Africa.

It was not, however, the only animal at the time with such a lifestyle. Prior to its arrival, the role of "large scavenger" had been taken in Europe by Percrocuta, an animal that somewhat resembled hyenas, but that was not directly related, and was still not fully adapted to cracking bone. That changed in the Late Miocene, as Percrocuta (already larger than any living hyena) was replaced by the even more fearsome Dinocrocuta.

While broadly hyena-shaped, this was about the size of the largest of black bears, and had exceptionally large teeth. These most certainly could crush bone, and very effectively at that, so the animal must surely have scavenged on any carcasses it could find. Interestingly, its fossils are never found in the same localities as Adcrocuta, suggesting that the two animals probably competed with one another for food, so that where one prospered, the other did not.

It was, however, also an active predator, although it probably couldn't run very fast and may have ambushed prey, rather than chasing it down. Some evidence suggests that young Dinocrocuta may have taken longer to develop the full bone-cracking abilities of adults than living hyenas do, but, more significantly, a fossil of an adult rhinoceros found in China shows bite marks consistent with an attack by a Dinocrocuta that the rhino subsequently survived. While it's certainly possible that the animal received the injuries while driving a predator away from its calf, rather than being the direct target of the attack, it clearly indicates that the latter went for living prey of some kind.

Yeah, a bear-sized hyena-like animal that hunted rhinos. You probably wouldn't have wanted to mess with it.

Dinocrocuta left no descendants, but another predator that arrived in Europe around the same time left a more lasting impact: it was the first of the true sabretooth cats. Machairodus had evolved, probably from the longer-toothed species of the primitive cat Pseudailurus, somewhere around Turkey towards the end of the Middle Miocene. Now it entered Europe, where, perhaps surprisingly, it lived alongside the similarly sabretoothed Sansanosmilus for over a million years. The latter, however, was not a true cat, but a nimravid, a holdover from an earlier time before true cats had evolved.

Machairodus was a relatively large predator, about the size of a lion, but with limbs that were closer in proportion to those of a tiger, perhaps suggesting that it hunted in a similar fashion. The name Machairodus was originally applied to just about every sabretooth cat, and many creatures that we now know to be something else, but, as these were split off into their own genera or even families, we were left with only a few fragmentary pieces that belonged to the 'real' animal.

Fortunately, that changed around the turn of the 21st century, with the discovery of a large number of relatively complete fossils from Spain. These showed that this, perhaps the earliest true sabretooth, lacked many of the skull features we would normally associate with such animals and that distinguish them from more modern looking cats. Despite this, they had undeniably large upper canine teeth, pretty much the equal of those of most of their later relatives, as well as a few other sabretooth features such as a large dewclaw and a long, muscular, neck. Taken together, this mix of features suggest that the 'sabreteeth' evolved before some (but not all) of the other characteristics that would later aid in their use.

Clearly, they were able to employ the elongated teeth effectively enough, but as millions of years passed, the course of evolution refined their features, slowly grading Machairodus into later, more derived forms, which now carry names such as Amphimachairodus and Paramachairodus. Of those, the first was another lion-sized animal, found across Eurasia, as well as in both Africa and North America. At least in the males, it had sabre teeth that were wider and more solid than the slender blades of many of its relatives, which may have made them more resistant to breaking when sunk into the flesh of a struggling victim.

Paramachairodus, however, was smaller, about the size of a leopard and with a long face and slender hind-limbs that would have enhanced its resemblance to that animal. It's plausible, for example, that, like leopards, it could climb trees, perhaps to eat its meal in peace. A more unusual feature is that, in Paramachairodus, the lower canines, as well as the upper ones, were enlarged and elongated, albeit to a much lesser extent. This has resulted in comparisons to the living clouded leopards, southeast Asian species distinct from the regular, spotted, sort, which have similar, if less dramatic, teeth. It's possible that the two animals share a similar lifestyle and approach to hunting, in which case the extinct one was presumably somewhat different from later sabretooths such as the more famous Smilodon.

As with Machairodus, our knowledge of this particular animal has been greatly enhanced by recent discoveries in Spain. We now have enough fossils, for instance, to show that males and females were probably about the same size, implying that the former did not have to fight much for access to the latter, perhaps because of a relatively sociable lifestyle.

One evolutionary model suggests that Amphimachairodus gave rise to later sabretooths such as the powerful Homotherium, while Paramachairodus gave rise to the likes of Megantereon, and, ultimately, Smilodon. At around the same time, however, a third lineage appeared in Europe that would not last for quite so long. This was represented by Metailurus, and, as the Late Miocene passed into the early Pliocene, Dinofelis.

The unusual thing about these particular sabretooth cats was that they didn't really have very long canine teeth - at least, by the standards of their relatives, since they were still longer than those of living cats. Metailurus was about the size and shape of a cougar, with long limbs suited for both pouncing and running, which likely means that it frequented open woodland, rather than either dense forest or grassy plains. Later forms of Metailurus, some of which are now classified in the newly named genus Yoshi, became lighter, and more slender, with an increasingly short face, reminiscent of a cheetah, which may suggest a similar diet, although not, presumably, the same means of obtaining it.

One further kind of cat that lived in Europe at the time also deserves mention. This is Pristifelis attica, and it's significant precisely because it wasn't a sabretooth, having conical teeth that resembled those of modern cats. (For a long time, indeed, it was placed in the genus Felis, alongside most of today's small cats). The smallest cat of its day, it was about the size of a bobcat, although with a more slender frame. It was probably effective at both chasing prey and climbing trees, rather than striking from ambush and rapidly overwhelming their victims, as we believe sabretooths did. Miopanthera, a very poorly known animal that may be related to today's big cats, also lived in Turkey around this time.

Sabretooths survived for a very long time, especially in North America, where they made it into, and almost through, the Ice Ages. But, from our modern perspective, we can see that the less specialised hunting strategies of lynxes, cougars, lions, and leopards eventually outlasted those of their longer-toothed kin.

This series, however, has really only covered one corner of the Miocene world so far. With a fairly detailed look at the European animals of the epoch out of the way, it's time to glance across the Atlantic, and see what was happening in Miocene North America...

[Photo by "ghedoghedo", from Wikimedia Commons.]

No comments:

Post a Comment