Sunday, 13 September 2020

Miniature Marsupial Lions

Thylacoleo carnifex
The largest carnivorous marsupial alive today is the Tasmanian devil (Sarcophilus harisii). Noticeably smaller than a European badger, it's still almost twice the weight of the next nearest contender for that title, or, indeed, of the omnivorous Virginia opossum so well-known to Americans. Most other living predatory marsupials can more accurately be described as "insectivores", due to their small size.

Unsurprisingly, this wasn't always so. Most famously, perhaps, there was the thylacine or Tasmanian wolf, which officially went extinct in 1936. There were also the "marsupial sabretooths" of South America, which died out during the Pliocene about three million years ago. However, these are no longer thought to technically be marsupials in the sense of being descended from the last common ancestor of the living species, even if they were definitely in that branch of the mammalian family tree.

The marsupial lion (Thylacoleo carnifex) most definitely was a marsupial, however, and was probably the largest carnivore from that group ever to have lived. It lived in Australia during the Pleistocene epoch, and died out a little over 40,000 years ago, most likely due to human activity. It has been estimated to have weighed around 160 kg (350 lbs), which puts it in the same size range as an adult lioness today. However, this fearsome predator didn't come from nowhere, and it was just the last in a line of similar marsupials to inhabit the land down under.

The group to which this animal belongs is technically named the thylacoleonids and the term "marsupial lion" is sometimes applied to the group as a whole, not just to the one genuinely lion-sized species that was its last survivor. Indeed, much of the evolutionary history of the group consists of a more-or-less steady increase in size, something that took place over the course of roughly 20 million years.

Perhaps surprisingly, marsupial lions were not closely related to either thylacines or Tasmanian devils. Instead, they represented a branch within the diprotodont order of marsupials - a group that today consists almost entirely of herbivores. In fact, their closest living relatives are thought to be wombats and koalas, neither of which are at all known for their predatory habits. 

The shift in diet seems to have happened very early on, with the large rabbit-like incisors that their relatives (and presumed ancestors) possessed becoming transformed into sharpened dagger-like blades at the front of their mouths. Lacking the canine teeth that are so significant in real lions, over time one pair of premolar teeth also shifted in shape towards a long and narrow cutting blade while, just as in real cats, most of the molar teeth used by many mammals to chew up plant matter became smaller and eventually vanished altogether.

This presumably made marsupial lions effective predators, with the largest ones able to feed on prey far larger than anything a Tasmanian devil could tackle. Yet there have never been a great variety of thylacoleonids, with perhaps no more than three species living in Australia at any one time, and often less. The majority of known species fall into just two genera, of which Thylacoleo is the more recent, including both T. carnifex and its leopard-sized Pliocene predecessors. The other is Wakaleo, which became replaced by the physically larger genus somewhere around 5 million years ago. There was a considerable change in the size of the various Wakaleo species over the course of their evolution, but one example has been estimated at 23 kg (51 lbs), similar to a Eurasian lynx in weight, if not bodily proportions.

In 1987, a third genus was added to the list and named Priscileo. This lived even earlier than the other forms, around 25 million years ago at the end of the Oligocene epoch. It was both smaller and more "primitive" than the later species, still having a full set of molar teeth, whereas even the earliest known species of Wakaleo had managed to lose at least one pair. 

Or so it was thought.

Because, new fossils described in 2017 revealed that a particularly early species of Wakaleo did, in fact, still have a full set of molar teeth, despite having numerous other features that clearly placed it in the better-known genus. Lacking any distinctive features to set it apart, the genus Priscileo was deemed invalid and its species re-assigned to Wakaleo. So, now, we're back with just two genera again.

Except, well, no, not quite. For one thing, by this point, a second species of Priscileo had been identified. It, too, was remarkably small, and it had the same tooth pattern as the existing species, which was about all that really distinguished at the time. With nothing much else to go on, it was therefore assumed to be a close relative of the 1987 species. That was in 1997 but, last year, its discoverer came back with a detailed description of a new, more complete, fossil of the same species. This, by her analysis, tells a rather different story.

The details, as you might expect, are rather technical. For the record, though, the key distinguishing features relate to the shape of the first molar tooth in the upper jaw, the shape of a bone surrounding the inner ear, and the unusually small size of a canal through which a nerve or blood vessel would have passed in a bone located behind the eyes. Apart from that last one, which is apparently a bit odd, these, and other similar features, are closer to what we'd normally expect to see in other wombat-like animals, but not in marsupial lions - in other words, they are "primitive" features that were lost as marsupial lions diverged further from their kin.

The upshot of this is that this species has now been given its own genus, Lakaneleo. (It can't be given Priscileo back again because the definition of that is tied to the species it was originally named for). But what can we say about this very early "marsupial lion"?

Well, for one, it didn't look very lion-like. Among its primitive features is the presence of only a relatively weak sagittal crest, the ridge down the centre of the skull to which jaw muscles are attached in mammals that need a particularly strong vertical bite. This is especially large in carnivores, from real lions to the likes of Thylacoleo, and the fact that it isn't here suggests a less powerful bite and a more horizontal chewing motion. That probably means a more omnivorous diet, although the details are hard to know for sure.

The authors of the new paper also give an estimate of the animal's size, and here again, the resemblance to lions is not great. The estimate they come up with is about 3 kg (7 lbs), which is slightly smaller than the average house cat. This, they note, is small enough that the animal could have climbed trees, which may be significant, given that the area it lived in was heavily forested at the time, and could arguably be described as a jungle.

Whether it actually did climb trees, perhaps to hunt, isn't something we can know from the fossil remains we currently have, since they don't include the limbs. The authors do point out, though, that this would help to explain how it could live alongside the early species of Wakaleo, which were larger and far more likely to have hunted by chasing prey along the ground. Mind you, it seems to me that a more omnivorous diet and eating smaller prey items when it did hunt could have had a similar effect, so we're still in the realm of speculation until better fossils come along.

Omnivory would make sense of another feature of the animal; that it's known fossils are of very different ages, suggesting that this Lakaneleo survived for at least 9 million years from the late Oligocene into the early Miocene, with very little change over that time frame. That's what we'd expect of an adaptable, successful animal, a description that fits generalist omnivores better than it does specialised predators. Wakaleo, for instance, changed noticeably over that time, becoming larger and more predatory, and, for that matter, so did some contemporary specialist herbivores.

The fact that it lived alongside the more predatory forms does rather rule it out as their ancestor, but it also turns out that Lakaneleo isn't the most primitive "marsupial lion" known. That honour goes to a third species that lived alongside it, belonging to yet another genus, only identified in 2016. The remains of this animal are more fragmentary, but, despite apparently belonging to an adult, this animal, Microleo attenboroughi (and yes, it is named after who you think) is thought to have weighed as little as 590 g (21 oz.).

Not so much a marsupial lion as a marsupial kitten...

[Photo by William Harris, from Wikimedia Commons.]

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