Metatherians (Part 1 of 2): Extinct Megafauna

Marsupials.

The immediate association most people have with the term ‘marsupial’ is that of fantastical, adorable, fluffy beasts in the far-away magical land of Oz, equipped with built-in fanny packs for storing their tiny, even more adorable, offspring. Bounding, big-eared kangaroos, sleepy koalas, and perhaps a hyperactive sugar glider or a waddling opossum might cross their minds. Not too far beyond this is where the train of thought pulls into its final stop, and suddenly they’re caught up in the romanticism of Australia itself; the sun-baked, tawny Outback scabland, didgeridoo droning in their mind’s ear, impossibly colorful fish flitting about the Great Barrier Reef, and perhaps Hugh Jackman or Nicole Kidman (whatever their fancy) driving cattle across the Northern Territory during The Dry.

While this idealization is all well and good, there is actually a lot more to these pouched animals than what fits on the in-fold of a Qantas brochure.
Marsupials are really bizarre by mammalian standards, and have a rich and relatively unrecognized evolutionary history that spans back 125 million years. This entry is one of two that will be devoted to these weird little creatures, focusing first on their unrealized illustrious past, and then on lesser known representatives of their clan in the present.

First, it may be helpful to actually define what, the grand scheme of things, zoologically, we are talking about. Extant (or currently existing) mammals roughly break down into three groupings, which many folks learned back in their middle or high school science classes. Of these three, the one that split off earliest in the mammalian family tree are the Monotremes; egg-laying mammals. These anachronistic sideshows, the platypus and but a few species of echidna or ‘spiny anteater’ make up the entirety of what remains of this subclass. The other two, which last shared a common ancestor more than 150 million years back, are the Metatherians and the Euherians. Eutheria, meaning ‘true beasts’, contains the placental mammals (meaning that in fetal development, a placenta is present allowing nutrient and waste transport between the mother and offspring) that we are very familiar with. This is the group that contains everything else with fur and warm-blood that doesn’t lay eggs or have a pouch. Dogs, elephants, rabbits, bats, us…we are all placental, eutherian mammals.

The last group, the metatherians, meaning ‘behind beasts’ or ‘near beasts’, contain the marsupials (although, there used to be several sister groups under the metatherian umbrella, but they are now extinct). The most glaring feature that distinguishes metatherians are their reproductive characteristics. The most obvious of which is the early birth of underdeveloped, jelly-bean sized young that remain in a pouch, or ‘marsupium’, firmly attached to their mother’s teat until they reach a more independent stage of development. This premature young, called a ‘joey’ is typically born after no more than five weeks inside the mother’s uterus. The marsupium itself is little more than a fold of skin specialized to cover the nipples of the mother and to safely house the joey as it continues its development.

Marsupials, and likely all metatherians, have urological and reproductive systems that would strike our Puritan eutherian hearts as alien. For example, female marsupials have two uteri, each one with its one vaginal canal, but the whole thing terminates in a single opening. A third vaginal canal, which can be permanent or temporary, in the middle of these first two, is solely used for birthing the joey. Corresponding to the two lateral vaginas, male marsupials have a forked penis. It should also be noted that marsupials have a single orifice for waste excretion, called a ‘cloaca’, that is also found in reptiles and birds. So, the penis in male marsupials is purely a sexual organ, and the careful re-routing of the urethra through it only arose in placentals like ourselves.

There are some other minor skeletal differences. For instance, marsupials tend to have more teeth than placentals; specifically in the number of pairs of molars. There are also some physiological differences, as marsupials tend to have a lower body temperature than placentals, as well as a shorter lifespan and comparatively slower metabolism.

Metatherians are thought to have split off from the mammalian line in the early Cretaceous, in Asia. Around 100 million years ago, the former supercontinent of Pangaea had rended itself into two main pieces, Laurasia in the north (containing North America and Eurasia) and Gondwana in the south (South America, Africa, Australia, India, and Antarctica), and Gondwana was being pulled further south and was splitting up into east and west factions. It is thought that early marsupials spread from Asia into Gondwana when it was still relatively close by, either directly, or through North America into South America (which were connected as late as 65 million years ago). When the asteroid hit, and all the dinosaurs were powderized, mammals were suddenly free to exploit a large number of previously occupied ecological niches. Placental animals did very well across Laurasia, and in certain places in the old fragments of Gondwana, specifically Australia and South America, marsupials went through their evolutionary diversification.

The thing is, whenever placental mammals were introduced into ‘marsupial territory’ over these next 65 million years, usually by the aid of tectonic rearranging of plates, they would sweep in and outcompete their marsupialian brethren. There are plenty of theories as to why marsupials lost over and over again in these competitions, and I will go over them in part 2 of this series. Marsupials never really got a good foothold in Eurasia, Africa, or North America; places where major groups of placental mammals, like dogs, antelope, big cats, rodents, primates, elephants, etc., really got off the ground and exploded in diversity. Metatherians in general did, however, make a respectable presence in South America in splendid isolation along with some equally weird placentals, and of course, most spectacularly, in Australia, which was almost completely devoid of a placental presence (outside of bats) up until around 50,000 years ago…when we charming lot crossed the Indo-Australian Land Bridge with half-domesticated dogs, fire, and weaponry.

Only 334 species of metatherian remain, compared to the more than 5,000 placental mammal species, and essentially all of these are centered around Australia, New Guinea, and South America (with one exception living in North America, a relatively recent transplant from the south, the Virginia opossum). The largest of them are kangaroos, roughly the size of a human being, and most living are small herbivores, insectivores, and some omnivores, with most carnivore lineages made extinct either by direct human extermination, or by introduced placental competition. Of those that do remain, many are threatened by habitat loss from human encroachment and climate change, communicable disease (chlamydia in koalas, facial tumor disease in Tasmanian devils), and simply being slaughtered by non-endemic species that humans have brought in.

However, metatherians have a grand, and undeniably odd, evolutionary past that has littered the fossil record with scores of highly-specialized, fantastic species that are now long-gone, but nonetheless illustrates the great power of geographic isolation to facilitate evolutionary experimentation.

Let’s take a little tour, shall we?

The crouching (and possibly a little too thin) fellow above is Procoptodon goliah, alternatively known as ‘the giant short-faced kangaroo’ or more hilariously, ‘the whopper hopper.’ P. goliah was the largest macropod (kangaroo) ever to bound across the hot plains of Australia, and belonged to a now extinct subfamily of kangaroos, the Sthenurinae, of which it was the most extremely out-sized and specialized member. P. goliah’s genus dates back to throughout the Pleistocene epoch (up to around one million years ago), and suddenly went extinct around 50,000 years ago.

P. goliah, like all the sthenurines, was very large and robust compared to modern kangaroos, and was a browsing animal rather than a grazer like today’s kangaroos. It had an unusually short, flat face, and forward-facing eyes, possibly giving it an oddly primate-esque appearance. Unlike today’s kangaroos, it had only a single, hoof-like digit on its springlike hind legs. Assisting its diet on high-growing leaves and twigs were two elongated claws on each hand, attached to very long, flexible arms that it likely used to pull down branches towards its unsettlingly monkey-like face. These limbs stand in contrast to conventional kangaroo arms, which are comparatively short, stiff, and good for little else but scratching oneself and boxing. It probably helped, as well, that P. goliah, probably used its thick, powerful tail as a third leg for support when it raised up on its narrow feet to drag down vegetation, like some sort of alien, athletic ground sloth.

Procoptodon also had a killer hook shot.

Long, rangy limbs and big ol’ meaty claws were definitely an asset to the browsing lifestyle, but the fact that Procoptodon goliah was a gigantic fucker was likely more important. A modern red kangaroo, the largest living marsupial, can have exquisitely large males that reach heights of six feet, but are more typically in the four or five foot range. A particularly heavy red kangaroo can be 200 pounds. The whopper hopper, by comparison, would have had to duck slightly to fit through the front door to your house, and with its extended reach, could easily jam a basketball through the hoop without so much as a hop. Females of the species were large enough to stuff a 10-year old human child in their fathomless pouches and carry them off. In life, it was far meatier than the poor excuses for kangaroos we have today, and tipped the scales at more than 500 pounds.

In short, if modern kangaroos are deer, then this thing was a damn moose.

Fossils have been found all over mainland Australia, so it is likely that the giant short-faced kangaroo was relatively commonplace. With all that heft, it would have been undoubtedly been a coveted meal for any predator, and that may include humans.
Procoptodon, like so many other large marsupials in Australia, went extinct at roughly the same time as humans showed up. This seems to suggest that humans were the main cause, but such things are never easy to determine. For one thing, no bones have ever been found at butchering sites, or with markings that indicate human hunting and processing for consumption. So, it’s not even known definitively if humans actually hunted them at all. But, they would have been easy targets, that’s for sure. Big, easy to spot, and more bulky and slow then their modern cousins, they would have been a relatively easy thing to hit with a spear. In the event of somehow cornering one in close quarters, however, they would be exceedingly dangerous. One well-placed kick would turn a human abdomen into a scarlet waterfall of viscera.

Whatever the reason, Procoptodon vanished from Australia very suddenly, denying us the experience of seeing this massive animal in life. Australia still has plenty of kangaroos, but as is often the case with comparing modern counterparts to extinct prehistoric relatives (see: T-rex vs. sparrows) none of them are quite as imposing, unique, badass, or ‘sexy’ as the pug-nosed, late-whopper hopper.

“Draw me like one of your French girls.”

Another type of kangaroo that shared the arid landscape with Procoptodon was Propleopus. However, this kangaroo came from very different stock, being of the same lineage that the modern animals known as ‘rat-kangaroos’ of the Potoroidae belong. It was much larger than its diminutive, bunny-like modern descendants, growing up to around 150 pounds, making it about the same size as a typical red kangaroo. It lived around the same time as the whopper hopper, but its fossils have been very rare. So, you’re probably saying to yourself right around now, ‘ok, big rat-kangaroo (whatever the hell THAT is)…what’s the big deal?’

Well, here’s the kicker (so to speak); Propleopus ate meat.

Evidence for this comes from the most logical place to find out information about what an animal ate; the teeth. Propleopus had some eerie modifications to the typical plant-munching chompers of most kangaroos, including the addition of serrated edges on the normally universally blunt cheek teeth, as well as blade-like lower incisors that jutted out from the jaw like a bayonet.

All the better to spear you with.

While the concept of a carnivorous kangaroo leaping after prey and maliciously tearing it to shreds is surreally hysterical, and a bit unnerving, all this dentition means is that it was very capable of eating meat; it doesn’t mean it actually hunted prey.

Propleopus also retained the capacity to process plant matter like fruit and soft leaves, so it may have been something more of an omnivore or an opportunistic scavenger of kills or reptilian or bird eggs. Either way, judging by the shiv sticking out of its mouth, and its general size, Propleopus must have been a soundly nasty creature if crossed. It’s possible that Propleopus was something like the baboon of Africa’s savannahs, a temperamental omnivore wandering the open country, searching for whatever it could mash up for food, even if it had to make it bleed first.

Propleopus is survived by ‘rat-kangaroos’ and ‘bettongs’, small macropods which rarely exceed fifteen pounds in weight and reside in Australia’s eastern coastal forests. These animals are, ecologically, something like the ‘rabbits’ of Australia, dining on herbivore fare, but will regularly indulge in darker tastes and harken back to their bigger, scarier ancestor by taking an insect or two, or perhaps scavenging on a sheep carcass.

Don’t turn your backs on these things. Pure evil.

While kangaroos were certainly more interesting and diverse several tens of thousands of years ago in Australia, there are more varieties of marsupial than the hopping kind. Many people are familiar with wombats; furry, short, squat, burrowing marsupials that somewhat resemble a cross between a teddy bear and a marmot, but can be about the size of a large badger. Wombats, members of the family Vombatidae, are fairly closely related to kangaroos, as they share the massive marsupial order of Diprotodontia. Wombats, however, are even closer in relation to koalas, and this can be seen in their similarly stubby, cuddly morphology…and yes those are scientific terms. Wombats, although meek and rodent-like today, had close relatives, even closer than koalas, that were the largest marsupials known to science. These giants lived alongside Procoptodon and Propleopus in this Pleistocene era of an Australian marsupial maximum of diversity.

Meet Diprotodon, the largest marsupial that ever laid a foot of its pouched self on this planet. Diprotodon existed across Australia from about 1.6 million years ago, until, unsurprisingly again, around the time humans arrived, about 50,000 years ago.
It was immense by marsupial standards. It grew to be six and half feet at the shoulder, ten feet in length, and as much as three tons in weight. This is comparable to the size of a hippopotamus, one of the largest land animals alive today. We know that it inhabited areas close to water, unlike the desert-loving Procoptodon, and dined on the varied plants of the open woodlands, forests, and grasslands along the perimeters of the Australian continent. Being this large of a creature, it would have a very hard time surviving in an area without readily available sources of water.

Hair impressions on fossilized footprints tell us that like wombats, Diprotodon had a dense coat of hair. It was thickly, stoutly constructed, with awkwardly pigeon-toed front feet, armed with large claws that it probably used to dig up subterranean roots.
Superficially, it would have resembled a furry, hornless rhino, with a gigantic noggin and toes instead of hooves. Even more notably, it had large, retracted nasal bones, which seem to suggest either the presence of a fleshy trunk of some kind, or a bulbous, leathery, koala-like, sac-shaped nose. Either way, Diprotodon was bound to be sort of ridiculous-looking to any humans that encountered it as they made their way into the newly discovered continent eons ago.

Like a Muppet…but the size of a mini-van.

A creature so impressively large must have had some sort of cultural impact on the first humans that crossed over from Asia, and many scholars believe that this impact can be discerned today. The ‘bunyip’, a mythological beast that typically is described as a large, billabong-wallowing, generally mammalian animal that’s especially malicious towards humankind, is a cultural item that is common throughout many tribes across Australia. It may be that the mighty Diprotodon has been passed down through the millenia as a cultural memory, transformed into the mythical, deadly, hippo-like bunyip in the folklore. Some Aboriginal groups, when shown Diprotodon bones, refer to them as bunyip remains. It may be that the bones themselves inspired the bunyip tale, or early humans really did encounter the animal, and pass down the stories for many thousands of years, and eventually these stories were warped into a mythical form by the modern era.

Again, humans are the suspected culprits in this lumbering beast’s ultimate demise, as within a few thousand years of humans’ arrival on the continent, Diprotodon was gone. Diprotodon was probably agonizingly slow, and had no chance against the sharp weapons humans had brought along with them. It was literally a shuffling bag of wool-covered flesh; no defenses, and no offensive capabilities to speak of…save for simply looking silly enough to cause a laughing fit in any would-be hunters.

Another possibility for their extinction, and perhaps more likely, is an indirect effect through the practice of fire-stick farming that ancient Australian aboriginal peoples practiced. Fire-stick farming is the intentional burning of scrubland vegetation to assist hunting practices (cornering animals in a constructed brushfire, or simply burning them alive), as well as to facilitate the growth of edible plants, and the development of ecosystems that directly benefit human hunting and gathering practices. Indigenous Australians practiced fire-stick farming for many thousands of years, gradually turning large areas of Australia into grassland, from native scrubland; evidence for this can be found in the geologic record via a marked increase in the amount of ash and charcoal deposits. It may be that this relatively dramatic, and quick, ecosystem alteration, combined with active hunting of megafauna like Diprotodon, was the cause of such widespread extinction when humans arrived in Australia.

Some people contend that they’ve SEEN weird animals that resemble what Diprotodon is thought to have looked like. Oddly enough, descriptions of these creatures usually change with new revelations on life reconstructions of Diprotodon…funny that.

While very, very little evidence exists that suggests that some small populations of the world’s largest marsupial may still live in Australia, there have been rumors of unidentifiable, bulky, big-schnozed animals being spotted in France, of all places.

Gerard Diprotodieu

Relatively a lot is known about Diprotodon in comparison to the next extinct metatherian I’m addressing; possibly one of the most enigmatic, and historically troublesome, creatures to be unearthed out of the Australian Pleistocene.

This is Palorchestes.

“…’sup brah?”

Palorchestes means ‘ancient leaper’, but judging by the reconstruction above, this animal was probably less into the leaping and more into the getting-baked-and-eating-oreos-with-peanut-butter side of things. Palorchestes‘ story begins with a profound misidentification, and many decades of radical ‘adjustment’ as new fossil evidence trickled up from the rock below.

The famous British anatomist Richard Owen (the same globular-eyed Victorian gent who coined the word ‘dinosaur’) first described fossils of this creature in 1873. Only a meager hunk of bone from the front of the skull was sent from Australia (still very much a part of the Empire at the time) to Owen back in Britain. From this small piece, Owen surmised that it had to be a kangaroo, based on the tooth morphology; not only that, but that it must have been the biggest kangaroo yet discovered…potentially a ten-foot tall giant. Owen was actually relatively close; Palorchestes does reside in that same big marsupial order of Diprotodontia as kangaroos, and is somewhat related. But, only as closely as koalas, wombats, and the gigantic Diprotodon. In reality, Palorchestes was a distinctly un-kangaroo-like critter, but it would take nearly a century for this information to come to light.

The vision of Palorchestes as a titanic kangaroo persisted for the first half of the 20th century. At the Australian Museum in Sydney, Palorchestes was represented by a towering to-scale sculpture of a 10 foot tall kangaroo for more than thirty years; it was a big hit with visitors, as one can imagine. Doubts in the paleontology community about the phylogenetic placement of Palorchestes started to swirl about in the late 50s, culminating in a highly persuasive argument put forth by J.T. Woods in 1958 that the animal was definitely more of a wombat than a kangaroo. In response, the Australian Museum trashed their beloved mega-kangaroo sculpture (literally; it’s been said that they shattered it and then buried the pieces) in order to avoid embarrassment.

More fossil evidence turned up in the 70s that showed that Palorchestes in fact had heavy claws on its forefeet, as well as a bizarrely shaped skull and deeply recessed nasal cavity, meaning that it probably had some sort of trunk. Not exactly wombat-like.
Reconstructions varied for the following few decades. It was re-imagined as a lanky, llama-shaped browser, a scythe-clawed piggish looking animal, and finally, as most modern depictions show, a surreal mash-up of a tapir and a ground sloth.

Most modern paleontologists that have studied Palorchestes place it in its own family, distinct from the ‘giant wombats’ like Diprotodon, but most certainly a cousin. Palorchestes, due to the presence of tough, high-crowned, resilient teeth and a flexible proboscis of some kind, and a deep well in the lower jaw for a prehensile, giraffe-like tongue to reside, was undoubtedly a forest and woodland living creature that browsed on rough leaves, twigs, and other hardy vegetative material. Palorchestes had ridiculously strong, well-muscled forelimbs, and this is especially true in the forearms. When living, it probably looked like it spent every second of free time ripping phone books in half. The combination of all this forearm strength and the five mining picks attached each hand have led scientists to the conclude that Palorchestes was really into excavating. It could have used these claws for digging down into the soil for roots, or for stripping trees of their bark and exploiting the tender cambium layer, or both. Palorchestes has been described as a ‘tree wrecker’; an herbivore specializing in simply beating the shit out trees by digging out their bases in search of roots and tubers, yanking down branches with its mighty arms, and flaying the poor plant for good measure with its grabbling-hook paws, all the while stripping it of its delicious leaves with its nimble trunk and tongue.

I shudder to think of the dendrocidal atrocities this poor joey has seen…

As is popular with making comparisons between isolated marsupials and their placental counterparts, in order to illustrate the majesty of convergent evolution, Palorchestes was considered a ‘marsupial tapir’ (in much the same way that the thylacine, or Tasmanian tiger, was called a ‘marsupial wolf’) on account of its trunk and browsing way of life. However, it may be accurate to say that Palorchestes was, ecologically, even more strange; more of a ‘marsupial ground sloth’.

Palorchestes never reached the elephantine sizes of their contemporaries, the placental ground sloths of South America, but were large for marsupials, nonetheless. One species, P. azael, was roughly the size of a cow, and could have made short work out of any small to mid-sized tree.

Kangaroos the size of NBA players, wombats heavy enough to launch a Subaru off a see-saw, and the Seussian acid trip that is Palorchestes are all nice…but where are the carnivores? Surely, you ask, with all these big, hulking plant eaters around, there were some badass pouched predators to take them down, right? Although many of the largest and underwear-soilingest predators were reptiles and birds during the golden age of marsupials in Australia and South America, specifically 20-foot long monitor lizards and long-legged terrestrial crocodiles in Australia and 8-foot tall flightless ‘terror birds’ in South America…there were some opportunities for nasty, apex predators of the metatherian variety to evolve.

And they were horrifying.

One such predator is Thylacoleo of mainland Australia. The genus name means ‘pouched lion’, or ‘lion with a pocket’…which alternatively sounds like a children’s book. Thylacoleo, in the style of the ‘marsupial wolf’ and the ‘marsupial tapir’, has been dubbed the ‘marsupial lion.’ While obviously not actually related to lions, Thylacoleo was so named due to its size and vaguely cat-like shape, and well, the fact that it was incredibly scary looking…

The demonic fucker above is Thylacoleo carnifex…the ‘carnifex’ means ‘murderous’ or ‘meat-cutting’. Not even a joke. As far as we know, it was the largest carnivorous marsupial that ever lived, and the only one with enough balls to take down a Diprotodon or a Procoptodon. This is an animal that has given us very complete remains in the fossil record, and yet we still know very little about how it lived or hunted…mostly because it is, quite honestly, unlike anything living.

A huge reason for this is that it is a hypercarnivore with what appears to be the ‘wrong’ ancestry. Every marsupial mentioned so far in this post has been a member of the order Diprotodontia, a diverse order that currently is the most speciose among marsupials and has a even richer history. Diprotodontia houses most familiar marsupial herbivores; kangaroos and wallabies, koalas, wombats, and the Australian ‘possums’ (including sugar gliders). Members of this order are united by unique dental characters, including the presence of just two large procumbent (sticking straight out from the tip of the jaw) incisors on the lower jaw, as well as a lack of canine teeth (there’s a conspicuous ‘gap’ in the mouth where they should be). Most members also have syndactylous second and third digits in the hind feet. ‘Syndactyly’ essentially means that the toes are completely fused together, and while this is a common birth defect in humans, in this group of marsupials, creepy mutant feet are normal. This condition is most easily seen in koalas, which use their fused toes creatively in their climbing technique, but even kangaroos have the feature.

And this little piggy doesn’t understand the concept of ‘personal space.’

With a complete lack of canine teeth, and a mouth instead full of teeth for trimming and mashing plants…it makes sense that for much of the history of the order, Diprotodontia has been an overwhelmingly herbivorous lineage. There are some rogue agents, like the rat-kangaroos like Propleopus and its occasionally insect-munching descendants, but true carnivorous habits were never a real part of the Diprotodont strategy. The real marsupial ‘carnivores’ of Australia are the Dasyuromorphia, the ‘dasyurids’, which includes the Tasmanian devil, many small insect-eating shrew-like animals, and the extinct thylacine. These are animals with dentition more geared towards processing other animals; sharp canines, meat-ripping cheek teeth, etc.

Ecologically, Diprotodonts are to deer, antelope, and rabbits as Dasyuromorphia are to cats, dogs, and weasels. So, when I say that a super-predator like Thylacoleo was actually kin to the Diprodontia clade, there’s a smack of incredulity. Finding an animal like Thylacoleo is like finding a sabertoothed zebra in the fossil record…very unusual.

The largest species of Thylacoleo, T. carnifex, could have weighed up to more than 300 pounds and was about the size of a jaguar. Much like a big cat, it was blanketed in rippling muscle, but its skeleton suggests that it wasn’t nearly as fleet over the ground as its placental namesake. To say this animal was robustly built would be an understatement. Thylacoleo‘s skeleton anchored an ungodly amount of muscle, and while this was a critical adaptation for holding down large and difficult prey, it made chasing things with any amount of speed completely unfeasible. It is thought, instead, that Thylacoleo was an ambush predator; quietly sneaking up on its prey, and then jumping out of nowhere and dispatching it on the spot, using its strength to muscle it to the ground. And, given the morphology of its feet, this ‘lie in wait’ spot may have been up in the trees. The marsupial lion had very flexible feet, and the front paws had a semi-opposable thumb, much like that of a koala or a possum, which would have both assisted in climbing and holding onto prey. The back feet also had a roughened pad underneath, like a possum, that would have given Thylacoleo more friction when clambering up a tree trunk. The marsupial lion may have actually been more of a ‘marsupial leopard’, stalking prey from high vantage points, dropping down, and then dragging the kill back up the tree to dine in solace.

I can see you wish to eat alone. Pardon my intrusion. Please don’t kill me.

Thylacoleo had a number of superb adaptations for wrestling with large, potentially equally as powerful, prey items. One of them was its tail, which was reinforced with bony chevrons that protected blood vessels and nerves in the event that it had to rear up on its hind limbs to give its next meal a less-than-friendly final embrace, potentially propping itself up on its tail, kangaroo-style.

Another, far more obvious adaptation, as you’ve likely noticed by now, was on those lovely hands. Thylacoleo‘s unnervingly primate-like thumb was dramatically oversized, and came equipped with a truly heart-stoppingly giant claw. It’s other claws, also quite sharp, were actually retractable to avoid wear and tear, much like a cat’s claws; this trait is found in no other marsupial. However, the Big One was out and about, and its function is not entirely clear.

My guess? Convincing every animal on the continent to do whatever the hell Mr. Thylacoleo wanted.

At first, it was thought that the marsupial lion used these tools as killing weapons in themselves. It was imagined that Thylacoleo would rear up on its haunches, and basically shred the fuck out whatever was unfortunate enough to look tasty that day, disemboweling it before it even hit the dirt. However, many scientists have moved away from the ‘Slice ‘n Dice Katana Hands’ theory for something a bit more probable, honestly; that Thylacoleo used the claws both as a means to simply dig into its prey while it struggled, and as a way to increase its grip as it ascended and descended from trees…possibly while carrying hundreds of pounds of fresh kill.

If the idea of an animal with built in grappling hooks, evolved specifically to keep you from squirming away from its hungry stomach, wasn’t enough to send a river of urine down your leg…well, take a look at the head.

Fucking Christ.

You’ll notice that the marsupial lion’s skull doesn’t look a whole lot like other carnivores, like dogs, cats, bears, etc. And that’s to be expected, since Thylacoleo‘s evolutionary trajectory is vastly different. The skull of this creature is superb example of how evolution ‘makes due’ with the starting materials. How do you make a mammal without any canines, that is part of a long lineage of herbivores, into a damned beast slayer? Evolution answers this question through improvisation, and it in the case of Thylacoleo it turned out elegantly.

In placental carnivores like big cats and wolves, those canine teeth are crucial. They serve as deep grip when the predator is crunching down on the neck of its prey, and they provide  some ability to rip and pull at the meat once the prey item has been killed. The numerous little peg-shaped incisors in the front help with this too, and the back cheek teeth are narrow and sharp for shredding the meat into ribbons before it’s swallowed. In these animals, there is a concert of teeth working together and in an assembly line.

Thylacoleo, being a Diprotodont, doesn’t have this diversity of teeth, so instead, the two pairs of front incisors have become pseudo-canines. They tilt towards each other to form a sharp ‘beak’, which quite effectively penetrates soft giant wombat flesh at the very beginning of the bite. Immediately, following that, the mouth itself has evolved to drag the entirety of bulk of the animal back into the throat of Thylacoleo with the closing action of the jaw. What this does is automatically place the flesh between the pairs of dramatically enlarged, sharpened premolars in the middle of the mouth, where it gets cleaved. These huge premolars dominate the mouth of Thylacoleo, and are among the most specialized carnivore teeth ever seen in animals alive or extinct. As the mouth closes, the curved, bladed surfaces of the premolars form complementary crescents that narrow until the teeth slide past each other. In essence, they work together like biological shears, cleanly slicing off mouthfuls of meat, bone, and cartilage in a single go.

The effect of these perfectly constructed teeth is enhanced, of course, by the sheer force by which Thylacoleo can bite down. As you can see above, there is plenty of broad, thick, sturdy bone for muscles to attach to on that head. The amount of muscle hanging off the jaws of the marsupial lion, proportionally, is unprecedented in mammals, even when compared to modern big-biters like hyenas and ‘real’ lions. Biometric studies of the skulls of even small Thylacoleo specimens show that they would have been able to crunch down on prey with the equivalent bite force of a lion or tiger well over twice their weight. A 220 lb marsupial lion could easily outperform a 500 lb African lion in a bite force test. In fact, Thylacoleo possessed, pound for pound, the strongest bite force of any mammal.

Thylacoleo‘s killing strategy would have stood in stark contrast to the placental big cats it resembled. Tigers, lions, and leopards grapple their prey, and then clamp down on the jugular with an array of sharp teeth until the animal suffocates or bleeds out, usually over the course of a long, arduous dying process. Thylacoleo would have been an animal that wasted very little time. After an initial take down, those powerful jaws would dive in, plunging through flesh and bone like great bolt cutters.

Lions and tigers pin, maul, and bleed their prey to death. Thylacoleo probably could have decapitated its prey if it wanted to.

The marsupial lion was a nightmarish blend of jaguar, steroids, a deli cutter, and a Velociraptor, all perfectly assembled into one brutal bastard of a marsupial.
The combination of the deadly jaws, highly-derived teeth, thumb claws, and strengthened tail bones have led many paleontologists to conclude that despite its herbivorous pedigree, Thylacoleo is likely the most highly specialized carnivorous mammal ever. For being a ‘primitive’ marsupial, that is quite the distinction.

However, its excessive specialization may have contributed to its extinction. It too died out right around the time humans made their way into Australia. Humans probably did interact with Thylacoleo, as the following cave painting suggests:

While humans may have met up with marsupial lions, it is doubtful they killed them directly, and especially not for food. Thylacoleo would have laughed off a spear, and followed up by swatting the limbs from your body in anger. It is very likely that early Australian aboriginals gave Thylacoleo a wide berth, and it is possible that humans were specialty items on the menu more often than not.

What’s the last thing you’d want to see in Australia, 50,000 B.C.? Survey says ‘a Thylacoleo looking down at you and licking its lips.’

Instead, humans likely precipitated the extinction of the marsupial lion by killing off all its prey. Once big, lumbering Diprotodonts like the giant wombats and short-faced kangaroos were gone, Thylacoleo‘s relatively limited career of taking down larger animals would be over. Australia was hit by a blitzkrieg of highly intelligent, resourceful predators that invaded from a foreign land. It is no wonder that widespread ecosystem disruption was a consequence a short few thousand years after our kind got there.

As I’ve said before in this post, Australia was a roiling cauldron of metatherian evolutionary experimentation…but to some extent, so was South America. South America was a far more wet and forested place for much of the 65 million post-dinosaurian years than Australia, which progressively dried out until the majority of the continent was a single desert. This allowed for the proliferation of different types of endemic metatherians evolved to occupy very different niches from those found on the big, dry island across the Pacific.

Fundamentally, South America was a dramatically different place, with a different natural history. There was an established placental mammalian presence in South America, made up of weird groups like the now-extinct ungulates the ‘litopterns’ and their relatives, as well as ground sloths, giant armored glyptodonts, and very, very large rodents. So, metatherians couldn’t just run wild, evolutionarily. Many of the top predator niches in South America were occupied by the fantastically, and appropriately named, ‘terror birds’; massive flightless relatives of the rails that were the dominant predator group for 58 million years on the island continent. One radiation of metatherians in South America was particularly successful at competing with the ‘terror birds’ in predator niches. While up until now I’ve been solely speaking of ‘crown group marsupials’, there actually used to be other metatherian groups that split off the main line early in metatherian evolution. These groups are all extinct now, and the only metatherians we can see today are ‘true’ marsupials, but one group, the Sparassodonta, reigned in South America up until a few million years ago.

The difference between sparassodonts and marsupials is mostly one of semantics, phylogeny, and technicality. Sparassodonts had many of the same features as marsupials, and definitely still gave birth to underdeveloped young and had pouches and all of those things. Mostly, the distinction is made because sparassodonts split off from the line gave rise to all the marsupials we know and love significantly early on. For this reason, while they can be thought of as ‘marsupials’, this isn’t technically true…and this is why I entitled this post Metatherians and not just Marsupials. Calling a sparassodont a marsupial is a bit like calling an alligator a lizard. Not really true at all, although they generally share a lot of the same characteristics, physically.

Sparassodonts are not well-understood. For many years, they were considered to be a part of the Didelphimorphia, the purely South American radiation of ‘opossums’, as they had some morphological similarities. Many of them were vaguely dog-like, but also had nimble, climbing-adapted digits and semi-opposable thumbs. They were exclusively carnivorous, and the image of Sparassodonta being a lineage of giant, predatory opossums made sense…and let’s be honest, it sounds badass. But, later studies have placed Sparassodonta as a separate metatherian entity, and currently they rest as stem-marsupials at best.

Many Sparassodonts were lean-snouted, weasel or dog-like predators, but there was quite a bit of diversity within the order, especially later on, during the Pliocene epoch (between about 5 and 2 million years ago). For example, there were the highly successful borhyaenids, which were short-legged, and yet often bear-like, bone-crushing monstrosities. And, most notably, there were the thylacosmilids…the ‘marsupial sabertooths’.

Thylacosmilus atrox, the largest and most well-studied member of the ‘marsupial’ sabertooth family is an eerily textbook example of convergent evolution. Convergent evolution refers simply to two or more groups of organisms evolving the same trait despite having differing lineages. An often used example of this is a comparison between the body shape of dolphins, sharks, and the extinct icthyosaurs. Dolphins are mammals, sharks are fish, and icthyosaurs are reptiles…and yet they have all converged on the most efficient body plan for being fast predatory animals in the water; streamlined body, rigid pectoral fins, tail flukes/fins, and even a dorsal fin. Gliding membranes have developed independently in flying squirrels (rodents), sugar gliders (marsupial possums), and colugos (primate-like animals in Southeast Asia). There are multiple extant groups of ‘anteaters’ and ‘termite eaters’ that all have similar adaptations (long snouts and tongues, reduced teeth, strong clawed forelimbs); true anteaters of South America, pangolins in Africa and Asia, numbats (marsupials) in Australia, aardvarks in Africa, echidnas in Australasia, armadillos in the Americas, and the sloth bear of Southern Asia.

Convergent evolution runs rampant on our planet, funneling organisms into the most efficient forms for a given occupied niche. The sabertooth strategy (precise laceration of major arteries using huge, fragile canine teeth) sits at the bottom of one of these funnels, as there have been sabertoothed cats, ‘false sabertooth’ nimravids (somewhat close relatives of cats), reptilian ‘gorgonopsids’ that pre-dated the dinosaurs, and of course…Thylacosmilus…the ‘pouched saber.’

Thylacosmilus was roughly the same size as a small lion or a jaguar, essentially tying with Australia’s marsupial lion for the title of largest metatherian carnivore ever. With a long, supple body and broad paws, it probably looked superficially cat-like…likely even more so than the marsupial lion. However, it lacked the retractable claws found in cats, and with its semi-opposable thumbs on the front feet and plantigrade (meaning that when walking, the whole foot presses against the ground, not just the toes, as in cats and dogs, which are ‘digitigrade’) locomotion it wouldn’t have the same ‘spring’ in its step. It likely plodded along silently and cautiously, not unlike the carnivorous opossum it was first suspected of being. Due to its feet, as well as its generally short legs and thickly muscled body, the marsupial sabertooth, like the marsupial lion, was not built for running down prey, and was also a likely ambush predator. It’s important to note that ‘real’ sabertooths are thought to have the same hunting strategy, as they were also too bulky to chase down prey for extended periods like a lion or cheetah.

When one gets to the head of Thylacosmilus, what at first seems to be a normal carnivore skull starts to morph into something far stranger.

There is a lot going on that is…’off’…with the head of Thylacosmilus.

First, those impressive saber teeth. Unlike ‘true’ sabertooths, the sabers of the ‘marsupial’ sabertooth grew throughout its lifetime, a bit like the teeth of rodents. Thylacosmilus had invested so heavily in this particular sabertooth-based killing strategy that the teeth had literally reshaped the skull of the animal, turning it into a misshapen hunk of bone. The saber teeth were so far embedded in the skull that the roots in the maxillae had extended up past nasal cavity, and up above the eyes and above the braincase…giving Thylacosmilus a curiously convex dome between its eyes. The intrusion of these teeth was so extensive that it obliterated any attachment points for any of the front top incisors, giving the marsupial sabertooth a decidedly unthreatening gap between its gigantic sabers.

Unlike many placental carnivores with saber teeth or powerful bites, there was no sagittal crest (a ridge running down the skull that allows jaw muscles to attach) on top of the skull. Instead, the back of the skull was a knotted burl of protuberances that provided muscle anchoring points that likely gave Thylacosmilus and impressive enough bite, but also, and more importantly, flexibility, strength, and finesse in the neck, as sabertoothed animals drive the force of the piercing blow from the neck and shoulders, not the bite itself.

Perhaps most strikingly were the two bony flanges running down the sides of the jaw, corresponding to where the sabers would lay when the mouth was shut. These would protect the delicate teeth by providing extra support in the event a substantial force hit the tooth from the side.

Yes. Thylacosmilus had goddamned biological scabbards for its saber teeth growing from the sides of its face.

Thylacosmilus also had somewhat smallish eyes completely encased in a ring of bone, which is very different from the cradle in which placental carnivore eyes sit.

Humans never saw this odd animal, as it is thought to have gone extinct shortly after the Isthmus of Panama formed, causing the Americas to be joined for the first time in tens of millions of years; this happened about two million years ago, long before our species even existed. The result of this joining of two very different lands was the Great American Interchange, in which various groups of organisms invaded/migrated across the isthmus, dramatically impacting ecosystems on either side, and forever changing the biogeography of the Western Hemisphere. Ground sloths, terror birds, porcupines, and armadillos invaded from the South. Otters, tapirs, horses, camelids (llamas, etc.), condors, and wolves invaded from the North.

Also along with the migration from North to South America were the sabertoothed cats.

It is thought that the extinction of Thylacosmilus was the direct result of sabertoothed cats like Smilodon coming into South America and completely outcompeting it by occupying precisely the same ecological niche. Two predators with the exact same hunting strategy, targeting the same types of prey, and utilizing the same tools, cannot coexist for long.

Why Thylacosmilus potentially lost out in this competition is unknown, but it may have something to do with all that specialization surrounding the saber teeth.
Thylacosmilus put all its evolutionary eggs in one basket; those perfected fangs. It invested so much into the saber teeth that it sacrificed any additional expansion of the forebrain because the roots of the fucking teeth were in the way. It’s possible that Thylacosmilus simply didn’t have the brainpower to compete against a predator like Smilodon, and the once grand advantage of having expensive, well-designed killing tools became a species-ending hindrance.

Specialization can only be supported in stable ecosystems. As soon as things change, it is always the species that are most burdened by their own perfected adaptations that perish.

This is partially the lesson all of these extinct metatherian megafauna had to learn. They were superbly adapted to their environments. Pouched perfection. The culmination of more than 120 million years of mammalian evolution. All of that crumbled when the floodgates were opened, and alien invaders, whether they were sabertooth cats or humans, swept in and ruined the party. But, this is how extinctions tend to happen on Earth. Lineages reach the dizzying heights of diversity and specialization, catastrophe hits, and it’s all over, and a new set of players takes up the mantle of the old ecosystem.
Although we lost some amazing, terrifying, and dumbfounding metatherian creatures to the permanence of extinction, there are still plenty of intriguing pouched animals alive today…and many of them go equally underappreciated as their long-gone megafaunal cousins…


© Jacob Buehler and “Shit You Didn’t Know About Biology”, 2012-2014. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Jacob Buehler and “Shit You Didn’t Know About Biology” with appropriate and specific direction to the original content.

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10 thoughts on “Metatherians (Part 1 of 2): Extinct Megafauna

  1. Thank you, this made me laugh out loud on a Friday afternoon, informative, and funny. Especially finding Gerard Diprotodieu, didn’t know it still existed. Also liked the Keas.

    x Anne

  2. Absolutely stunning article! I’m greatly interested in marsupial megaphauna and found here so much new and interesting! Thank you for writing and posting this. What do you think of supposed palorchestes depictions in Northern Australia’s rock art? Could they be real?

    • Thank you! :D
      Yes, I’ve heard of the supposed Palorchestes rock art. Honestly, I’m not convinced. The art is so vague that it could be any quadrapedal animal, including one that hasn’t been discovered yet that more accurately fits the figure drawn on the rock wall.

  3. Pingback: A hilarious and informative series on metatherians « Why Evolution Is True

  4. Thanks for your article Jake. I’m convinced that the Palorchestes rock art is actually a placental mammal. One possibility is a female Baba Rusa (pig-deer) from Indon islands to the north. They have only 2 teats as in the rock art and look very similar. Just what early immigrants would have brought with them as domestic stock (but not male BR’s)…perhaps why there are none in Oz now. Have you noticed the exquisite Thylacine head in the lower right quadrant? Almost like a photo. Also the salty in the upper left.

  5. Hi, love this page and often refer to it. One thing you may not know is that in Australia, we love teasing visitors with stories of dropbears – koala-like predators that drop out of trees to attack their prey. I think that dropbears may well be a kind of folklore, to warn us about the dangers of Thylacoleo. For more information on the dropbear, google Janssen 2012, Australian Geographer 43(4): 445-452.

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