In the early 1980s, paleontologists studying the fossils of giant kangaroos and rhino-sized wombat-relatives from the Pleistocene epoch of Australia found something odd.

Some of the bones bore deep, slicing bite marks, unlike those made by any living predator or scavenger.

The researchers realized that only one Australian animal from that time period had teeth capable of leaving such distinctive damage.

It was the biggest and most powerful predatory marsupial ever known to have existed - one without any parallel in its environment today: the marsupial lion, Thylacoleo carnifex.

This creature was so fearsome that the first scientist to describe its fossils called it "one of the fellest and most destructive of predatory beasts.” But while most mammalian apex predators around the world evolved from a long line of already deadly carnivores, Thylacoleo and its relatives had a much more humble origin.

Because here, in isolation from the rest of the world, evolution built a family of deadly predators by taking a group of cute, harmless herbivores and turning them murderous.

Typical Australia.

Now, it’s basically a running joke at this point that Australia’s ecosystems are particularly strange and distinct from most of the rest of the world, due to its long isolation.

And one especially unusual thing is that, today, the whole place seems to be missing big, apex predator mammals.

Generally, in the rest of the world, these sit near or at the top of the food chain, and prey on medium and large-sized herbivores.

But while Australia has plenty of those ideal potential prey, there are no big, carnivorous mammals to hunt them.

It’s an ecological void that simply isn't being filled by any of the continent’s native marsupials.

But it’s now clear that this vacancy is actually only a very recent development in the evolution of Australia’s ecosystems.

Because numerous fossil finds over nearly two centuries tell of a time when that now-missing mammalian apex predator once ruled the continent.

Thylacoleo carnifex translates to “meat-cutting marsupial lion.” It was named by British naturalist Richard Owen in 1859, based on fossils that were sent to him.

He was only working with a damaged skull and some teeth, and it would take over a century for us to fully reconstruct the rest of its body.

But those partial remains were enough to convince Owen that this was the continent's missing mammalian apex predator.

The fossils did take him a little by surprise, though, because the mouth of the marsupial lion was highly specialized and had an unusual dental set-up.

See, in most of the rest of the world, predatory mammals – which are mostly in the order Carnivora – have small incisors and big canines.

But Thylacoleo had gone in the complete opposite direction on its journey to carnivory.

Its upper canines were small and non-functional, and it had lost its lower ones entirely.

And its incisors were huge – more like those of a rodent than a lion – but they might’ve filled a similarly stabby role to the canines of carnivorans.

The rest of its dentition was made to shear through flesh, too, like specialized premolars that resembled bolt cutters – kinda like the extreme version of the scissor-like carnassial teeth of carnivorans.

And in the years since Owen first formally described the species, researchers have actually found that it was probably even more ‘fell and destructive’ than he initially realized.

Because, based on the structure of its skull and jaws, it’s been estimated to have had the strongest bite force of any mammal relative to its size.

And this is even more impressive considering that we now also know that it’s not really related to any of the predatory Australian marsupials that you might’ve heard of, like the Tasmanian devil or the now-extinct thylacine.

Those and other smaller meat eaters are part of one order of marsupials called Dasyuromorphia.

But the family Thylacoleo belonged to stemmed from a completely different marsupial group, instead – a group that is otherwise totally herbivorous, called the Vombatiformes.

This group includes the closest relatives of Thylacoleo’s family, the koalas and wombats.

So it looks like the family of marsupial lions descended from a lineage of ancient proto-wombats that were almost certainly small, harmless plant-eaters, too.

But how did this big evolutionary change happen?

How does something like a wombat become an apex predator?

While Thylacoleo carnifex was the first member of the marsupial lion family to be discovered, the fossil record of this family stretches back tens of millions of years.

Smaller-sized relatives in the genus Wakaleo, or ‘little lions,’ first appeared in the Late Oligocene epoch, around 25 million years ago.

They were probably arboreal hunters that chased prey through the treetops.

With little to no mammalian competition for those predatory niches, this new way of life allowed early members of this family to thrive and diversify.

But while species of Wakaleo gradually increased in size over the course of the Miocene epoch, probably mirroring increases in prey size during this period, some of their relatives actually went in the opposite direction… Like Microleo attenboroughi from around 18 million years ago, which weighed in at only 600 grams.

It was first discovered in 2016 and was described by one of the researchers as “the cute, but still feisty kitten of the family.” Microleo shared the rainforests of the Australian Miocene with larger relatives the size of dogs and cats, but they all probably specialized on different prey at different levels in the rainforest canopy.

But these humid, heavily forested ecosystems gradually diminished through the Miocene and into the Pliocene epoch as Australia became drier over time.

And this led to the extinction of many of  the species that specialized  in those environments.

The genus Thylacoleo alone was able to survive through the Pliocene into the Pleistocene epoch, where it eventually gave rise to Thylacoleo carnifex – the last and largest member of its family, which survived until around forty thousand years ago.

As the biggest of the marsupial lions, Thylacoleo carnifex was uniquely suited to feasting on the grazing megafauna that roamed Pleistocene Australia.

So, as we’ve seen over and over in natural history, if there’s an open ecological niche, even the most unassuming group can end up evolving to fill it if they find themselves in the right place at the right time.

This ecological opportunity drove the evolution of the marsupial lions, resulting in their strange patchwork of carnivorous traits that culminated in Thylacoleo carnifex.

And while its skull and teeth alone told Owen that the missing apex predator had been found, the rest of its skeleton being incomplete meant that questions about how it actually lived were hotly debated for a long time.

Details about an extinct animal’s paleobiology are often very tricky to figure out, especially if we only have partial remains and no living analogues to compare it to.

But thanks to the discovery of several more complete specimens found in a series of caves over the last couple of  decades, in 2018 researchers  unveiled the first full skeletal reconstruction of Thylacoleo carnifex.

It had a few superficial similarities with big cats, like its overall size, which put it somewhere between a leopard and a female lion, along with its short-snouted skull and forward-facing eyes.

But their analysis of its overall skeleton - including its rigid back and stiff tail - suggested that, in many ways, it was actually built more like a giant Tasmanian devil than a big cat.

Rather than being a slender, fast runner that chased down its prey, it was a stocky and heavily built ambush predator and scavenger, also like the Tasmanian devil.

And the structure of its strong forearms and semi-opposable thumbs with retractable claws suggest that it would've been capable of grappling with large, struggling prey, while also being a pretty good climber, too.

We’ve even found claw marks of juveniles on steep rock surfaces near the inner entrance of a cave in southwest Australia.

These claw marks are consistent only with the size and shape of young marsupial lions, and confirm that they were excellent climbers, even in the dark.

Plus they tell us that,  like many other carnivores,  they seem to have reared their young in caves at least some of the time, probably leaving them sheltered there as the parent went off to hunt or scavenge.

So what happened to the last of the marsupial lions?

Well, like so many of the continent's megafauna that also went extinct toward the end of the Pleistocene, it's hard to tell for sure.

Pressure from humans may have played a role, along with the continuing trend towards increased aridity and the shrinking of forest habitats.

So perhaps with fewer big prey animals available and with less forest cover from which to ambush them, it, too, eventually died out.

But the story of Thylacoleo carnifex and its relatives shows us that inside even the least likely species is the evolutionary potential to become an apex predator, given enough time and the right selective pressures.

Since that niche is now empty once again, who knows what unassuming mammal might rise to become the continent's next most fell and destructive predatory beast, millions of years from now...