Saturday, 25 April 2015

Invasion of the Killer Wombats

Thylacoleo carnifex is a weirdo. Palaeontology contains a lot of weirdos (and I don’t just mean the specimens), with their duck bills, fractal body plans and ninja killer-thumbs. Thylacoleo looks like the love-child of a rat and a bear, conceived in the shadow of Fukushima. That’s the right side of the earth at least: this >100kg beastie is an Aussie through-and-through, a pouched mammal probably descended from the wombats but grown to the size of a leopard. Unlike them, it got big and it got nasty: carnivore nasty.

The skull is crazy: blunt, broad, with those goofy front teeth. And have you seen what’s going on in the cheeks? Did someone misplace their scissors? Those premolars project unusually far outwards (laterally) and are so straight and blade-like, with little to none of the bumpy dentition most of us are used to see when we look at a jaw.

Side view of the decidedly strange Thylacoleo. Note the unusual cheek
teeth. Image: Wikimedia commons.

 Oh, and did I mention the claws? Each front paw was armed with a massive, killy, death-thumb (that’s the scientific description of course). When you say “killer-wombat” you don’t expect something quite this impressive. Hilariously, this nasty weapon is known as a dew-claw, a dainty name for such a great bit of carnivore kit.

My favourite reconstruction of this beast, showing off those killer thumbs. Image by Adrie & Alfons Kennis via NatGeo.
Although I’d heard of it, this oxymoronic “marsupial lion” didn’t really grab my interest until I got to handle some of its cranium at the Natural History Museum in London recently when my supervisor and I were gathering data for my current research project. We decided to have a rummage in the drawers of the fossil mammal collection and found this feisty murderer lurking alongside some impressive whole-skull casts. This material was first described by Sir Richard Owen, so it was a real privilege to see it. My supervisor went nuts when she found the bones, and wisely grabbed the photo opportunity. She and a colleague have been working on one of the big puzzles of the extinct Australian mega-fauna: how did Thylacoleo kill and eat its prey?

My supervisor and I handle some nasty wombat at the Natural History Museum, London..
Up until the mid 1900s scientists argued that it was a bad-ass herbivore, or at worst a scavenger of carrion and the odd egg or two. Since then, in-depth analysis of the function of the skull and dentition point to it being carnivorous, as do microwear studies of the teeth (Wells et al. 1982 if you are interested, though you'll struggle to get hold of it). Carbon and nitrogen isotopes, and strontium and zinc levels both supported a flesh-based diet. Wroe (2008) reconstructed the bite forces of the skull and concluded it had a bite approximately 80% the strength of a lion’s – pretty impressive for a giant wombat. The odd elongated back teeth seemed to be especially strong, and so the suggestion was made that Thylacoleo used them to slice and cut into its prey; but was that to kill the animal, or just to eat it?

Thylacoleo compared with a lion using finite element analysis (FEA). In this instance, the test is for strain when pulling back with the head. Note the low stress on the cheek teeth of Thylacoleo. Wroe (2008).

Palaeontologists love an analogue. The problem with analogues is that they tend to come in packages, forcing you to accept all or nothing. And so the disagreements about Thylacoleo’s killing modes began. Did it kill like the modern big cats: using those ratty incisors like a cat’s canines to smother or suffocate the prey? Or was it like a sabre-tooth cat, holding an animal down and perhaps pulling out chunks of flesh to cause massive blood loss?
This leopard goes in for a throat bite to suffocate the gazelle. It grips with the biting canines, and afterwards will process the meat with those shearing carnassials. Images: Wikimedia Commons.
Big cats, with the exception of the cheetah, have some twisting movement in the forelimb so that they can hold their prey while they suffocate it with their jaws. This is in contrast with animals like dogs, which have very little twisting, and instead rely on delightful methods such as ripping the poor animal to bits while it is still alive (I’m more a cat person to be honest). Both of these methods hinge on the well-known arrangement of teeth in the Carnivoran skull: the biting canines and the shearing carnassials. But Thylacoleo didn’t have the same arrangement of teeth, so could it have killed like a modern member of the Carnivora? And what about the massive killer death-thumbs, were they just to pin animals down, or could it have used them to slash then open?
Thylacoleo probably ate a lot of kangaroo, like this
Procoptodon. Image: Mauricio Anton
Having read through several arguments, my main thought is that we are limiting ourselves with these analogues. The idea that it used rippy death-thumbs to kill appeals to my desire for us to think differently about animals that are physically very different from any modern species, but having said that, nature has a tendency to repeat itself so it is sensible science to use modern species as benchmarks for understanding long lost creatures. The problem comes when, for example, we say an animal like Thylacoleo couldn’t have suffocated its prey with a throat bite because it didn’t have modern-carnivoran-like canines. We can’t be sure that it didn’t just do it differently.

There is something in palaeontology called many-to-one mapping. Simply put, it suggests that features that differ between species could serve the same function (many features, one function). Conversely, one-to-many mapping is the model that explains instances when the same physical features can carry out different functions in different species (one feature, many functions). How can we identify which model is correct in any given scenario? Sometimes we can’t, at least not with the evidence as it stands. With luck and time, eventually enough information will be available, or new methods developed to allow us to remove some of the options and whittle it down until we are confident we have the ‘right’ answer.

Complete Thylacoleo skeleton from Nullarbor cave. This kind
of find is rare and can tell us a lot about an extinct animal. Image:
Western Australia Museum
My personal opinion as things stand is that this giant antipodean wombat-zilla pinned its prey down – the claws giving extra support to compensate for the differently arranged teeth – and bit into the neck like a modern cat - perhaps with a bit of flesh-chunk removal. It then ripped into its dinner, using both the rat-teeth and the scissor teeth in its cheeks to slice the flesh.

But that’s just my theory. For now, Thylacoleo remains a mystery. 

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