Colours of Wildlife: Thylacosmilus, the Sabretooth Sparassodont
Created | Updated May 3, 2020
Thylacosmilus, the Sabretooth Sparassodont
Willem is a wildlife artist based in South Africa. He says "My aim is simply to express the beauty and wonder that is in Nature, and to heighten people's appreciation of plants, animals and the wilderness. Not everything I paint is African! Though I've never been there, I'm also fascinated by Asia and I've done paintings of Asian rhinos and birds as well. I may in future do some of European, Australian and American species too. I'm fascinated by wild things from all over the world! I mainly paint in watercolours. . . but actually many media including 'digital' paintings with the computer!"
We stay with sparassodonts for a while. Refer to the Borhyaena article for more on sparassodonts … briefly, they were a group of exclusively South American predators that were close relatives of the modern marsupials. They went extinct mostly before the influx of 'modern' placental predators that happened when the land bridge between North and South America was completed.
Our sparassodont of the day is Thylacosmilus atrox, the name meaning 'atrocious knife pouch', one of the most amazing examples of convergent evolution ever. Convergent evolution, as you'll know if you've been reading these articles, is the phenomenon where not-closely-related species come to resemble each other because of being 'shaped' by evolution for similar lifestyles. In the case of Thylacosmilus, the resemblance is between itself and the so-called 'sabretooth cats'. It had the same kind of lengthened canine or eye teeth as the sabretooth cats did, with a very similar skull and body shape as well. It differed in a few respects, such as having a long tail (sabretooth cats all had very short tails) and in its limb proportions. But likely its hunting technique was almost exactly the same.
The sabretooth hunting technique is something that no present-day animal uses; sabretooths existed until very recently, but are now all gone. Sabretooths had jaws that could open very wide, to clear the long teeth, but as a result, they had weak jaw articulations and jaw muscles. The teeth other than the canines were typically reduced or even absent. The neck wass typically long and very strong. The body was compact and powerful, the limbs short and sturdy. From all this, and practical considerations, it would seem that the long sabre teeth were used for a quick kill, by opening the jaw very wide and then 'ramming' the teeth into the victim's flesh. The sabretooth might target its prey's neck or soft underbelly. But it must inflict a very deep and rapidly fatal wound. It can't hold on for long as the teeth are actually quite brittle and may break if the prey animal struggles with the teeth still embedded in it. The strong body is for grappling the prey and holding it steady long enough for delivering the killing bite. Because the sabretooth wasn't made for fast running, it likely set an ambush for its prey, rushing or leaping out as soon as it came within striking distance. Sabretooths seem to have gone for very large prey animals, like elephants and mammoths, bison or wild oxen, camels, rhinoceroses, horses and so.
While we mostly speak of sabretooth 'cats', there were many other sabretooth hunters with the same killing style. The first were among the so-called 'mammal-like reptiles' or pre-mammalian synapsids, the most diverse of which were the gorgonopsians. That was even before the first dinosaurs, so the lifestyle has a very ancient pedigree. During the age of mammals, sabretooths evolved from among the nimravids, the barbourofelids (close relatives of cats), the true cats, and the sparassodonts.
Now Thylacosmilus itself was one of the most extremely adapted of the sabretoothed mammals. Its sabre teeth were triangular in cross section, with a sharp rear edge maintained by the upper canine grinding against the lower as the mouth was opened and closed. The canines had extremely long roots embedded deep inside the skulls, and going back to above and behind the eyes. The canines kept growing throughout the animal's life. The lower jaw had huge and wide bony flanges to protect the upper canines when the jaw was closed; there was likely lip tissue that partially or fully 'sheathed' the canines when the mouth was closed. Thylacosmiluscould open its jaws wider than even the longest-toothed of the true sabretooth cats. Its cheek teeth were much reduced in size; unfortunately right now we don't know what its front or incisor teeth looked like since these are so far missing in all known fossils.
The rest of Thylacosmilus' body was powerful. It was long and rather low in build, with feet that trod with the soles flat on the ground. Reaching a weight of 80-150 kg/175-330 lbs, it was about as large as a jaguar. Muscles were concentrated on the front of its body, to hold struggling prey and absorb the impact when striking with its teeth. Its hind legs were strong too and it might have been able to rear up on them. Being a marsupial-relative, the female likely gave birth to very underdeveloped young which she subsequently nursed in a pouch like that of a kangaroo.
The fossils of Thylacosmilus were found in Argentina. It lived from the late Miocene into the Pliocene, 9-3 million years ago. The environment at the time was likely savannah-like, with open patches of grass as well as bushes and trees that it could use for hiding and stalking its prey. On the open grasslands the huge terror birds would have been the top predators, so that Thylacosmilus would have avoided them. Its prey would have included various large South American hoofed mammals like toxodonts and litopterns such as Macrauchenia. Its extinction happened just before the influx of true sabretoothed cats into South America, and was likely due to climate and habitat change.
