Some animals are not as weird as we often think they are (zebras, for example, are really just horses with stripes, a not-uncommon color pattern among mammals). Some animals are far stranger than people tend to realize (elephant trunks are like no other structure that has ever evolved). Among my favorite animals, though, are those that people tend to recognize as being strange but that are, in fact, even more bizarre than they might appear at first glance. The prime example among living animals is the platypus, which is aberrant in almost every single way possible. This month's featured fossil vertebrate provides a nice paleontological example of an animal who's outer coating of weirdness just hides more profound weirdness underneath. Thylacosmilus atrox lived in the savannas of South America in the Mio-Pliocene of South America. A reasonably well-known fossil mammal, it is often referred to as the marsupial sabertooth, a name that highlights the two superficially strange things about it. Thylacosmilus had among the largest sabers of any predator ever to have lived, and this trait has drawn comparisons to saber-toothed cats and to the cat-like Barbourofelis. Unlike cats and barbourofelids, though, Thylacosmilus was not a carnivoran. In fact, it wasn't even a placental mammal, but rather a metatherian more closely related to kangaroos and opossums than to cats. So, was Thylacosmilus a marsupial that evolved convergently with cats on a continent without carnivorans? This is certainly how it is often described and depicted (perhaps most notably by the great Czech paleoartist Zdenek Burian), but this view of Thylacosmilus does not do its weirdness justice. To begin with, it wasn't actually a marsupial, but a member of the Order Sparassodonta a group of stem metatherians endemic to South America. This means that, while Thylacosmilus was more closely related to marsupials than to placentals, it and its sparassodont kin were only distantly related to both groups. Secondly, while Thylacosmilus certainly had its share of cat-like features, it was not a perfect felid analog. As Julie Meachen has shown, there is more than one way to be a sabertooth: some carnivorans have sturdy sabers and gracile arms, while some have long, delicate sabers and robust arms, and some (such as Xenosmilus) don't fall neatly into either category. With its long, thin sabers, it is tempting to think of Thylacosmilus as the South American version of Smilodon or Barbourofelis, but does the rest of the body follow suit? As mentioned in a previous post, my student Laurel Perper has been looking into this question by visiting the type specimen of T. atrox at the Field Museum. Her results show that some aspects of the forelimbs of Thylacosmilus are similar to those of saber-toothed cats, but the robustness of its humerus turned out to be literally off the charts, meaning that Thylacosmilus had arms so beefy that they compare better to those of bears than to even the burliest of cats. So why is Thylacosmilus so similar to cats in some ways but so different in others? Is it because it was descended from a very different group of mammals, from which it would have inherited a very different "evolutionary toolkit" than did cats? Were differences in the environment and climate between North and South America responsible for the weirdness of Thylacosmilus? Were its robust arms an adaptation to preying upon the unique megafauna of the Pampas, which would have included armored glyptodonts, enormous toxodonts, and sloths and rodents orders of magnitude larger than their living relatives? Perhaps Thylacosmilus evolved as it did because of competition with other South American predators, most notably the giant "terror birds." Between the measurements Laurel has made at the Field Museum and those that I intend to make in my upcoming trip to South America, my students and I are hoping to have sufficient data to be able to start answering these questions soon, so if they piqued your curiosity as much as they did mine, stay tuned!