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Final for Thylacosmilus atrox. In the 1920s Captain Marshall Field funded two expeditions to South America which were undertaken by the Field Museum of Natural History in Chicago, Illinois; these expeditions were launched under the hopes of finding fossils of mammals and other animals from the Cenozoic Era. In 1926, during the second expedition, a team was searching the Ituzaingo Formation in northern Argentina when they came across the remains of three animals thought to be never before discovered species of ancient marsupial. 

South America was prime real estate when it came to new discoveries due to it having been almost completely cut off from the rest of the world since the beginning of the Cenozoic and so expectations were high when it came to finding some uniquely adapted mammal specimens, it wasn’t until 1933 when paleontologist Elmer S. Riggs named and described the specimens found on the Marshall Field Expeditions that science would realize just how right they were about the mammals of that time and place.

Thylacosmilus (meaning: “Pouched Knife”), was a member of member of a group called Sparassodonta, once thought to be true marsupials it is now considered to be a closely related group. Despite their resemblance to later placental predators like Smilodon fatalis, they were not closely related to them at all. Their saber teeth are an example of convergient evolution, where different, unrelated species develop similar traits to fill similar niches. Unlike felids, it’s teeth grew backwards into their skull the older they got, they also did not possess retractable claws like most cats. Due to their short legs it’s likely T. atrox were ambush hunters.

It was roughly 4.9 feet in length and weighed around 330 pounds and it lived during the Late Miocene to Pliocene, roughly 9 to 3 million years ago.

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Since Sparassodonts such as Thylacosmilus are as far as we know, probably related to Marsupials, would they've had pouches and reproduce somewhat similar to them?

We don’t know for certain if sparassodonts and other non-marsupial metatherians had pouches. It’s definitely a possibility, but they could also have had a different strategy similar to some modern marsupials that don’t have true pouches:

  • Quolls only have pouches during the breeding season, developing them from folds of skin on the abdomen.
  • Numbats don’t have pouches at all, and just carry their joeys around dangling from their bellies.
  • Shrew opossums also don’t have pouches, but don’t constantly carry their young, instead leaving them behind in a nest.
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South America’s Saber-tooth

At first glance this weird mammal might look like a saber-toothed cat like the infamous Smilodon, but it is not a cat at all, but a large cat-like animal related to marsupials. Even more surprising this beast was not from Australia, but thrived in Paleo-Pliocene South America. Its large saber teeth have been noted as a great example of convergent evolution as these “marsupials” evolved separately from their felid friends up north. Several species that span six different families have been described, the most famous however was Thylacomilus atrox, which was a species found in Argentina. Thylacomilus had one of the longest canines out of all Sparassodonts, these teeth were so long that they had to be held in a special scabbard-like pouch on the lower jaw, which protected its teeth when its mouth was closed, giving the animal its name “pouch sabre”. Sparassodonts were one of the top predators of their time, but unfortunately they became extinct during the Pliocene. This is thought to be due to competition from actual saber-tooth cats like Smilodon that came down from North America during the Great American Interchange.

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