Thylacosmilus was a carnivorous marsupial that lived in South America during the Miocene and Pliocene epochs, approximately 9 to 3 million years ago. It had long, curved upper canines, reminiscent of saber-toothed tigers, but it was not a feline, but rather a marsupial mammal.

Its skull featured bony sheaths in the lower jaw that protected prey, a unique trait among mammals. Its body was robust, with short, muscular legs, adapted for ambushes and quick, close-range attacks rather than long chases.

At about 1.2 meters long and weighing close to 100 kilograms, Thylacosmilus was one of the main apex predators in South American ecosystems before the arrival of large carnivores from the north.