Illustrator signature must be included in all reproductions Thylacosmilus, illustration. This extinct prehistoric mammal was a sabre-toothed metatherian (marsupial). Its fossils are found in what is now Argentina. It lived between 9 and 3 million years ago during the Late Miocene and Pliocene. It's continually growing canine teeth were protected by bony protrusions of the lower jaw. It was similar in size to a jaguar.