Saber-toothed mammals, now all extinct, were cats or “cat-like” forms with enlarged, blade-like upper canines, proposed as specialists in taking large prey. During the last 66 Ma, the saber-tooth ecomorph has evolved convergently, at least in five different mammalian lineages across both marsupials and placentals.
Thylacosmilus atrox, the so-called “marsupial saber-tooth,” is often considered as a classic example of convergence with placental saber-tooth cats such as Smilodon fatalis. However, despite its superficial similarity to saber-toothed placentals, T. atrox lacks many of the critical anatomical features related to their inferred predatory behavior—that of employing their enlarged canines in a killing head strike.
Thylacosmilus had massive, ever-growing canines, was considered as a considerably more horrendous predator than the placental carnivores it superficially took after, for example, Smilodon.