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About the size of a jaguar, the bizarre looking Pliocene Argentinian marsupial carnivore, Thylacosmilus, is a superb example of the the concept of convergent evolution -- descendents of unrelated organisms presumably under similar selection pressures "converging" on each other in form over time. Its skull looks remarkably like those of placental sabre-toothed cats, to which it was only distantly related. Key differences, however, did exist (e.g. long, rudder-like extensions of the chin protected the extremely long, dagger-like teeth).