From didelphid ancestors certain South Americanmarsupials specialized into aggressive carnivores during Tertiary times. These were the borhyaenids, of which the Miocene genus Borhyaena was typical and resembled a large wolf. The skull was very dog-like, with the canines enlarged as piercing and stabbing teeth, and some of the molars modified into shearing blades. The body was long, limbs exceptionally strong and the feet were equipped with exceptionally sharp claws. Others such as Thylacosmilus was as large as a tiger; possessed a short skull and tremendously elongated bladelike upper canine tooth, whereas in the lower jaw there was a deep flange of bone to protect this tooth when the mouth was closed. These carnivorousmarsupials became extinct with the influx of placentalcarnivores from North America.