to tell from machairodonts. Several defining characteristics are a lack of a coronoid process, many sharp "premolars" more akin to pegs than scissors, and very long skulls. Despite their large canines, however, most gorgonopsians probably lacked the other specializations found in true saber-toothed predator ecomorphs. Two gorgonopsians, Smilesaurus and Inostrancevia, had exceptionally large canines and may have been closer functional analogues to later sabertooths. The second appearance is in Deltatheroida, a lineage of Cretaceous metatherians. At least one genus, Lotheridium, possessed long canines, and given both the predatory habits of the clade as well as the generally incomplete material, this may have been a more widespread adaptation. The third appearance of long canines is Thylacosmilus, which is the most distinctive of the saber-tooth mammals and is also easy to tell apart. It differs from machairodonts in possessing a very prominent flange and a tooth that is triangular in cross section. The root of the canines is more prominent than in machairodonts and a true sagittal crest is absent. The fourth instance of saber-teeth is from the clade Oxyaenidae. The small and slender Machaeroides bore canines that were thinner than in the average machairodont. Its muzzle was longer and narrower. The fifth saber-tooth appearance is the ancient feliform (carnivoran) family Nimravidae. Both groups have short skulls with tall sagittal crests, and their general skull shape is very similar. Some have distinctive flanges, and some have none at all, so this confuses the matter further. Machairodonts were almost always bigger, though, and their canines were longer and more stout for the most part, but exceptions do appear. The sixth appearance is the barbourofelids. These feliform carnivorans are very closely related to actual cats. The best-known barbourofelid is the eponymous Barbourofelis, which differs from most machairodonts by having a much heavier and more stout mandible, smaller orbits, massive and almost knobby flanges, and canines that are farther back. The average machairodont had well-developed incisors, but barbourofelids' were more extreme. The seventh and last saber-toothed group to evolve were the machairodonts themselves. Diet The evolution of enlarged canines in Tertiary carnivores was a result of large mammals being the source of prey for saber-toothed predators. The development of the saber-toothed condition appears to represent a shift in function and killing behavior, rather than one in predator-prey relations. Many hypotheses exist concerning saber-tooth killing methods, some of which include attacking soft tissue such as the