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Hyaenodontidae (superorder Ferae, order Creodonta) An extinct family of carnivorous mammals that appeared early in the Eocene and flourished throughout most of the Tertiary. They were slimly built, with elongated jaws; probably they possessed claws, and many had well-developed carnassials, differing from those of modern carnivores in being formed from the first or second molars of upper and lower jaws (rather than from the upper premolar and lower molar). Some forms (e.g. Machaeroides and Apataelurus) had jaws similar to those of the later sabretoothed cats. Hyaenodonts ranged in size from animals the size of a weasel to some large enough to have preyed on large herbivores.