extant viverrids. Larger forms had greater shearing capacity and the capacity increased over time. Arfia, one of the most common carnivorous mammals in early Eocene North America, developed a more open trigonid on M3 over the course of the Early Eocene, increasing the shearing ability of the carnassials. A similar development can be seen by comparing Oxyaena, Prototomus and Limnocyon with the smaller, more generalized feeders among the creodonts. Extinction Several theories have suggested that hyaenodonts and oxyaenids became extinct because they were outcompeted by the newly-evolved Carnivora. However, there is no direct evidence that the existence of large Carnivora caused the extinction of these taxa, and in some cases (for instance in Africa), hyaenodonts thrived in environments in which large carnivorans were also present. Theories that suggest they were outcompeted by the Carnivora include that their smaller brains limited their intelligence. Other speculations focus on their limb structure, which limited leg movement to a vertical plane, as in ungulates; they were unable to turn their wrists and forearms inward to trip, slash, or grab prey as modern carnivores can. Creodonts had to depend entirely on their jaws to capture prey, which may be why creodonts generally had a larger head size in relation to their bodies than carnivores of similar stature. In the Carnivora, the last upper premolar and the first lower molar are the carnassials, allowing the rearmost molar teeth to evolve adaptations for feeding on non-meat foods. In creodonts, either the first upper and second lower molars, or the second upper and third lower molars, were the primary carnassials, and the rear teeth formed a carnassial series. This structure committed them to eating meat almost exclusively, which may have limited their ability to exploit mesocarnivore and omnivore ecological niches. These differences may have caused environmental changes to affect hyaenodonts and oxyaenids differently than modern carnivores. References Further reading Cenozoic mammals of Europe Obsolete mammal taxa Tortonian extinctions Paleocene first appearances Fossil taxa described in 1875