Hyaenodon
Hyaenodon ("hyena-tooth") is an extinct type genus of carnivorous mammals from tribe Hyaenodontini within subfamily Hyaenodontinae (in family Hyaenodontidae) that lived in Eurasia and North America during the middle Eocene to early Miocene, existing for about . Description. Typical of early carnivorous mammals, individuals of "Hyaenodon" had a very massive skull, but only a small brain. The skull is long with a narrow snout - much larger in relation to the length of the skull than in canine carnivores, for instance. The neck was shorter than the skull, while the body was long and robust and terminated in a long  tail. Compared to the generally larger (but not closely related) "Hyainailouros", the dentition of "Hyaenodon" was geared more towards shearing meat and less towards bone crushing. Some species of this genus were among the largest terrestrial carnivorous mammals of their time; others were only of the size of a marten. Remains of many species are known from North America, Europe, and Asia. The average weight of adult or subadult "H. horridus", the largest North American species, is estimated to about and may not have exceeded . "H. gigas", the largest "Hyaenodon" species, was much larger, being and around . "H. crucians" from  the early Oligocene of North America is estimated to only . "H. microdon" and "H. mustelinus" from the late Eocene of North America were even smaller and weighed probably about . Tooth eruption. Studies on juvenile "Hyaenodon" specimens show that the animal had a very unusual system of tooth replacement. Juveniles took about 3–4 years to complete the final stage of eruption, implying a long adolescent phase. In North American forms, the first upper premolar erupts before the first upper molar, while European forms show an earlier eruption of the first upper molar. Paleoecology. The various species of "Hyaenodon" competed with each  other and with other hyaenodont genera (including "Sinopa", "Dissopsalis", and "Hyainailurus"), and played important roles as predators in ecological communities as late as the Miocene in Asia and preyed on a variety of prey species such as primitive horses like "Mesohippus", early camels, oreodonts and even primitive rhinos. Species of "Hyaenodon" have been shown to have successfully preyed on other large carnivores of their time, including a nimravid ("false sabertooth cat"), according to analysis of tooth puncture marks on a fossil "Dinictis" skull found in North Dakota. In North America the last "Hyaenodon", in the form of "H. brevirostrus", disappeared  in the late Oligocene. In Europe, they had already vanished earlier in the Oligocene. Classification and phylogeny. Phylogeny. The phylogenetic relationships of genus "Hyaenodon" are shown in the following cladogram