New carnivoraforms (Mammalia) from the middle Eocene of California, USA, and comments on the taxonomic status of Miacis gracilis

The middle Eocene constitutes an important period for understanding the early evolution of carnivoraforms and the origin of crown-group carnivorans. Here I describe two new genera of carnivoraforms from the Uintan North American Land Mammal Age of southern California, and report hitherto-undescribed specimens of Procynodictis progressus, 'Miacis' gracilis, 'Miacis' hookwayi, and two undeterminate taxa to bridge some of the gaps in taxonomic knowledge. Specifically, the fossil materials described here support the placement of P. vulpiceps and P. progressus in the same genus but suggest their distinct identities at the species level. Similarly, 'M.' gracilis, which has been synonymized with P. vulpiceps by some authors, is considered a separate species from both P. vulpiceps and P. progressus. A minimum of 11 carnivoraform taxa are recognized in the middle Eocene of southern California, including nine from the late Uintan to the earliest Duchesnean (ca. 43-41 Ma ago). Of the latter, at least six taxa are apparently endemic to this region, thus conforming to the high provinciality of North American mammalian faunas during this time. In addition to the carnivoraforms, four taxa of creodonts and a mesonychid are known. The notably high taxonomic richness of middle-Eocene mammalian carnivores in southern California is associated with high diversity of non-carnivorous mammals known from there; this pattern agrees with the correlation between mammalian predator and prey diversity that has previously been observed at various spatiotemporal scales. The stratigraphic distributions of the late-Uintan taxa reported here are consistent with the recently revised ages of vertebrate assemblages from the Sespe Formation of Ventura County, California, in the parsimonious sense.