Borophagine canids of the Monarch Mill Formation (Middle Miocene), Nevada, U.S.A.

The non-marine Middle Miocene Monarch Mill Formation (Nevada, U.S.A.) outcrops in the Middlegate Basin and preserves fossil mammals that lived during the establishment of the present-day basin and range topography in central Nevada. The Monarch Mill Formation and its mammal fauna overlies the Middlegate Formation and its flora, and together the biota provides an informative window into ecosystem composition at the geographic crossroads between the Great Plains and the coastal regions of North America during the Middle Miocene. Here we report on previously undescribed and/or unfigured canids from the Monarch Mill Formation. To the previously known borophagine Tomarctus brevirostris and the fox-like canine Leptocyon we add two borophagine genera to the Eastgate Local Fauna of the Monarch Mill Formation. Specimens of the hypocarnivores Paracynarctus and Cynarctus are described and provide an enhanced understanding of the Middlegate Basin canid assemblage, now comprising at least four genera. Together with floral evidence, this canid assemblage is indicative of mesocarnivore-sustaining vegetation and locally limited open environments, with no hypercarnivorous canid occurrences. Rapid regional subsidence was an overarching factor in the shift towards present-day hyper- and meso-carnivore dominated canid assemblages in the Basin and Range Province, and the appearance of hypercarnivores in post-Barstovian times may reflect contemporaneous regional topographic shifts in the Great Basin at large.