The lowest part of the Monarch Mill Formation in the Middlegate basin, west-central Nevada, has yielded a middle
Miocene (Barstovian Land Mammal Age) vertebrate assemblage, the Eastgate local fauna. Paleobotanical evidence
from nearby, nearly contemporaneous fossil leaf assemblages indicates that the Middle Miocene vegetation in the area
was mixed coniferous and hardwood forest and chaparral-sclerophyllous shrubland, and suggests that the area had been
uplifted to 2700–2800 m paleoaltitude before dropping later to near its present elevation of 1600 m. Thus, the local
fauna provides a rare glimpse at a medium- to high-altitude vertebrate community in the intermountain western interior
of North America. The local fauna includes the remains of fish, amphibians, reptiles, birds, and 25 families of mammals.
Carnivorans, the focus of this study, include six taxa (three of which are new) belonging to four families. Canidae
are represented by the borophagine Tomarctus brevirostris and the canine Leptocyon sp. indet. The earliest record and
second North American occurrence of the simocyonine ailurid Actiocyon is represented by A. parverratis sp. nov. Two
new mustelids, Brevimalictis chikasha gen. et sp. nov. and Negodiaetictis rugatrulleum gen. et sp. nov., may represent
Galictinae but are of uncertain subfamilial and tribal affinity. The fourth family is represented by the felid Pseudaelurus
sp. indet. Tomarctus brevirostris is limited biochronologically to the Barstovian land mammal age and thus is consistent
with the age indicated by other members of the Eastgate local fauna as well as by indirect tephrochronological dates
previously associated with the Monarch Mill Formation. Actiocyon parverratis sp. nov. extends the temporal range of the
genus Actiocyon from late Clarendonian back to the Barstovian. The Eastgate local fauna improves our understanding
of mammalian successions and evolution, during and subsequent to the Mid-Miocene Climatic Optimum (~14–17 Ma).
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