Latest Arikareean sediments of the upper Arikaree Group in southeastern Wyoming produced rare fossils of large, early Miocene amphicyonid carnivorans for field parties of the Frick Laboratory, American Museum, from 1932 to 1940. Recent geologic field investigations, including mapping, have discovered additional remains of these carnivorans, clarifying their geographic and stratigraphic distribution, and permitting a more informed description of the earlier collections.
These carnivorans come from tuffaceous sandstones of the Upper Harrison beds, the terminal formation-rank unit of the Arikaree Group in southeastern Wyoming. A large species of the amphicyonid Daphoenodon (D. falkenbachi, n. sp.) occurs in northern Goshen and southeastern Platte Counties, Wyoming, and in the Niobrara Canyon, Sioux County, Nebraska. A smaller species of this genus (D. skinneri, n. sp.), probably ancestral to D. falkenbachi, is known from a single individual from southern Niobrara County, Wyoming. A third amphicyonid, Adilophontes brachykolos, n. gen., n. sp., is closely related to Daphoenodon, and is reported from northern Goshen County and east-central Platte County, Wyoming. The three species share a similar basicranial anatomy, including the form of the auditory bulla, and can be referred to the endemic North American amphicyonid subfamily Daphoeninae. They are found only in Upper Harrison strata in southeastern Wyoming and northwest Nebraska, and are presently unknown elsewhere in North America.
Postcranial remains indicate that these large predators lacked the extreme digitigrade specializations of the feet and limbs encountered in living canids and large felids. They are characterized by relatively short lower-limb segments, and paraxonic feet with slightly spreading digits, which might be termed subdigitigrade. These skeletal traits and their carnassiform dentitions with prominent canines, premolars, and shearing carnassials indicate a carnivorous diet probably obtained by a rapid rush from cover and a short, powerful pursuit of ungulate prey in open grassland and riparian stream settings east of the Rocky Mountain uplifts.
A new formal term, Anderson Ranch Formation, is proposed as a replacement for the Upper Harrison beds of Peterson (1909) that yielded the carnivores discussed in this study.