BIOCHRONOLOGIC CONTEXT

Faunal assemblages included in this study are arranged biochronologically in Figure 19. Ages are after Tedford et al. (2004); Nebraska faunal age relationships after Voorhies (1990a; 1990b). Further informal subdivisions of the late Barstovian (Ba2a, Ba2b, and Ba2c) based on previously identified biozones are described below. "Primary faunas" are previously published and/or have well-defined external age control.

Barstovian. The oldest faunal assemblage included in this study is the Kleinfelder Farm locality, Wood Mountain Fauna. Storer (1975) recognized elements of the Wood Mountain Fauna in common with the early Barstovian Lower Snake Creek Fauna of western Nebraska as well as with the somewhat younger Norden Bridge Fauna (lowermost Valentine Formation, Niobrara River valley, north-central Nebraska), and considered the Wood Mountain Fauna to be "just older than the Norden Bridge local fauna" (p. 127). Tedford et al. (2004) further noted a close resemblance of the Wood Mountain Fauna to the Keota Fauna of the upper Pawnee Creek Formation in northeastern Colorado (14-14.5 Ma). The Wood Mountain Fauna is here considered earliest late Barstovian in age (informally designated Ba2a, Figure 19). This interval is sparsely represented in northeastern Nebraska by the Miller Creek Fauna (Voorhies 1990a).

The Norden Bridge Quarry Fauna and Nebraska faunas of equivalent age from the Cornell Dam Member of the Valentine Formation (Achilles, Lost Duckling, and Egelhoff Quarries; Voorhies 1990a, 1990b), are slightly younger than the Wood Mountain and Keota Faunas by as much as 1 Ma, based on ages of 13.5-13.6 Ma on the Hurlbut Ash within the Cornell Dam Member (Tedford et al. 2004). The Norden Bridge Quarry Fauna, as well as the faunal assemblage from Immense Journey Quarry (southwestern Nebraska) also dates to this interval (Voorhies et al. 1987), here informally designated as Ba2b in age (Figure 19).

The Valentine Railway Quarries Fauna in the Crookston Bridge Member of the Valentine Formation represent the youngest informal division of the late Barstovian used here, Ba2c (Figure 19). In north-central Nebraska, the Crookston Bridge Member lies in superposition above the Cornell Dam Member of the Valentine Formation, and the Valentine Railway Quarries Fauna is constrained below by the 13.5-13.6 Ma Hurlbut Ash and above by the ~12.2 Ma Swallow Ash in the lower part of the overlying Ash Hollow Formation (Tedford et al. 2004). Vertebrate fossils from individual Valentine Railway Quarries are derived from the same paleochannel deposits within the Crookston Bridge Member (Voorhies and Timperley 1997). The faunal assemblages from Garner Bridge South locality and Myers Farm are considered equivalent in age to the Valentine Railway Quarries (B.E. Bailey personal commun., 2002; Voorhies 1990a respectively). The Cunningham Hill Fauna, Colter Formation, western Wyoming, has rodent and horse taxa in common with both the Norden Bridge and Valentine Railway Quarries Faunas (Barnosky 1986) and cannot be further constrained from Ba2b and/or Ba2c in age.

Clarendonian. Tedford et al. (2004) reported an Ar-Ar age date of 11.58±0.05 Ma for a tuff just beneath the small mammal locality of Fish Lake Valley, placing it within the first half of Cl2. Tedrow and Robison (1999) referred specimens from the Scrappy Hill faunal assemblage in the Salt Lake Formation, northwest Utah, to H. galbreathi; the presence of H. galbreathi was used by the authors to infer an approximate correlation with the Fish Lake Valley Fauna.

The faunal assemblage from Tiensvold Ranch locality, collected from Ash Hollow Formation sediments in Sheridan County of northwestern Nebraska, is undescribed but a faunal list was compiled by Bair and Voorhies (1998) under the name 'Gordon Fish Site' (fish are the most abundant fossil vertebrate known from the locality). All large mammalian species present at Tiensvold Ranch are present in the Minnechaduza Fauna described by Webb (1969, p.18-19) indicating an early Cl2 age, constrained by age dates of the Swallow Ash at ~12.2 Ma below and Davis Ash at 11.6 Ma above (Tedford et al. 2004). The faunal assemblage collected directly below the volcanic ash bed at Poison Ivy Quarry, northeastern Nebraska, is dated to 11.93 Ma by Perkins (1998), also early Cl2 in age (Figure 19). The geologically youngest specimens of Hesperolagomys studied are from the late Clarendonian Pratt and Bluejay Quarries in the Merritt Dam member of the Ash Hollow Formation of north-central Nebraska (Cl3 of Tedford et al. 2004).