Geochronological and Taxonomic Revisions of the Middle Eocene Whistler Squat Quarry (Devil’s Graveyard Formation, Texas) and Implications for the Early Uintan in Trans-Pecos Texas

Abstract

The Whistler Squat Quarry (TMM 41372) of the lower Devil’s Graveyard Formation in Trans-Pecos Texas is a middle Eocene fossil locality attributed to Uintan biochronological zone Ui1b. Specimens from the Whistler Squat Quarry were collected immediately above a volcanic tuff with prior K/Ar ages ranging from ∼47–50 Ma and below a tuff previously dated to ∼44 Ma. New 40Ar/39Ar analyses of both of the original tuff samples provide statistically indistinguishable ages of 44.88±0.04 Ma for the lower tuff and 45.04±0.10 Ma for the upper tuff. These dates are compatible with magnetically reversed sediments at the site attributable to C20r (43.505–45.942 Ma) and a stratigraphic position above a basalt dated to 46.80 Ma. Our reanalysis of mammalian specimens from the Whistler Squat Quarry and a stratigraphically equivalent locality significantly revises their faunal lists, confirms the early Uintan designation for the sites, and highlights several biogeographic and biochronological differences when compared to stratotypes in the Bridger and Uinta Formations. Previous suggestions of regional endemism in the early Uintan are supported by the recognition of six endemic taxa (26% of mammalian taxa) from the Whistler Squat Quarry alone, including three new taxa. The revised faunal list for the Whistler Squat Quarry also extends the biostratigraphic ranges of nine non-endemic mammalian taxa to Ui1b.

Geochronology of the middle Eocene Purple Bench Locality (Devil’s Graveyard Formation), Trans-Pecos Texas, USA

“…The spread in ages between the lower (Rhino's Loaf, 45.4 +0.6 / -1.1 Ma; Krister Step, 45.5 +0.2 / -1.4 Ma) and upper units (Peaches, 42.8 +1.0 / -0.4 Ma) sampled indicates the intervening sediments accumulated over a period of between 0.2 -3.1 Myr. The absolute ages calculated here for Rhino's Loaf (44.3 -46.0 Ma) and Krister Step (44.1 -45.7 Ma) are in good agreement with the sanidine 40 Ar/ 39 Ar age estimate (44.88 ± 0.04 Ma) reported for the Whistler Squat Quarry fossil locality (TMM 41372;Campisano et al, 2014). Like Rhino's Loaf and Krister Step, the Whistler Squat Quarry occurs in the lower member of the DGF (Wilson, 1986), but the relative stratigraphic positions of the three localities are currently unclear.…”

“…The imprecision of these dates places the fossil assemblage from the Purple Bench locality within a 3.6 Ma time interval (i.e., 41.1 -44.7 Ma). Additionally, the Purple Bench locality occurs stratigraphically above a series of localities in the lower member of the DGF (Figure 2) that have yielded a mammalian fauna distinctive of the early Uintan (Ui1b) NALMA (Campisano et al, 2014;Stocker and Kirk, 2016). Recent biotite and sanidine 40 Ar/ 39 Ar radiometric dating of volcanic tuffs constrain the Ui1b Whistler Squat Quarry of the DGF to be between 45.04 ± 0.05 and 44.88 ± 0.04 Ma (2s standard error; biotite and sanidine respectively; Campisano et al, 2014).…”

“…Additionally, the Purple Bench locality occurs stratigraphically above a series of localities in the lower member of the DGF (Figure 2) that have yielded a mammalian fauna distinctive of the early Uintan (Ui1b) NALMA (Campisano et al, 2014;Stocker and Kirk, 2016). Recent biotite and sanidine 40 Ar/ 39 Ar radiometric dating of volcanic tuffs constrain the Ui1b Whistler Squat Quarry of the DGF to be between 45.04 ± 0.05 and 44.88 ± 0.04 Ma (2s standard error; biotite and sanidine respectively; Campisano et al, 2014). Mammalian taxa characteristic of the Duchesnean NALMA occur stratigraphically higher than the Purple Bench locality in the DGF (Wilson and Stevens, 1986;Robinson et al, 2004).…”

Mammals from the earliest Uintan (middle Eocene) Turtle Bluff Member, Bridger Formation, southwestern Wyoming, USA, Part 1: Primates and Rodentia

“…However, the fauna of the TBM also includes a large number of Bridgerian holdover taxa and is lacking certain characteristic later Uintan taxa including Amynodon Marsh, 1877, and seleonodont artiodactyls (Gunnell et al, 2009;Campisano et al, 2014). These facts strongly support Gunnell et al's (2009) recognition of an earliest Uintan Ui1a biochron for the stratotype section of the TBM.…”

“…Wilson (1986) documented the faunas from the Devil's Graveyard Formation (DGF) of Texas. Two local faunas are recognized from the lower member of the DGF; the Basal Tertiary Local Fauna (Ui1a) from Junction (TMM Locality 41443), 0.6 miles east of Junction (TMM Locality 41444) in the basal Tertiary conglomerate and the laterally equivalent Hen Egg Mountain localities in the lower member of the DGF; and the Whistler Squat Local Fauna (Ui1b) from stratigraphically higher in the lower member of the DGF (Wilson, 1986;Walton, 1992;Gunnell et al, 2009;Campisano et al, 2014). Recently, Campisano et al (2104) reevaluated the geochronology and taxonomy of the fauna from the Whistler Squat Quarry wherein they provided high precision 40 Ar/ 39 Ar dates of 45.04 ± 0.10 Ma for the lower tuff and 44.88 ± 0.04 Ma for upper tuff at the quarry.…”