Evidence for an Early Sagebrush Ecosystem in the Latest Eocene of Montana

Abstract

Late Eocene strata at Little Pipestone, Montana, preserve paleosols and an undiverse but abundant trace fossil assemblage dominated by adhesive meniscate burrows placed into the ichnogenera Taenidium. The assemblage as a whole is most consistent with the Scoyenia ichnofacies. The reconstructed paleohydrological and paleoenvironmental setting that results from the combination of sedimentological, paleopedological, and ichnological data is of a seasonally desiccated, well-aerated floodplain characterized by low flood frequency and intensity, slow sediment accumulation rates, and an overall subhumid/semiarid climate very similar to a modern sagebrush steppe. This would represent the oldest documented sagebrush steppe ecosystem in Montana.

Coupling of marine and continental oxygen isotope records during the Eocene-Oligocene transition

“…In other continental Eocene-Oligocene transition successions (Fig. 2), locally evaporative conditions are indicated by aridification (Sheldon and Retallack, 2004;Abels et al, 2011;Passchier et al, 2013;Boardman and Secord, 2013), by shifts in the assemblage of paleosol types (Terry, 2001), and by shifts in the trace fossil assemblage (Sheldon and Hamer, 2010). None of these features is observed in the Hampshire Basin, and rather than aridification through the Eocene-Oligocene transition, wetter conditions have been reconstructed ( Fig.…”

Eocene vegetation and ecosystem fluctuations inferred from a high-resolution phytolith record

“…Taken together, these various lines of evidence indicate that climate change was not the primary driver of fl oral variation at Timberhills A. Regionally, cooler and drier conditions are not inferred locally until ~5 m.y. later, associated with the Eocene-Oligocene transition (Sheldon and Retallack, 2004;Retallack, 2007;Sheldon, 2009;Sheldon and Hamer, 2010). The presence of palms and gingers throughout the section also supports a stable, warm, humid climate during this time.…”

“…In contrast to Retallack (2007), no evidence of pedogenic carbonate (calcite or siderite) was found in the 1 GSA Data Repository item 2012247, 10 parts, including descriptions of phytolith compound variables; tables of raw and compound variable phytolith counts, and stable isotope data; and fi ve supplemental fi gures, is available at http://www.geosociety.org/pubs/ft2012 .htm or by request to editing@geosociety.org. 1000-10,000 yr *Paleoclimate estimations are based on regional data from Axelrod (1998), Graham (1999), and Sheldon and Hamer (2010). First value is mean annual precipitation (mm yr -1 ), and second value is mean annual temperature (°C).…”

“…Global paleoclimate (Zachos et al, 2001) and regional vegetation (Graham, 1999;Strömberg, 2005;Sheldon and Hamer, 2010) reconstructions suggest that this region was in a long-term transition from warm, wet paratropical forest to a dry, open ecosystem. At Timberhills A, the majority of the horizons record forest vegetation with tropical elements refl ected by the presence of palm and Costus-type phytolith morphotypes.…”

“…Whereas previous phytolith work indicates that open, grass-dominated vegetation (woodland to grassland) was widespread in Montana by the late early Miocene (Strömberg, 2005), trace fossils and paleosols suggest that Montana was an open and sagebrush-type landscape by the latest Eocene (ca. 34 Ma; Sheldon and Hamer, 2010).…”