Effects of climate change on mammalian fauna composition and structure during the advent of North American continental glaciation in the Pliocene

by 1973- Ruez, Dennis Russell

Hagerman Fossil Beds National Monument (HAFO), Idaho, is internationally significant because it encompasses hundreds of fossil localities representing many of the most important terrestrial Pliocene sites known. This study establishes the background for comparisons between localities in the Glenns Ferry Formation within HAFO by describing the nature of the fossiliferous deposits, using published data to provide revised age estimates for HAFO localities, and better marking the relative difference in elevation for particular time horizons. Fossils from the Hagerman Horse Quarry, anthills, and blowout localities are considered to be essentially at the original stratigraphic level of deposition. Species of modern ants belonging to Pogonomyrmex do gather fossils from more than the immediate area, but the estimated maximum vertical movement is within the resolution of elevation possible at most HAFO localities. The microstratigraphy of blowout localities is described here for the first time, with vertebrate fossils derived exclusively from layers of 11 about 12 cm thickness. Fossils recovered as surface float generally should be excluded from stratigraphic comparisons. Based on a combination of paleomagnetic and radiometric studies, the maximum age for the top of the Glenns Ferry Formation exposed at HAFO is 3.11 Ma, and the minimum age for the lowermost exposures is 4.18 Ma. It is unlikely that there is any Glenns Ferry Formation sediment younger than 3.04 Ma or older than 4.29 Ma at HAFO. Finally, using marker beds and published stratigraphic sections, the necessary change in elevation to compare all Glenns Ferry Formation fossil localities at HAFO against an idealized composite section is established. Within this framework fossil sites can be placed in their proper stratigraphic context and faunal change can be identified more precisely.