BIOGEOGRAPHIC HISTORY OF LATE PLEISTOCENE CONIFERS IN SNOWMASS VILLAGE, COLORADO

Nearly two hundred Late Pleistocene fossil ovulate cones were recovered from a new fossil site near Snowmass Village, Colorado, in 2010 and 2011. The site, known as the Ziegler Reservoir fossil site, is a 12-hectare basin that sits at 2705 m that was formed by a glacier during the Bull Lake glaciation. Cosmogenic, radiogenic and OSL dating constrain the sediments in the lake to have been deposited between ~130 and 40 ka, spanning 90 k years of the Late Pleistocene. However, these dates plus the stratigraphic model suggest that the earlier part of this record is best represented in the basin. Fossil conifer species were identified using morphometric analysis of the seed cones, cone scales, seeds, and bracts. The taxonomic placement of the fossil conifer species were confirmed using specimens of extant conifer species from western North America. The same morphological characters were specifically studied, including distinctive morphological features of the bract apicula. Four fossil conifer species were identified. Abies concolor and Pseudotsuga menziesii fossils were recovered from stratigraphic units corresponding to the earlier interglacial sediments preserved at the site. The occurrence of Abies concolor is notable because this species had not previously been reported from the late Pleistocene in Colorado. This has provided new insights into biogeographic history Abies concolor showing that the species present range is a smaller than its historic range. In the upper and younger stratigraphic units, which correspond to more glacial conditions at the site based on other proxies, Picea engelmannii fossils were recovered. This occurrence of P. engelmannii in these sediments fits well with the species current range and climatic tolerances. Finally, an as yet identified species Picea, which is morphologically most similar to Picea pungens, was found throughout the sediments at the site. While the species taxonomic placement remains uncertain, it speaks to the diverse conifer assemblage that existed at the Ziegler fossil site between ~130 and 40 ka.