PALEOBIOLOGY OF SNOWMASS PROBOSCIDEANS (PLEISTOCENE, COLORADO)

At an altitude of almost 3km in the Colorado Rockies, the Zeigler Reservoir fossil site gives a rare look at a high-elevation ecosystem from the late Pleistocene (ca. MIS-5 through MIS-4) of North America. Remains of at least 35 mastodons and 3 mammoths dominate the macrofossil assemblage at the site. Mastodon remains are conservatively referred to the well-known, continent-wide Mammut americanum, though differences in tusk morphology from populations in the Great Lakes region raise the possibility of some other assignment. Mammoth remains are attributable to Mammuthus columbi. Mastodon remains are intercalated among three major lake-margin slump deposits and must therefore represent events that are to some degree separated in time. Nonetheless, their demographic profile originally raised the possibility of a series of catastrophic events, each affecting multiple individuals. Whether catastrophic on some scale or attritional in nature, the mastodon assemblage is the largest collection of bones of this taxon from one site and offers a rare opportunity to study a western, high-altitude population from the Sangamonian. Mastodon mandibular tusks are abundant at the site and appear to represent both males and females from calves to senescent adults. Our study presents the first attempt to use microCT, thin-section, and isotope records from these compact lower tusks to interpret life-history data. Further efforts will attempt to place individual growth records in the context of Snowmass environmental reconstructions. We recognize an up-section trend in δ¹⁸O profiles toward higher values suggestive of warmer temperatures. Through this sequence, mastodon growth histories show low mean sensitivities suggestive of only low levels of environmental stress. This work will help frame expectations for assessing environmental pressures on terminal Pleistocene populations. Study of the Snowmastodon remains promises to enhance our understanding of mastodon paleobiology and indirectly, of factors that led to their extinction.