Tusk pairs in the Ziegler Reservoir mastodon (Mammut americanum) assemblage: Implications for site taphonomy and stratigraphy

The Ziegler Reservoir (ZR) mastodon assemblage consists of remains from dozens of individuals that represent one or more populations living in a high-elevation region of the Colorado Rocky Mountains during Marine Isotope Stage (MIS) 5. Several left-right pairs of mastodon mandibular tusks were discovered in different stratigraphic units, challenging previous assumptions that depositional units preserving mastodon remains sample a comparable succession of regional populations through time. It is now recognized as likely that at least some of the specimens contained in these layers were transported from original deposits located on the lake margin, and that successive slumps transported parts of the same skeleton on different occasions, depositing them in different stratigraphic units. Assigning both tusks of a pair to the lowest depositional unit in which either occurs alters the temporal pattern in season of death for ZR mastodons and somewhat modifies a census of individuals, but does not significantly affect the secular trend previously detected in average oxygen isotope values. Two tusks with matching isotope profiles were previously treated as potential evidence of a catastrophic death event that had been proposed to explain the assemblage of remains, but these are now considered to be from the same individual. Newly paired records highlight the importance of careful taphonomic studies for any synthetic analysis of paleoecological history.