- Copyright © 2016, SEPM (Society for Sedimentary Geology)
The Waco Mammoth National Monument (WMNM) potentially represents the only recovered Columbian mammoth (Mammuthus columbi) herd to date, but the “herd” interpretation is dependent on the demographics of the accumulation as well as a catastrophic kill mechanism. The demographics are consistent with an extant elephant herd that is lacking only infants, and the generally accepted cause of death is a catastrophic flood and rapid burial based primarily on fossil articulation and associations with an ancient river channel and aquatic fauna. Herein we present new ichnological evidence of post-mortem biogenic bone modification contrary to a flood scenario with rapid burial. Traces on bone include branching furrows (Corrosichnia type), paired grooves (Machichnus regularis and M. bohemicus), arcuate grooves that penetrate the cortical material (Brutalichnus brutalis), roughly triangular punctures with jagged margins (Nihilichnus nihilicus), and hemispherical borings (Cubiculum isp.). The branching furrows are interpreted as root-dissolution features, whereas the remaining suite of traces demonstrate scavenging of the mammoths by rodents, carnivores, and hide beetles during a period of dry-decay and prolonged subaerial exposure. We propose that a drought scenario is a more plausible kill mechanism for this particular assemblage because: (1) a diminishing watering hole concentrating the local fauna explains the high taxonomic diversity; (2) migration to a distant water source explains the absence of M. columbi calves; and (3) a drought provides a parsimonious explanation for the site history in light of new observations regarding vertebrate and invertebrate scavenging. Under this scenario, the mammoths of WMNM represent at least one social group that perished during an anomalously dry season.