Season of death of the Bowser Road mastodon

Abstract

A sample of dentin and cementum removed from the growing end of the left tusk of the Bowser Road mastodon was analyzed to document structural variation and interpret season of death for this individual. Within this tusk sample, we investigated three different patterns of structural variation: repeated step-wise changes in X-ray attenuation of dentin along transects perpendicular to the surface of the tusk pulp cavity (the surface of dentin apposition), highly variable but roughly cyclical variation in the thickness of structural increments of dentin (again measured normal to the surface of the pulp cavity), and topographic features expressed on the external surface of the tusk, encircling its root and paralleling its proximal margin. These three patterns of structural variation are linked in that each is initially manifested in dentin, and each occurs over a spatial scale determined by annual rates of dentin apposition and tusk extension. Based on previously documented associations between the structural features observed here and individual seasons comprising the annual cycle, we treat our annual increments as extending from one winter-spring boundary to the next. We also confirm that the structural increments for which we document the pattern of thickness variation formed periodically, spanning intervals of about one fortnight (two weeks) each. In a transverse thin section from a position near the middle of our sample slab, we observed two complete years of growth and a final interval of fifteen fortnights. This implies an early autumn season of death for the Bowser Road mastodon. In prior work, autumn deaths were probabilistically associated with active hunting by humans.