A deposition mechanism for Holocene miring bone deposits, South Island, New Zealand

Abstract

Localised deposits of Late Pleistocene and Holocene bird bones occur in wetlands throughout New Zealand. These are characterised by dense accumulations of mostly disarticulated bones, with assemblages dominated by large, flightless bird taxa; in particular the extinct ratite moa (Aves: Dinornithiformes). A wide range of deposition mechanisms were historically proposed for these sites, including large floods and stampedes during wildfires. We outline a simple method for analysing the orientation and spatial distribution of bones within these deposits using GIS software, and apply this method to the interpretation of three such deposits from South Island, New Zealand. The results are consistent with non-catastrophic, periodic miring of individual moa. Long bones within these sites were preferentially orientated and subhorizontally inclined, indicating post-deposition disarticulation and movement of the bones within the sediment by sediment liquefaction and raking from the legs of mired birds, with a possible influence from water flow. Small, light skeletal elements were significantly under represented in the deposits. This may be due to post-mortem scavenging or weathering of vertebra and crania, 'pumping' to the surface of light, buoyant elements during liquefaction events, or crushing of these elements by subsequently mired birds.