THE BISON AND THE BLOW FLY: USING PUPARIA OF THE BLACK BLOW FLY (PHORMIA REGINA: DIPTERA, CALLIPHORIDAE) TO CONSTRAIN THE SEASON OF DEATH AND TAPHONOMIC HISTORY OF AN EARLY HISTORIC-AGE BISON, CARSON CITY, NEVADA, USA

Abstract: We report the occurrence of abundant dipteran puparia of Phormia regina, the black blow fly, in association with an early historic-age bison skeleton excavated near Carson City, Nevada. Cut marks on some of the bones indicate that the bison was butchered and probably skinned by humans. Radiocarbon dating provides two possible age intervals for the death of the bison: (1) latest seventeenth to early eighteenth century or (2) early nineteenth to early twentieth century; we consider the more recent age to be more plausible. The purpose of this study is to explore how the presence of puparia of this well-studied, necrophagous fly species can be used to help constrain the season of death and inform the interpretation of the taphonomic history of the bison. The life cycle of P. regina requires a minimum of 8.8 days within a temperature range of 14°C to 35°C, so the bison carcass must have been exposed to the air for at least that long within that temperature range. However, of the thousands of recovered puparia, 35% remain closed and did not produce adult flies; of this cohort, only a tiny percentage exhibit small exit holes attributable to parasitoid wasps. Cold temperatures, and not parasitoid wasps, are the most probable cause of the high pupal mortality. Climate data for the region, along with P. regina temperature constraints and streamflow and flooding records for the Carson River watershed, indicate that the bison died and was colonized by black blow flies in the spring, when night-time temperatures were low. A short time later the skinned and butchered skeleton was buried by floodplain sediments. Blow fly puparia can contribute useful information for the taphonomic analysis of vertebrate fossil sites.