Taphonomic study of the cave bears (Ursus cf. deningeri and U. spelaeus) from the Sima I of the El Polvorín cave (Northern Iberian Peninsula)

Cave bears used caves for hibernation and giving birth. Caves maintain stable conditions of temperature and humidity, which facilitates the preservation of fossil accumulations. Causes of mortality in caves are related to starvation during hibernation, which affects mainly juveniles and old adults, predation and accidents, affecting bears of all ages. The Iberian Peninsula is located in one of the extremes of the geographical range of this species and has yielded an abundant cave bear fossil record, particularly in karstic systems in the northern fringe. The origin of the accumulation of bear fossil sites in Iberia has been investigated mostly based on selected paleobiological variables (sex-ratio and age-at-death profiles) and qualitative taphonomic assessment. The objective of this paper is to delve into the origin of a bear accumulation with a large proportion of hyaena remains, the Sima I of El Polvorín cave (Biscay, Northern Iberian fringe, Spain). For that end a combination of quantitative paleobiological (age-at-death and sex) and taphonomic (bone fracture patterns and surface modifications) data is used. This analysis has ruled out humans and hyenas as the origin of the accumulation but, due to the limited number of fossil remains available it is not possible to ascertain whether the bear accumulation was the result of a natural trap within the cave, natural mortality during hibernation, or a combination of both causes. This study underscores the necessity of integrated methodological approaches and the potential of old paleontological collections to provide new important paleobiological and taphonomic information under these approaches.