Taphonomic analysis of Batallones-10, a Late Miocene drought-induced mammalian assemblage (Madrid basin, Spain) within the Cerro de los Batallones complex

Abstract

The Late Miocene Batallones-10 site (Madrid Basin, Spain) contains several authochthonous multitaxic assemblages. A total of 15 large-mammal species are documented, including hipparionine horses (Hipparion sp.), giraffes (Decennatherium rex) and moschids. Three discrete fossiliferous levels have been identified with the aid of artificial intelligence systems. Although there are some statistical differences in the faunal composition of each level, paleontological, geologic and taphonomic data from all three Batallones-10 levels point to similar taphonomic histories. Mortality data, with abundant young individuals and the presence of pregnant females, indicates a catastrophic process of bone accumulation mainly driven by prolonged drought. Relatively high proportions of partially preserved or fully-articulated skeletons, and absence of carnivore feeding marks on bones, suggest that carcasses were undisturbed by predation. Rapid desiccation of soft tissues kept many of the joints intact, resulting in a high proportion of articulated remains. During drought episodes, large mammalian herbivores congregated around a shrinking water-hole, depleting the local vegetation, such that weakened individuals died of starvation and miring rather than dehydration. When rainfall resumed, the lack of vegetation and presence of low-permeability surficial sepiolite-rich marls, resulted in rapid run-off. This buried most of the unweathered, unabraded remains, whilst slightly orienting or completely removing others. After burial, faulting and slickensides produced diagenetic fractures, which favoured the precipitation of manganese oxides on bone surfaces and the diagenetic corrosion of remains, which is more pronounced in the uppermost Level III. This work, which focuses on the taphonomy of a herbivore-dominated site, improves our overall knowledge of the formation of the Cerro de los Batallones palaeontological complex.