Formation processes of Late Miocene Hipparion Red Clay vertebrate assemblages from northern China: taphonomic constraints on palaeoecological and biochronological interpretations

Hipparion Red Clay of northern China represents one of the most crucial terrestrial mammal records of the Eurasian Neogene. Despite their long-standing relevance to biostratigraphy and palaeoecology, the taphonomic processes governing the formation and preservation of these assemblages remain largely unexplored. Here, we present the first quantitative taphonomic analysis of two Late Miocene sites embedded within the Hipparion Red Clay of the Chinese Loess Plateau: Daidian and Dongmen. These sites are geographically separated (~100 km) and diachronous yet occur within the same regional stratigraphic unit. Large herbivores dominate both assemblages but differ markedly in taxonomic composition, with equids ( Hipparion spp.) prevailing at Daidian and rhinocerotids ( Chilotherium habereri ) dominating Dongmen. Mortality profiles of the dominant taxa indicate attritional accumulation at both sites, characterised by juvenile-biased age structures and the absence of catastrophic mortality signatures. Skeletal representation and completeness patterns are broadly comparable, with a predominance of appendicular elements and consistent underrepresentation of axial and girdle bones. Bone surface modifications related to biostratinomic processes are rare, whereas extensive fragmentation coupled with high skeletal completeness indicates pervasive fossildiagenetic breakage. Statistical analyses reveal no significant differences in bone modification patterns between sites. These results support a depositional model in which vertebrate remains accumulated gradually on low-relief open landscapes through attritional mortality and were repeatedly incorporated into the sedimentary record by very low-energy aeolian dust deposition. The Hipparion Red Clay, therefore, emerges as a taphonomically consistent archive capable of preserving time-averaged vertebrate assemblages with high palaeontological fidelity across space and time.