Abstract:

The astragalus is one of the tarsal bones and anatomically subdivided into three distinct regions: the head, neck, and body. This critical bone serves as the primary osseous interface between the distal tibia and proximal tarsal bones, forming the talocrural articulation that facilitates the characteristic hinge-like motion of the tetrapod ankle joint. Furthermore, comparative osteological analyses reveal that the astragalus typically exhibits greater structural density compared to other skeletal elements, a morphological adaptation likely associated with its weight-bearing function within the ankle complex. This increased bone density, combined with its strategic anatomical position protected by adjacent articulating surfaces, significantly enhances its preservation potential in both modern and paleontological contexts. The exceptional preservation frequency of astragali in vertebrate fossil records has consequently established this element as a crucial anatomical marker for phylogenetic studies and functional morphology analyses. In addition, since the structure of the astragalus is directly related to the locomotion patterns of animals and reflects the range of motion of the hind limbs, it is often used to analyze the characteristics of species and their ecological and functional adaptability. The Hualongdong Site is an important comprehensive ancient human site from the late Middle Pleistocene in China, where a large number of mammalian fossils have been unearthed. This paper takes 316 specimens of mammalian astragalus fossils excavated at the site from 2014 to 2024 as research materials. The investigation methodologically integrates the measurement, the identification of morphological characteristics and the analysis of surface traces, This systematic approach facilitates a comprehensive exploration of the animal group, the ancient human behaviors, and the background of burial. The mammalian astragalus bones unearthed at the Hualongdong reflect a fauna composition dominated by the Cervidae and the Bovidae, accounting for 84.5% of the identifiable specimens and 80.5% of the minimum number of individuals respectively. Sus lydekkeri and carnivores, predominantly from the Ursidae also account for a certain number. There are also a few specimens from the Rhinocerotidae, Equidae, Tapiridae and Stegodontidae. The presence of the austral Ailuropoda-Stegodon faunal elements is represented by Ailuropda, Stegodontidae, and Tapiridae, while the presence of boreal taxa is represented by Cervus (Sika) grayi, Equidae, Stephanorhinus sp. In fact, the NISP and MNI of the boreal taxa are higher than the austral taxa. The astragalus measurement data show that Bos (Bibos) sp., Equidae, Rhiocerotidae, and Sus lydekkeri at the Hualongdong site represent the larger body sizes comparative to their relatives during the same period. Overall, the composition of the animal group reflected by the astragalus indicates a relatively open forest-grassland landform, with the presence of mountain animals. Burial analysis indicates that the astragalus is less affected by natural forces such as weathering and erosion by water flow. A thorough examination of the surface marks reveals that the astragali of the Cervidae and Bovidae families exhibited artificial cutting marks, chopping marks, carnivorous biting marks, and rodent gnawing marks. This indicates the involvement of humans, carnivores, and rodents in the accumulation of astragali in the cave. This study is the first to systematically reveals the composition of the animal group and the evidence of human behavior reflected by the astragalus fossils at the Hualongdong site. It provides reference materials for the research on the survival strategies and animal resource utilization of ancient humans in East Asia during the late Middle Pleistocene.

Key words: Hualongdong site, astragalus, morphological structure, surface traces, late Middle Pleistocene