Bone remodeling of the Homo heidelbergensis mandible; the Atapuerca-SH sample.

Growth by bone remodeling is one of the key mechanisms responsible for skeletal morphology. This mechanism consists of the coordinated activity of two cellular groups: osteoblasts and osteoclasts, which are responsible for bone deposition and resorption, respectively. Information obtained from the study of these remodeling growth fields allows us to understand how species-specific craniofacial form is achieved. These data can help to explain the facial growth differences among Primates, both extinct and extant. The aim of this study was to obtain the distribution of growth remodeling fields of the Homo heidelbergensis mandible (Atapuerca-SH sample), and to infer the growth processes responsible for its specific morphology. A Reflected Light Microscope (RLM) was used to identify the microfeatures of the bone surface related to bone deposition and resorption. Results show that H. heidelbergensis presents a specific growth field distribution, which differs slightly between immature and adult individuals. Interpretation of these maps indicates that the mandible of H. heidelbergensis presents noteworthy variability in the symphyseal region. Two distinct patterns of growth are seen, one of those unique for this species and the other similar to that of Homo sapiens. The lingual side of the mandibular corpus has a resorption area found only in this species and one that includes a variable extension in immature and adult individuals. Finally, the mandibular ramus is characterized, among other aspects, by a large resorption field on its buccal surface. Considering the mandible as a whole, the bone remodeling pattern obtained in this work shows that lower facial growth in H. heidelbergensis is dominated mainly by forward growth, illustrated by the strong inward displacement of the ramus, which is in agreement with the Enlow's "V" growth principle.