Fusion of the occipitomastoidal synchondrosis as a developmental marker in the Sima de los Huesos Crania (Atapuerca, Spain).

The basicranium contains multiple synchondroses potentially informative for estimating the developmental stage of individuals. The basilar synchondrosis has been routinely used for this purpose in bioarchaeological, forensic and paleoanthropological research, and studies carried out in modern human populations have shown a close relationship between the fusion of the occipitomastoidal synchondrosis and developmental processes. This synchondrosis articulates the jugular process of the occipital bone with the jugular surface of the temporal bone. As the process of fusion of the synchondrosis progresses, the jugular surface undergoes a series of alterations whose study allows to establish the state of fusion of the synchondrosis when the individual died. The extraordinary preservation of the jugular surface in a large number of individuals represented in the fossil hominin sample from the middle Pleistocene site of the Sima de los Huesos (SH) has made it possible to carry out the first systematic study to assess the usefulness of occipitomastoidal synchondrosis in the establishment of the state of development in fossil hominins. Our results show that the complete closure of the occipitomastoidal synchondrosis occurred toward the end of the growth period in the SH fossils. This result opens up the possibility of using it to determine the developmental stage of fossil hominins for which no other information is available, such as the state of the dentition or the degree of closure of the basilar synchondrosis. This has allowed us to infer a state of development for three SH crania where it could not previously be established with certainty.