Sexual dimorphim in the human calvarium. A geometric morphometric approach

In human evolution the assessment of sex in fossil specimen is performed by transposing the information relative from extant species (e.g., Homo sapiens, Pan paniscus, Gorilla gorilla) to extinct ones. The cranium is considered a key component in establishing others types of biological information including age, state of health and provenience and is considered the second important skeletal structure for a determination of sex after the pelvis [1]. In this communication, we analysed the human calvarium in know-sex specimens to detect which anatomical traits are more sexual dimorphic. To reach the aim of this project, we collected digital models of 165 adult specimens and on each skull we acquired 50 landmark on the entire cranial morphology. The only use of landmarks does not allow to study some anatomical traits due to lack of anatomical point. For this reason, we opted to collect also a surface patch of 500 semi-landmarks. We performed the analysis in R environment by using the Morpho, Arothron and geomorph R packages [2]–[4]. After semi-landmark placing and sliding, we split the semi-landmark configuration into three sub-regions: the entire calvarium, the frontal bone and the supraorbital torus. On each sub-region, we perfomed the General Procrustes Analysis (GPA) and the rotated configurations were subjected to Procrustes Anova to calculate the relation between shape and sex. Subsequently, we calculated the shape variations associated to the female and male morphology. All the sub-regions resulted related to sex variable, the R-squared from Procrustes Anova are the following 0.014, 0.024 and 0.048 for calvaria, frontal bones and supraorbital torus respectively. The shape variation associated to female morphology highlights the presence of more vertical frontal bone, short brain case and more shortening part of posterior parietal bones. On the contrary the male morphology is characterized by more horizontal profile of frontal bone, larger brain case above all in posterior part of parietal bones. In sum a geometric morphometric approach applied on the study of sexual dimorphism resulted very useful in the determination of sex because the accuracy value are higher (82% for the entire calvarium, 85% for both frontal bone and supraorbital torus) than currently (i.e., traditional) methods used to establish sex in unknown sample. Future research