Shape Variation of Linea Temporalis and Crista Nuchae in Mesocephalic Dogs

Dog domestication began around 40,000 years ago in Eurasia, following their divergence from wolves. Once domesticated, dogs were selectively bred for functions such as guarding, herding, and hunting. The classification of modern breeds was later formalized in Victorian England, emphasizing both morphology and working purpose. Cranial features like the linea temporalis and crista nuchae serve as critical osteological landmarks, reflecting muscle attachment, bite force, and neck stabilization. In this study, geometric morphometric analyses were performed on 81 mesocephalic dog skulls representing five functional groups (hunting, guarding, companionship, herding, sledging). Using three-dimensional models from Czeibert et al. (2024), 43 landmarks and semilandmarks were digitized along these cranial structures. Procrustes superimposition, PCA, Procrustes ANOVA, regression, and discriminant analyses were applied to evaluate shape variation, size influence, and functional differentiation. PCA revealed that most variation (PC1: 32.7%) occurred in the parietal portion of the linea temporalis and dorsal origin of the crista nuchae, with toy dogs forming a distinct cluster. Significant shape (p < 0.0001) and size (p < 0.0006) differences were found, while discriminant analysis showed partial overlap among functional groups. Overall, the study highlights how variation in these cranial landmarks reflects functional adaptation, selective breeding, and evolutionary development in domestic dogs.