Morphological Variation and Integration of the Ethmoidal and Prechiasmatic Regions in Sheep

This study investigated breed- and sex-related variation in two adjacent regions of the sheep cranial base on the endocranial surface, namely the ethmoidal region (fossa ethmoidalis-crista galli complex) and the prechiasmatic region centered on the sulcus chiasmatis. We hypothesized that breed-related morphometric differences would be detectable in this cranial base region and that the ethmoidal and prechiasmatic regions would show modular separation despite substantial covariation. Three-dimensional landmark data from 113 adult Akkaraman, Morkaraman, and Zom sheep were analyzed. Whole-configuration size differed by breed and sex, ethmoidal size mainly by sex, and prechiasmatic size mainly by breed. Shape analyses showed significant breed effects in the whole configuration and in both regional modules, whereas sex effects were weaker and limited to the whole configuration and prechiasmatic region. Allometry was significant at all levels and was strongest in the two regional modules, especially the prechiasmatic region. Modularity analysis supported the separation of the ethmoidal and prechiasmatic landmarks into two modules, whereas integration analysis indicated a high degree of covariation between them (r-PLS = 0.933), with most shared covariance concentrated on the first PLS axis. These findings indicate that variation in this endocranial cranial base region is detectable in both size and shape, with shape differences identified more consistently across the whole configuration and the two regional modules, although the associated effect sizes were modest. Overall, the results highlight that variation between adjacent cranial base regions is primarily expressed through shape and covariation rather than size alone, providing a morphometric framework for future studies of cranial structure.