Comparative cranial ontogeny in Sirenia (Mammalia: Paenungulata)

Abstract

Sirenians are mammals that exhibit multiple morphological specializations for an exclusively aquatic life and a strictly herbivorous diet. In this study, we investigate the cranial ontogeny of representatives of the two extant families, Dugong dugon (Dugongidae) and Trichechus manatus (Trichechidae). We analyzed a postnatal ontogenetic series of 17 dugong and 22 manatee skulls using 21 linear measurements. Principal Component Analysis was applied to visualize ontogenetic trajectories, and multivariate and bivariate allometric analyses were used to assess differential growth. PCA results revealed distinct ontogenetic trajectories for each species. In both species, cranial variables predominantly showed negative allometry, whereas rostral variables exhibited isometric or positive allometric trends. But key interspecific differences were identified: the dugong showed more rapid rostrum growth and elongation and a narrower zygomatic width, whereas the manatee displayed growth trends toward expanded zygomatic arches and positive allometry in tooth row lengths. These postnatal growth patterns directly correlate with the diverging feeding strategies of dugongs and manatees, showing the ontogenetic basis of dietary specialization.