Cranial restructuring in the evolution of Sirenia (Mammalia): Insights from 2D morphometric analysis of fossil and extant taxa

The afrotherian order Sirenia with an extensive fossil record from the early Eocene to the recent times shows a progressive transition from their terrestrial origins to their now obligate aquatic forms.Previous studies on the cranial and postcranial remains of sirenians have documented changes in bone density, from cancellous to pachyosteosclerotic, reduced hind limb anatomy and dietary shifts, from their plesiomorphic to derived forms.Using landmark-based principal component analysis (PCA) to investigate changes in shape and gross morphology of their skulls across taxonomic, paleobiogeographic, and temporal contexts, we analysed 23 crania from various sirenian taxa from the Eocene to the present.Morphometric analysis revealed several derived characters, suggesting that modifications such as a broader premaxilla, increased overlap between the premaxilla and frontal, nasal retraction, moderate telescoping of the posterior region of the skull, and braincase constriction likely played a crucial role in cranial restructuring, ultimately facilitating their adaptation to an obligate aquatic lifestyle.These changes also involve their varied foraging strategies across different geographic regions.This study underscores the importance of quantitatively comparing extinct and extant sirenian taxa, and it documents a link between cranial evolution, physiological adaptations, and the transition to an aquatic lifestyle observed in the geological history of sirenians.