Comparative Anatomy of Rare Marine Mammals

Abstract

Rare marine mammals -- including the most data-deficient cetacean and sirenian species globally -- present formidable anatomical research challenges: live specimens are rarely available for study, and much of what is known about their internal anatomy derives from stranding events, incidental fisheries bycatch mortalities, and opportunistic examination of museum skeletal material accumulated over decades of opportunistic collection. This study presents the most comprehensive comparative anatomical analysis of rare marine mammal species yet conducted, combining micro-CT scanning of 247 museum skulls from 18 rare cetacean and sirenian species, photogrammetric 3D surface models of 84 skeletal specimens, geometric morphometric analysis of 38 skull landmarks, and dissection records from 47 stranded individuals examined 2015-2022. Analyses reveal previously undescribed anatomical features in four beaked whale species (Mesoplodon spp.), confirm a significant divergence in the vomer bone morphology between Atlantic and Pacific populations of True's beaked whale (Mesoplodon mirus) consistent with cryptic species-level differentiation, and provide the first quantitative skull morphometric data for three Mesoplodon species with fewer than 20 confirmed specimen records globally. An anatomical character matrix of 124 osteological characters for the 18 study species provides a morphological systematics dataset for integration with molecular phylogenetic analyses. The study demonstrates that micro-CT of museum specimens enables anatomical research on species for which destructive sampling would be ethically and practically impossible.