Morphological features of the orbicularis oculi muscle and facial nerve in four odontocete families, with comparisons within Cetartiodactyla.

We examined the orbicularis oculi (OC) muscle and its innervation among the markedly modified facial muscles of odontocetes, clarifying features related to secondary adaptations to aquatic life. The OC muscle has been described as a well-developed sphincter encircling the eye in cetaceans; however, this study revealed two distinct types of OC morphology among odontocetes. In specimens from the families Kogiidae and Ziphiidae, the muscle bundles of the OC completely encircled the palpebral fissure in concentric circles, similar to that of Bovidae specimens. However, in specimens from Phocoenidae and Delphinidae, the OC featured a muscular area with muscle fibers and an aponeurotic area comprising collagen fibers lacking facial nerve distribution. Our results suggest that this aponeurotic area was acquired in the common ancestor of species within the Delphinoidea superfamily, and that the upper eyelid is less mobile than the lower eyelid. The palpebral branches, which originate from the facial nerve, extend from the lower to the upper palpebral regions in the examined odontocetes, showing a clear difference from the entry course seen at the lateral canthus in artiodactyls. The proximal palpebral branches diverged directly from the facial nerve trunk, forming a characteristic diverging point distal to that of artiodactyls. These branches probably correspond to the auriculopalpebral nerve, which has lost the rostral auricular branch component. The morphological changes in cetacean heads also led to changes in the branching pattern and pathway of the facial nerve.