Mandibular, hyoid, and hypobranchial musculature in skates (Chondrichthyes: Batoidea: Rajiformes): revised terminology and phylogenetic implications

About 300 valid species are classified within the order Rajiformes, the largest group of chondrichthyans. The monophyly of this order is highly supported by morphological and molecular inferences, but little is known about the mandibular, hyoid, and hypobranchial muscles of its members. This study aims to describe and illustrate the morphological variation in mandibular, hyoid, and hypobranchial muscles among rajiforms, review their terminology and discuss the phylogenetic and functional implications of conditions observed. Twenty-two ethanol-preserved specimens belonging to 19 of the 36 skate genera currently considered valid were manually dissected. Nine new characters are proposed and tested cladistically. The main differences observed are the relative width of muscle units of the levator palatoquadrati, insertion point and extension of the intermanibularis posterior, relative position of the facialis nerve and the levator hyomandibularis muscle, origin and insertion of the coracomandibularis, extension of the coracohyomandibularis, and occurrence of the preorbitalis medialis and intermandibularis anterior. When analysed together with other morphological features, muscle characters have shown to play a key role in the understanding of interrelationships among skates. Nevertheless, additional studies are needed to improve our knowledge of rajiform systematics and the functionality of mandibular, hyoid, and hypobranchial muscles.