Comparative Morphology and Homology of the Hyoid Apparatus and Associated Muscles in Beloniform Fishes (Teleostei: Atherinomorpha)

ABSTRACT

A comparative morphological investigation of the hyoid apparatus of atherinomorph fishes supports a new interpretation of homology of the branchiostegal rays. In the atherinomorph orders Atheriniformes and Cyprinodontiformes, and outgroups, the branchiostegal rays are arranged in a posterior group I that forms first in ontogeny and attaches to the lateral surface of the hyoid bar, and an anterior group II which develops later and attaches medially to the anterior portion of the hyoid bar. In contrast, the atherinomorph order Beloniformes is diagnosed by derived skeletal characters of the hyoid apparatus including the branchiostegal rays arranged in a uniform row exclusively along the lateral face of the hyoid bar. This arrangement suggests that the rays are homologous with those of group I and that there are no group II rays. We test this hypothesis with characters from muscle anatomy. In the Atheriniformes, Cyprinodontiformes, and outgroups, the branchiostegal rays of the posterior group I attach to the lateral surface of the hyoid bar with the most anterior rays of this group at the insertion of the interhyoideus . The two thinner, anterior branchiostegal rays (group II), attach medial to the interhyoideus . In beloniforms, the single group of branchiostegal rays on the lateral surface of the hyoid bar has its most anterior rays at the insertion of the interhyoideus . No rays attach medial to the interhyoideus . This evidence supports the hypothesis that beloniforms have solely group I branchiostegal rays. Group II rays never develop. Further modifications of the hyoid apparatus are phylogenetically informative within the adrianichthyoid beloniforms. A unique pair of muscle sheets—the ventroposterior section of the hyohyoideus superior —along the ventroposterior‐most portion of the hyoid bar and separated from the more anterior section of the hyohyoideus superior by a distinct gap diagnoses adrianichthyids, which also have a unique ligament that anterolaterally connects the ventral hypohyal and the basihyal cartilage. The ricefish genus Oryzias is diagnosed by modifications of the hyoid‐bar skeleton including an enlarged hypohyal cartilage.