Unique occipital articulation with the first vertebra found in pristigasterids, chirocentrids, and clupeids (Teleostei: Clupeiformes: Clupeoidei)

This study recognized a W-shaped occipital articulation associated with the first vertebra in pristigasterids, chirocentrids, and clupeids as a unique character among teleosts, based on an evaluation of 43 species within 40 genera of these three families of the Clupeoidei. This occipital articulation is accompanied by an anterior extension of the neural arch bases, which are autogenous with the first vertebral centrum. In chirocentrids and many of the clupeid species examined, the anterior extension occurs on the second vertebra, and similar occipital articulation is found between the first and second vertebrae. The W-shaped occipital articulation is not found in any other teleosts, including Denticeps (suborder Denticipitoidei), which is thought to be a sister group to the suborder Clupeoidei. The W-shaped occipital articulation is absent in the other family of the Clupeoidei, Engraulidae, based on an evaluation of 11 species in 10 genera. Instead, the convex anterior surface of the first vertebral centrum forms a condyle that articulates with the basioccipital, and the neural arches fuse with the centrum behind this condyle. Therefore, it is unclear whether the anterior extension of the first vertebral neural arch bases, which causes the W-shaped occipital articulation, occurs in engraulids. Based on an evaluation of the osteological development of Konosirus punctatus and Engraulis japonicus, the cartilaginous neural arch bases of the first and second vertebrae extend anteriorly at an early developmental stage in the former, whereas no anterior extension of the first vertebral neural arch bases occurs at any developmental stage in the latter. Therefore, the anterior extension of the neural arch bases, which causes the W-shaped occipital articulation, seems to be a unique character of pristigasterids, chirocentrids, and clupeids among teleosts. Within the recent phylogenetic context, this character may be a synapomorphy of these three families.