Palatoquadrate and its ossifications: Development and homology within osteichthyans.

The palatoquadrate and associated dermal bones have significant evolutionary transformations among teleostomes and provide numerous features that characterize teleostomian subgroups. The palatoquadrate forms the upper part of the mandibular arch and is present as a single cartilaginous element in the early ontogeny of teleostomes, except for some advanced teleosts such as siluroids where it is divided into pars autopalatina and pars pterygoquadrata. During ontogeny, the palatoquadrate may ossify as a unit, with a pars autopalatina (absent in Acanthodii), pars quadrata, and pars metapterygoidea in teleostomes (e.g., primitive acanthodians and actinopterygians, onychodonts, and rhipidistians). However, the palatoquadrate may remain cartilaginous (e.g., chondrosteans) or it may ossify as separate elements (e.g., autopalatine, metapterygoid, and quadrate) as occurs in advanced acanthodians, Polypterus and advanced actinopterygians, and advanced actinistians. From the single-unit pattern, separate autopalatine, metapterygoid, and quadrate evolve in parallel in the three teleostomian subgroups. Therefore, it is necessary to distinguish between actinopterygian and actinistian autopalatines and among acanthodian, actinopterygian, and actinistian metapterygoids and quadrates. A palatoquadrate fused with the neurocranium occurs in parallel in dipnoans. There are differences in the timing of ossification of the autopalatine, metapterygoid, and quadrate. The autopalatine ossifies late in ontogeny in Polypterus, Amia, and primitive teleosts (absent in lepisosteids and osteoglossmorphs), whereas both metapterygoid and quadrate ossify early in ontogeny. The early ossification of the autopalatine is characteristic of clupeocephalan teleosts. During ontogeny, tooth plates (not forming a separate dermometapterygoid) fuse with the metapterygoid in actinopterygians. Pars autopalatina, pars metapterygoidea, and pars quadrata are regions at the three corners of the single-unit palatoquadrate present in primitive teleostomes; there are no clear limits among these regions, but they may be identified by their processes, articular facets, and topographical relationships with surrounding bones and the orbit. Autopalatine, metapterygoid, and quadrate are chondral bones, perichondrally ossified. Dermal elements such as dermopalatine(s), entopterygoid, ectopterygoid, and tooth plates may cover the palatoquadrate medially. The predermopalatine that originates in front of pars autopalatina in Cladistia and the "dermopalatine" that lies medial to the ectopterygoid in Ginglymodi are specializations of these groups. A dermopalatine fused with the autopalatine is characteristic of clupeocephalan teleosts. Highly specialized tendon bone pterygoids are found in some teleosts (e.g., siluroids). The presence of both maxilla and lacrimal lateral to the pars autopalatina is synapomorphous of osteichthyans. The eye supported by the bony palatoquadrate is a teleostomian synapomorphy. Dermal elements support the eye in actinopterygians, the entopterygoid in advanced actinopterygians, but the ectopterygoid in lepisosteids. A quadratojugal is a synapomorphy of osteichthyans but exhibits a number of transformations in connection with the vertical pit-line and the preopercular canal; a quadratojugal bearing the vertical pit-line is the primitive condition for osteichthyans. Ontogenetic evidence does not support the homology of the membranous posterior process of the teleostean quadrate with the quadratojugal. The lack of a quadratojugal and the presence of the elongate posterior or posteroventral process of the quadrate is a synapomorphy of teleosts.