The dental plate on bichir pectoral fins: A unique dermal skeletal element bearing individual odontodes with tooth-like replacement.

Abstract

The dermal skeleton appeared early in vertebrate evolution in the form of mineralized skin denticles composed of tooth-like units-odontodes. This surface odontogenic competence later also expanded inside the oropharyngeal cavity where teeth are formed as modified odontodes possessing innovative replacement dynamics. Whereas in modern cartilaginous fishes, both the internal odontodes (teeth) and external odontodes (scales) exhibit generally the same shape and structure, the dermal skeleton of bony fishes was further modified by the fusion of odontodes forming so-called odontocomplexes. This ancient dermal armour was reduced in both ray-finned and lobe-finned fishes, or disappeared in tetrapods. Bichirs (Polypteridae) occupy a key phylogenetic position as the earliest extant ray-finned fishes retaining a massive dermal skeleton. We performed developmental and structural analyses of their odontocomplex elements comprising the cranial dermal bones, trunk scales, fin rays, and spines of the dorsal finlets, primarily using the Senegal bichir (Polypterus senegalus). All these elements are covered by a hypermineralised layer ganoine, considered to be a true enamel. Yet, during the development of these odontocomplex elements, individual odontodes could not be recognised. However, we also identified one unique dermal element with a dual structural nature combining the scale-like odontocomplex with individual odontodes. These so-called dental plates form a narrow series of repeating elements that extend in between the fin rays on bichir pectoral fins. Individual odontodes on these dental plates are organised into C-shaped rows attached to a scale-like element. Interestingly, these individual odontodes bear striking morphological and histological similarities to teeth, and their dynamics of replacement parallel that of teeth in bichir oral dentition. Dental plates occupy a distinct dermal skeletal domain on distal pectoral fins, where replacing odontodes form a spiky surface with apparent functional advantages when bichirs rest their pectoral fins upon the substrate.