Placoderm fishes, pharyngeal denticles, and the vertebrate dentition

The correlation of the origin of teeth with jaws in vertebrate history has recently been challenged with an alternative to the canonical view of teeth deriving from separate skin denticles. This alternative proposes that organized denticle whorls on the pharyngeal (gill) arches in the fossil jawless fish Loganellia are precursors to tooth families developing from a dental lamina along the jaw, such as those occurring in sharks, acanthodians, and bony fishes. This not only indicates that homologs of tooth families were present, but also illustrates that they possessed the relevant developmental controls, prior to the evolution of jaws. However, in the Placodermi, a phylogenetically basal group of jawed fishes, the state of pharyngeal denticles is poorly known, tooth whorls are absent, and the presence of teeth homologous to those in extant jawed fishes (Chondrichthyes + Osteichthyes) is controversial. Thus, placoderms would seem to provide little evidence for the early evolution of dentitions, or of denticle whorls, or tooth families, at the base of the clade of jawed fishes. However, organized denticles do occur at the rear of the placoderm gill chamber, but are associated with the postbranchial lamina of the anterior trunkshield, assumed to be part of the dermal cover. Significantly, these denticles have a different organization and morphology relative to the external dermal trunkshield tubercles. We propose that they represent a denticulate part of the visceral skeleton, under the influence of pharyngeal patterning controls comparable to those for pharyngeal denticles in other jawed vertebrates and Loganellia.