The evolution of vertebrate dentitions: phylogenetic pattern and developmental models

The classical theory that teeth evolved from dermal denticles linked with the origin of jaws no longer accounts for the diversity of new data emerging from agnathan and early gnathostome fossils. Tenets on which this is based and the concept of co-option of dermal denticles into the mouth are critically discussed. It is proposed that dermal and oral denticles exhibit patterning differences that imply divergence at loci deep within vertebrate phylogeny. Some whorl-like sets of oropharyngeal denticles, found in both fossil agnathans and gnathostomes, are proposed as a significant feature of morphological pattern. It is suggested that the developmental controls to produce these specialised arrangements in the oropharynx were co-opted in primitive gnathostomes for the dentition on the mandibular arch. These joined denticle sets could presage the tooth whorl sets spaced around the jaw margin, or files of separate teeth, each with replacement teeth aligned in a pattern of ordered size, shape and timing, as observed in primitive fossil and extant gnathostomes. Observations on dentitions of fossil adult primitive sharks and on ontogenetic pattern in extant sharks, lead to a developmental model for patterning the vertebrate dentition. This considers the germ layers ectoderm and endoderm as imparting real differences to the two systems (dermal and oral) through epithelial control of the primary signal to competent mesenchyme. Agnathan oropharyngeal feeding structures could be co-opted into denticle or tooth bearing feeding apparatuses in gnathostomes, quite independently of any prerequisite for dermal denticle-bearing ectoderm to line the primitive stomatodaeum. A model for metameric repeat in each quadrant of the dentition is proposed, in which compartments of tooth competence and incompetence are initiated from a medial, symphyseal signalling centre. It is concluded that data from phylogenetic and ontogenetic patterns in lower gnathostomes can influence the proposed model for patterning the vertebrate dentition and predict new and profitable data searches in higher gnathostomes.