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The long-established scenario of teeth evolving from dermal denticles in direct association with the evolutionary origin of jaws, is based on the observation that shark (chondrichthyan) dermal denticles and oral denticles are homologous and contiguous with teeth. This theory, however, no longer accounts for the diversity of new data emerging from the fossil record. Certain denticle-covered fossil agnathan ®shes (thelodonts) are now known to have well-developed oropharyngeal denticles. Moreover, the conodonts, a large and geologically extensive fossil group with a naked body but phosphatic mineralized feeding apparatus, are now thought to belong within the vertebrate clade. This projects the evolutionary origin of teeth, or at least a specialized suite of oral denticles, back to a point some 50 million years earlier than previously thought. These data suggest that `teeth’, in a broad sense of the term, may precede jaw evolution, and possibly precede the evolution of dermal armour in some primitive vertebrates. Alternatively, other new evidence shows that toothless armoured ®sh probably existed contemporaneously with conodonts. These include taxa with either dentine plus acellular bone-like basal tissue (Anatolepis, M. P. Smith et al., 1996), or acellular skeletal tissue, but no dentine (Young et al., 1996). Such discoveries challenge previously held views on the relative primitiveness of different tissues, and predict important stages in the evolution of signalling molecules for regulatory mechanisms during craniate mineralized skeletogenesis. Furthermore, the precise geological dates associated with these data, when placed in the context of recent phylogenies of early vertebrates, require hypotheses of signi®cant `ghost lineages’ (Norell, 1992: i.e. those lineages that are implied by emergent gaps in phyloge- netic trees plotted against a time axis) in order to account for a diversity of vertebrate types emergent
before the Cambrian±Ordovician boundary (see Figure 10.3). The whole concept of teeth linked to jaws as an integrated functional unit, constant throughout evolution and development, needs to be reassessed.
The theory that teeth only evolved with the imperative of biting jaws, states that marginal oral skin denticles became enlarged to function as teeth once the ®rst branchial arches became modi®ed to close tightly with a dorsoventral opposing action. Such enlarged marginal denticles were specialized to form in `advance of need’ from a subepithelial dental lamina, instead of `on demand’ as space became available in the skin. Furthermore, these denticles were endowed uniquely with morphological and pattern information, speci®c for the precise functioning of the dentition. This information could concern spatial morphological variation (heterodonty as in the Port Jackson shark: see Figure 10.4C), tooth location in particular regions of the oral cavity, and timing of formation linked to shedding of functional teeth and eruption. Control of tooth size is very precise, with dimensional increases for new teeth in an exact gradation relative to body size and morphological change (see section 10.6). The survival value of an ef®cient dentition throughout an animal’s life history is extremely high, and likely to be selected for and conserved in evolution, with similarly conserved essential developmental controls and regulatory mechanisms.
One of the key innovations at the origin of craniates was the developmental potential of a fourth germ layer, neural crest-derived mesenchyme, to make skeletal tissues in the head, in particular teeth (Gans and Northcutt, 1983). This population of cells, in migrating ventrally from a dorsal position in the neural ectoderm to

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