Introduction

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 fishes (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 fish 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.