EVOLUTIONARY ORIGINS OF THE VERTEBRATE DENTITION - PHYLOGENETIC PATTERNS AND DEVELOPMENTAL EVOLUTION

The theory that teeth evolved from dermal denticles linked with the or igin of jaws no longer accounts for the diversity of new data emerging from the fossil record. We have reviewed oropharyngeal dental pattern s in all fossil groups of early vertebrates to establish the primitive condition, in order to understand the polarity of change. The evoluti onary procedence of dermal denticles before teeth now seems less likel y; both may be alternative manifestations of a common morphogenetic sy stem. This developmental system involves regulatory changes affecting the odontode, a fundamental exoskeletal unit, and can explain skeletal diversity. However, tooth and denticle differences may have diverged at loci deep within vertebrate phylogeny, as real differences exist be tween them. Teeth were conceived as evolving from non-growing odontode s with regulation of precise increase in size, position, sequence of t ime of development, and polarity of shape. A characteristic feature of teeth is the ability to replace from a developing sequence, programme d with these parameters, prior to demand. Tooth whorls, a feature of d enticles in the oropharyngeal region, may be regarded as a preadaptati on of this tooth replacement mechanism. The new fossil evidence sugges ts that teeth may have evolved from these more specialised oropharynge al denticles in agnathan vertebrates.