ABSTRACT
Existing hypotheses for the origin of a vertebrate exoskeleton are reviewed and it is concluded that most are incompatible with hypotheses of early vertebrate relationships or are merely narrative explanations for patterns apparent in phylogenetic hypotheses. The relevance of conodonts to the debate over the origin of the vertebrate skeleton depends ultimately upon two factors: 1 homologies of the hard tissues from which conodont elements are composed 2 the phylogenetic position of conodonts
(these factors are not necessarily mutually exclusive). The evidence for homology of conodont and vertebrate hard tissues is reviewed and homology in the vertebrate skeleton discussed. Recent work indicates that conodonts are the most plesiomorphic members of the total-group Gnathostomata, lying between the living groups of jawless craniates, which lack a mineralized skeleton, and the extinct groups of jawless vertebrates, all of which possess an extensively developed external dermal armour. This suggests that the earliest condition for the vertebrate skeleton is, as represented by conodonts, in the form of an oro/pharyngeal raptorial feeding array. However, this view depends entirely upon the phylogenetic position of conodonts and we urge caution against reliance upon evolutionary narratives dependent upon unstable phylogenetic hypotheses.
