THE DAWN OF THE VERTEBRATES - CHARACTERS VERSUS COMMON ASCENT IN THE RISE OF CURRENT VERTEBRATE PHYLOGENIES

Armoured fossil jawless fishes, or 'ostracoderms', have long been rega rded as being ancestral to Recent hagfishes and lampreys. The latter w ere supposed to have lost the mineralized exoskeleton and undergone a 'degeneracy' linked with their burrowing or ectoparasitic modes of lif e. However, recent cladistic analyses suggest that most, if not all 'o stracoderms' are more closely related to jawed vertebrates than to eit her lampreys or hagfishes, although they are clearly jawless. These vi ews are very similar to those expressed by the early British palaeonto logists who made the first attempts at placing these extinct taxa in t he classification of the vertebrates. The chaotic history of the phylo genetic position of the 'ostracoderms' seems to be due to varying appr oaches to the use of either characters or common ascent in phylogeny r econstruction.