a1 Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ, UK and Department of Palaeontology, Natural History Museum, Cromwell Road, London, SW7 5BD, UK
The jawless Osteostraci are the sister taxon to jawed vertebrates and, thus, are integral to understanding the evolution of gnathostomes from a jawless ancestor. Our understanding of this episode is currently hampered by incomplete knowledge of osteostracan phylogenetics and character polarity. Previous hypotheses of osteostracan intrarelations are reviewed and synthesised into two conflicting hypotheses – ‘Tremataspis-basal’ versus ‘Ateleaspis-basal’ models, both related to the ancestral morphotype of the group. Previous analyses are shown to be limited in three main regards – taxonomic scope, phylogenetic methodology and character polarity. To address this problem, direct observations and reconstructions were made for every osteostracan genus. This enabled a taxonomically comprehensive analysis, utilising global parsimony and out-group analysis. The inclusion of a morphological range of taxa from both extant and extinct osteostracan-related clades (pteraspidomorphs, galeaspids, placoderms and gnathostomes), enables a test of osteostracan polarity and character evolution. A new phylogeny and classification for the Osteostraci is proposed, identifying three main orders – Benneviaspidida, Zenaspidida and Thyestiida. The ‘Ateleaspis-basal’ model is supported along with the homology of paired pectoral fins of Osteostraci and jawed vertebrates.