The taxonomy and phylogeny of British ‘cephalaspids’

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Abstract
Osteostracans are considered the sister group of jawed vertebrates and are phylogenetically the earliest branching vertebrate lineage to exhibit true paired fins with endoskeletal support. This means they take centre stage in understanding the jawless‐to‐jawed transition and the origin of paired appendages. Unfortunately, their macroevolutionary significance is obscured by current rudimentary understanding of their taxonomy and phylogenetic relationships. This is especially the case with the notoriously problematic wastebasket taxon ‘ Cephalaspis ’. Previous systematic studies of British ‘cephalaspids’ have relied largely on qualitative assessments of continuous traits, but it remains unclear to what extent these traits can reliably distinguish morphospecies. Here, we perform a geometric morphometric analysis to assess the morphological disparity of ‘cephalaspid’ osteostracans from the Midlands Valley and the Anglo‐Welsh Basin. Using this approach, we find that continuous traits cannot distinguish current/existing taxa within and across faunas, emphasizing the importance of discrete characters to support robust diagnoses. We revise ‘cephalaspid’ taxonomy accordingly. We establish four new genera: Platyrhakis , Saigheadaspis , Ymersaspis and Rochtarge , and propose the synonymy of five species with existing taxa. We estimate the phylogenetic affinity of these new genera using parsimony searches with ‘rogue’ taxon removal and Bayesian analyses. We find that while several groups are statistically well‐supported, they are united by highly homoplastic characters, highlighting the problematic nature of current osteostracan morphological datasets.

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