A new osteostracan fauna from the Devonian of the Welsh Borderlands and observations on the taxonomy and growth of Osteostraci

The Osteostraci are a morphologically diverse group of jawless fishes considered as sister taxon to jawed vertebrates. Fossil Osteostraci therefore have wide-reaching ramifications for our understanding of the origin and evolution of gnathostomes. Their utility in this context is currently impeded by rudimentary taxonomy of faunas from the United Kingdom. Principal among these is a well-preserved, previously undescribed fauna from the Early Devonian (Lochkovian) of Wayne Herbert Quarry, Herefordshire. Here we provide a description of new osteostracan material from this fauna. Anatomical observations of the character-rich, taxonomically informative headshield, combined with quantitative measurements, have permitted identification of 15 variable characters. From these, seven discrete morphogroups are recognized and used to establish the taxonomy of the material. This includes two new species of Zenaspididae (Zenaspis waynensis and Diademaspis janvieri), two new species of Cornuata incertae sedis (Janaspis newtonensis and J. punctata), two indeterminate species, and Pattenaspis whitei, which is redescribed on the basis of new material. Our analyses indicate that cornuate osteostracan species are likely over-split due to erection of new taxa based on poorly preserved type specimens, inadequate consideration of taphonomic character loss, and over-reliance on continuous rather than discrete characters. Furthermore, patterns of dermoskeleton ornamentation and tessellation of Diademaspis janvieri and Cornuata, gen. indet., sp. B are used to infer mechanisms of dermoskeletal growth. These new data allow incorporation of this fauna into scenarios of osteostracan evolution, considerations of gnathostome origin, and studies of global Lochkovian diversity. © 2012 Taylor & Francis Group, LLC.