Exploring phylogenetic relationships of Pteraspidiformes heterostracans (stem-gnathostomes) using continuous and discrete characters

Ostracoderms are a paraphyletic group of extinct jawless fishes comprising the gnathostome stem and are fundamental to our understanding of early vertebrate evolution. However, only a handful of these clades have robust phylogenies in place, hindering our interpretation of early vertebrate histories. A new phylogeny is proposed for the Pteraspidiformes – the largest and most studied clade of heterostracan ostracoderms. Difficulties such as large amounts of missing data and the limited morphological variability within the group has led us to explore different coding strategies such as the inclusion of quantitative data and implied weighting. We present a new comprehensive dataset including all described genera of Pteraspidiformes (47 taxa) analyzed using discrete characters only, a combination of discrete and continuous characters, and gap coding strategies (transforming the continuous into discrete characters), along with a bayesian analysis. Two representatives of the Psammosteidae (Drepanaspis and Psammosteus) are also incorporated within the analysis to elucidate their inclusiveness within the Pteraspidiformes. Well-resolved trees are only achieved under re-weighted (implied weighting) analyses. Here we show that many ‘classic’ Pteraspidiformes clades hold true under our different coding methods, with the implied weighting of discrete characters and inclusion of continuous characters giving very similar topologies. In all instances, the Psammosteidae, are found to belong within the Pteraspidiformes, nested with the Spitsbergen forms Doryaspis, Xylaspis and Woodfjordaspis. Gap coding, however, results in a different tree topology to other analyses, perhaps due to the high sensitivity to missing data. Our results indicate, when reconstructing relationships of homoplastic ostracoderms, that careful consideration and justifications should be applied to quantitative characters.