Phylogenetic relationships of the ‘higher heterostracans’ (Heterostraci: Pteraspidiformes and Cyathaspididae), extinct jawless vertebrates

Heterostracans are a clade of extinct jawless fishes that were a large component of early vertebrate ecosystems. They are key members of the gnathostome stem, and as such they are central to our interpretation and understanding of early vertebrate evolution. Our knowledge of heterostracan inter-relationships is incomplete, with previous phylogenetic analyses focusing on the intra-relationships of individual clades. The two largest clades of heterostracans, Cyathaspididae and Pteraspidiformes, comprise Janvier’s ‘higher heterostracans’ and have been interpreted as monophyletic sister groups. Here we present a new parsimony-based phylogeny for the Pteraspidiformes and Cyathaspididae including quantitative data. Our results indicate that Cyathaspididae are a paraphyletic assemblage with respect to a monophyletic Pteraspidiformes (which includes the Psammosteidae). Furthermore, we show that the Anchipteraspididae are sister group to Pteraspidiformes, when using pteraspid-type bauplan to interpret their anatomy. Application of stratigraphic range data and congruence measures was used to assess the performance of different treatments of phylogenetic data in terms of stratigraphic consistency; equal weighting of quantitative and qualitative characters was found to consistently outperform implied weighting. Our combined results indicate that inclusion of quantitative characters can greatly improve resolution of ostracoderm phylogenies and provide a framework to consider the intra- and inter-relationships of heterostracans more broadly.