Phylogenetic relationships of the ‘higher heterostracans’ (Heterostraci: Pteraspidiformes & 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 the Cyathaspididae are a
paraphyletic assemblage with respect to a monophyletic Pteraspidiformes
(which includes the Psammosteidae). Furthermore, we show the
Anchipteraspididae are sister group to the 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 provides a
framework to consider the intra- and inter-relationships of heterostracans
more broadly.