A Phylogeny for Heterostraci (stem-gnathostomes)

The armoured jawless fishes (ostracoderms) are major and widespread components of middle Palaeozoic ecosystems. As successive plesia on the gnathostome lineage, they reveal the early sequences of vertebrate evolution, including the assembly of the vertebrate skeleton. This is predicated however, on understanding of their diversity and interrelationships. The largest ostracoderm clade, the Pteraspidimorphi, is often reconstructed as sister taxon to other boney vertebrates yet they lack a phylogenetic framework, in particular the heterostracans. Problematic heterostracans with a tessellate headshield ('tessellate-basal' model) are often regarded as the plesiomorphic condition for the clade but no phylogenetic analysis has included these taxa. Here we review the Heterostraci and present their first comprehensive phylogenetic analysis (131 heterostracan taxa and 12 outgroup taxa). Heterostraci and Ordovician Pteraspidimorphi are recovered as sister-group to all other boney jawless vertebrates in parsimony analyses, however, in no instances do we recover a monophyletic Pteraspidimorphi. Tree visualization reveals lack of resolution results from two conflicting solutions for the heterostracan 'root'. Stratigraphic congruences provides support for the macromeric Ctenaspisdidae as sister taxon to all other Heterostraci rather than the tesselate-basal model. The results presented here are the first phylogenetic hypotheses of heterostracan relationships and it is hoped a first step into an accurate interpretation of character evolution and polarity in this crucial episode of vertebrate evolution.