A phylogeny for Heterostraci (stem‐gnathostomes)

Abstract The armoured jawless fishes (‘ostracoderms’) are major and widespread components of middle Palaeozoic ecosystems. As successive branches on the gnathostome stem lineage, they represent 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 Pteraspidomorphi, is often reconstructed as sister taxon to other bony vertebrates but they lack a cladistic 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 55 outgroup taxa). Heterostraci are recovered as monophyletic, and forming a clade together with the Ordovician Pteraspidomorphi near the root of the gnathostome crown group in parsimony analyses. The heavily armoured Amphiaspida are recovered as sister taxon to all other Heterostraci. Within the Heterostraci, the Psammosteida are nested within the Pteraspidiformes, which are sister group to the Traquairaspidida, while Cyathaspidida form a monophyletic group. 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 in this crucial episode of vertebrate evolution.