Phylogeny of the Pteraspidiformes (Heterostraci), Silurian-Devonian jawless vertebrates

The Pteraspidiformes are an order of armoured agnathans of Silurian and Devonian age composed of five families: the Anchipteraspididae, Protopteraspididae, Pteraspididae, Protaspididae and Psammosteidae. Relationships within the order are poorly known and although past attempts have been made to represent the state of knowledge in this area none have attempted a rigorous computer-based analysis. Here we present the results of the cladistic analysis on a matrix comprising 25(+1) taxa and 59 characters using Winclada. The first analysis was run without any representative of the Psammosteidae and supports the presence of five groups within the order although one, the 'Protopteraspididae', is paraphyletic and a second, the Gigantaspididae, is newly recognized. In a second analysis the addition of the psammosteid Drepanaspis gemuendenensis results in a better resolved tree in which the basal family, the Anchipteraspididae, are clearly monophyletic and in which the Psammosteidae, as represented by Drepanaspis, is the next less inclusive. This is contrary to the previous view that the psammosteids were an advanced clade within the Pteraspidiformes.