Phylogenetic relationships of psammosteid heterostracans (Pteraspidiformes), Devonian jawless vertebrates

Psammosteid heterostracans are a group (suborder Psammosteoidei) of Devonian-age jawless vertebrates, which is included in the order Pteraspidiformes. The whole group of psammosteids is represented by numerous species (more than 40); their phylogenetic relationships are still poorly known and deserve further study. Classical researchers of the psammosteids, such as D. Obruchev, E. Mark-Kurik and L. Halstead Tarlo, had different views on the phylogeny of the group (e.g. origins and evolution of Psammosteus). To check the modern hypotheses of psammosteid origins from various Pteraspidiformes and to clarify psammosteid interrelationships, the most complete phylogeny of this group (38 ingroup taxa + juvenile Drepanapsis) is presented here. Different methods of data analysis were used to explore the psammosteid data set, including equally weighted characters versus implied weighting. According to the results of the phylogenetic analysis, the monophyletic status of the group and their early development from the Pteraspidiformes are supported. The diagnoses and interrelationships of many taxa are clarified. Two new genera are proposed (Vladimirolepis gen. nov. and Elgaia gen. nov.), and two subfamilies are erected (Placosteinae subfam nov. and Psammosteinae Traquair).