Taxonomy, phylogeny and variability of Pseudisograptus Beavis

Manubriate isograptids from the Castlemainian and Yapeenian (Arenig) of New Zealand, Australian, and Chinese collections are described and analysed in terms of population systematics. Pseudisograptus manubriatus (T. S. Hall, 1914) is divided into five subspecies, of which P. m. koi, P. m. harrisi, P. m. janus, and P. m. texanus are new; P. bellulus sp. nov. and five other Pseudisograptus species are described. A tentative phylogenetic scheme is constructed from their stratigraphic occurrence and a cladogram of pseudisograptids and other forms with manubriate features. Similarities in proximal structure and thecal form link the pseudisograptids with the glossograptids, and the monophyletic suborder Glossograptina Jaanusson, here taken to include Apiograptus (syn. Exigraptus), Kalpinograptus (syn. Apoglossograptus), and Pseudisograptus, is down graded to family rank. The paraphyletic Pseudisograptus is a sister group to the monophyletic subfamily Glossograptinae. Details of proximal development and structure of the manubrium are described for P. m. janus and P. bellulus, using pyritized specimens preserved in full relief. From this data and that gleaned from flattened specimens of other taxa it is concluded that the manubrium structure described by Bulman, and taken to be a paradigm for the group, is in fact rare and atypical.