Evolution and phylogenetic classification of the Diplograptacea

The complex astogenetic patterns produced by the specialized, first few thecae (primordial thecae) of graptoloid rhabdosomes were conserved during evolution and provide a reliable guide to 'propinquity of descent' among diplograptids. Using this principle one can redefine their taxonomy, establish phylogenetically meaningful higher taxa, and obtain an improved understanding of diplograptacean phylogeny. The Diplograptacea comprises four major subclades. 1, Orthograptidae: archaic orthograptids (species of the 'Glyptograptus' teretiusculus species group), Orthograptus and Amplexograptus, together with archiretiolitids, lasiograptids, and 'Climacograptus' typicalis-'G.' lorrainensis groups; 2, Dicranograptidae: dicranograptids plus nemagraptids; 3, Diplograptidae: pseudoclimacograptids and Climacograptus s.s., together with Diplograptus s.s. and offshoots; 4, Monograptidae: 'G.' dentatus and descendants including Undulograptus paradoxus, G. euglyphus, 'C.' normalis, and all the Silurian diplograptids including the uniserial monograptines. The first three families dominated Ordovician faunas. Taxa with complex proximal end structures were succeeded during the Llandeilo and Caradoc by taxa with simpler astogenies. Following extinction of the dominant Ordovician taxa, monograptids (sensu lato) underwent explosive evolution in the Llandovery. New generic group taxa: Archiclimacograptus, Arnheimograptus, Diplacanthograptus, Eoglyptograptus, Geniculograptus, Hustedograptus, Oelandograptus, Pseudamplexograptus, and Urbanekograptus. New family group taxa: Eoglyptograptinae and Orthograptidae.