PHYLOGENETIC CLASSIFICATION AND EARLY EVOLUTION OF THE DICHOGRAPTACEA

Planktic graptolites arose close to the base of the Ordovician System from benthic dendroid graptolites of the order Dendroidea, but the transition and diversification of these is not well understood. The earliest planktic graptolites of the order Graptoloidea appear with the genus Rhabdinopora and the group rapidly diversified in the Tremadoc. Several major groups of the Dichograptacea evolved during this time interval, and their exact phylogenetic relationships are explored here.

Phylogenetic classification of Graptoloids requires placing high taxonomic importance on characters related to the early astogeny of the rhabdosome, e.g. the proximal end development. Phylogenetic classification of the Dichograptacea has proven difficult because of the relatively minor variation in these proximal end characters and poor specimen quality. An evaluation of isolated dichograptid specimens reveals previously unknown discreet features related to their proximal end development that can be used for a better phylogenetic interpretation. A cladistical analysis and reorganization of the Dichograptacea utilizing this updated data indicates their evolutionary history and morphological similarity in comparison to other graptoloid families and benthic outgroups.

The Anisograptidae with its transitional forms can be regarded as the stem group for all planktic graptoloids and are here included in the Graptoloidea. The Dichograptacea can be differentiated into the Dichograptidae and the Sinograptidae, based on distinct features of their sicula development and the geometry of branching at the proximal end. Proximal end structure also shows that the virgellate dichograptids (Xiphograptus, Pterograptus, Phyllograptus) are unrelated to the Diplograptacea and are nested within the Dichograptacea, making the retention of the Virgellina as a clade unnecessary.