A cladistic analysis aimed at understanding the phylogenetic relationships of basal (Early to Mid Ordovician) planktic graptolites resulted in a well-resolved majority rule consensus tree that displays a series of major monophyletic groups. This tree was used to produce a classification system for the planktic Graptoloida, one of the major groups of the Graptolithina. Resolution within the individual taxonomic units is low, however, as only few species of the individual groups were included in the analysis. Tree statistics such as the retention index show that the proximal end characters used in the analysis are more reliable indicators of synapomorphy, whereas distal rhabdosome characters show significantly more homoplasy on the consensus tree. This result supports previous qualitative assessments suggesting that the initial part of the colony appears to have been relatively conservative during the evolution of the graptolites, whereas the distal features are more easily modified. Thus, the proximal end structure is especially relevant for revealing shared common ancestry through the presence of distinct homologies. Based on the structure of the consensus tree, we recognize a series of nested holophyletic clades, primarily total clades (names of which are identified by the prefix Pan-) and crown clades. The proposed names conserve the traditional names applied to common graptolite higher taxa as much as possible and yet precisely describe the branching history of the clade. The Graptoloida is the clade that descended from the first graptolithine to possess a nematophorous sicula and planktic habit and comprises a stem lineage of "anisograptids" (e.g., Rhabdinopora flabelliformis and Anisograptus matanensis), plus the crown clade Supercohort Eugraptoloida. The Eugraptoloida includes all the remaining graptoloids, which form the sister taxa, Cohort Pan-Reclinata nov. and Cohort Pan-Sinograpta. Many familiar genera formerly included among the Dichograptidea form a stem lineage to the crown clade Subcohort Reclinata nov. The Reclinata includes the sister clades, Superorder Pan-Tetragrapta nov. and Superorder Pan-Bireclinata nov. The Pan-Bireclinata comprises stem group bireclinatids (including Isograptus) and the crown clade Order Bireclinata nov., which in turn consists of the sister total clades Suborder Pan-Glossograpta nov. and Suborder Pan-Axonophora nov. The Pan-Axonophora are represented in this analysis only by stem lineage species (Arienigraptus gracilis to Exigraptus uniformis), but the group contains the roots of the diverse and well-known diplograptids and monograptids, which dominate subsequent Middle Ordovician to Early Devonian graptolite faunas.•

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Detailed studies on Carboniferous species of the xenacanth