The classification of the Pterobranchia (Cephalodiscida and Graptolithina)

This paper presents a proposal for a taxonomic approach to the classification of the Pterobranchia (Cephalodiscida and Graptolithina) to be adopted for the revision of the Treatise on Invertebrate Paleontology, Part V (Hemichordata), currently in preparation. A combination of traditional Linnaean taxonomy, supported by cladistic analyses in some groups is proposed herein as a practical solution for the classification of the Graptolithina as for many groups a cladistic analysis has never been attempted and is unlikely to be undertaken in the near future. The number of ranked taxa has been kept as low as possible, with all genus level taxa referred to a family. All families and higher taxonomic units are discussed, but new taxa have not been introduced. Paraphyletic (but not polyphyletic) taxa are accepted as useful units in this classification. A number of recently introduced taxonomic units, based on cladistic analyses (e. g. Eugraptoloida, Pan-Reclinata, Pan-Bireclinata), are discussed in the context of this classification and the usefulness of these taxa is critically evaluated. The solution proposed here opts not to name a number of nodes from the published cladistic analyses that potentially could be named and in some cases have been named - not to inflate the hierarchy of the used taxonomic system. Taxa are kept as close as possible to their original definition and not unnecessarily expanded or restricted. The taxonomy proposed here for the Graptolithina indicates that the extensive use of higher level taxa, e. g. orders for small groups of genera as has been done for many benthic graptolite groups in the past is unnecessary and should be avoided.