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Evolution and phylogenetic classification of the Glossograptidae and Arienigraptidae (Graptoloidea): new data and remaining questions

New data on the proximal structure of several glossograptid species, including Paraglossograptus holmi, indicate that symmetry between sicula and the first theca is ubiquitous and primitive for the group. This isograptid symmetry is shared with species of Isograptus and Oncograptus, among others, as is a dicalycal second theca, and the initial downward growth of the first several thecal pairs. Consideration of these and other features of the rhabdosome structure leads us to reassess the phylogenetic relations among Glossograptus, Pseudisograptus, Isograptus, and their relatives. The Glossograptidae is a holophyletic clade comprising the common ancestor of Isograptus caduceus and I. victoriae and all its descendants, including the monopleural glossograptines. Incipient monopleural development is exhibited by the otherwise Isograptus-like Bergstroemograptus crawfordi. Derived glossograptids, such as Glossograptus and Kalpinograptus, share a fully monopleural rhabdosome with a prominent proximal bulge formed by the overlap of the early thecae. Contrary to some claims, none possesses a manubrium or manubrium-like structure. The manubrium and the arienigraptid suture are restricted to Arienigraptus hastatus and its descendants. These features unite the Arienigraptidae. The Arienigraptidae, based on their possession of isograptid symmetry and lack of any derived characters shared uniquely with the glossograptids, appear to be a sister group of the Glossograptidae rather than a stem group. These two families appear most closely allied to certain reclined, two-stiped species, such as Isograptus? dilemma, that retain tetragraptid proximal symmetry.