Phylogenetic analysis, systematics, and evolution of early Ordovician graptolites
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The exact phylogenetic relationships of many early Ordovician graptolites are unknown due to a lack of knowledge about the detailed proximal end structure of these groups. The proximal end structure of graptolite rhabdosomes are considered especially relevant for revealing shared common ancestry. During evolution the initial part of the colony is conserved while the more distal features are more easily modified by selection. Isolated specimens from the Cow Head and Table Head Groups in western Newfoundland were used for a cladistic analysis with an emphasis on characters related to proximal end structure. The results of this analysis are summarized as a 175 step majority rule consensus tree displaying a series of large monophyletic groups. Applying Linnaean hierarchy conventions to the dataset produced a high level taxonomic classification system that reflects the topology of the cladogram. The Order Sinograptoidea is a sister taxon to the Orders Dichograptoidea, Tetragraptoidea, and Glossograptoidea. The Order Dichograptoidea is a paraphyletic stem group to the sister taxa Tetragraptoidea and Glossograptoidea. Anisograptid graptolites are a stem group to the entire Graptoloidea, and isograptid graptolites are a stem group to the Glossograptoidea. All of these orders are monophyletic groups, with the exception of the Dichograptoidea. This classification also follows very closely with accepted nomenclature. Tree statistics such as retention index indicate that the proximal characters used in the analysis are more reliable indicators of synapomorphy. Distal characters show significantly more homoplasy on the consensus tree than do proximal end characters.