PHYLOGENETIC INTERPRETATION AND TAXONOMY OF CLIMACOGRAPTID GRAPTOLITES (DIPLOGRAPTACEA)

The precise phylogenetic relationships among species of the family Diplograptidae have been a subject of substantial disagreement among graptolite systematists. We have begun our investigation of this group with an analysis of fifteen species of the genera Climacograptus, Appendispinograptus, and Diplacanthograptus. We have compiled data on proximal structure, thecal shape, and colony form. Analysis of eighty-one characters found that 48 were parsimony informative. We employed Archiclimacograptus modestus, A. meridionalis, and Pseudoclimacograptus scharenbergi as potential outgroup taxa. A range of different branch-and-bound and heuristic searches involving alternative outgroups all found that the most-parsimonious trees were 181 steps. This tree length is entirely outside the bounds of 1000 random trees generated from this same data set, suggesting the presence of a meaningful phylogenetic signal among the data. Excluding informative characters, the CI is 0.5394 and RI is 0.5581. Thus, the most parsimonious trees include substantial homoplasy. Nevertheless, majority rule consensus trees and bootstrap support trees show strong support for several aspects of the cladogeny of this group: 1) There is strong support for the monophyly of the clade Climacograptus+ Appendispinograptus+Diplacanthograptus. This clade comprises a poorly resolved stem group (among which are the species Climacograptus bicornis, C. tridentatus, C. cruciformis, C. hastatus, and C. lanceolatus) which is sister to a monophyletic group of derived climacograptids (C. tubuliferus, C. uncinatus, C. nevadensis, and C. putillus, which lack proximal spines other than the virgella) and the clade Diplacanthograptus+ Appendispinograptus. The latter are themselves monophyletic sister taxa. This analysis suggests that the genus Euclimacograptus Riva (type species C. hastatus) is synonymous with Climacograptus Hall and that Ensigraptus Riva (type species C. caudatus) is synonymous with Diplacanthograptus Mitchell.