A PHYLOGENETIC ANALYSIS OF THE ORTHOGRAPTINAE (EARLY LATE ORDOVICIAN GRAPTOLITHINA)

Diplograptid graptolites underwent a major evolutionary radiation during the early Late Ordovician in which the derived sub-clades, the Climacograptidae, Dicranograptidae, and Orthograptidae, replaced the primitive diplograptaceans. A major component of this radiation was the very rapid expansion of the sub-family Orthograptinae, which subsequently dominated graptolite faunas for the remainder of the Ordovician Period. The rapid progression of this radiation has obscured the branching history of the orthograptines. In particular, the phylogenetic relationships of the genera Amplexograptus, Geniculograptus, Orthograptus, and Rectograptus are not well understood. In this study, we conducted a phylogenetic analysis of 14 representative taxa from these genera. Most parsimonious trees were located using the branch and bound search method within PAUP. Amplexograptus, Orthograptus, and Rectograptus exhibit a Pattern G proximal development pattern, and Geniculograptus has a Pattern F. These developmental patterns are thought to have been derived from orthograptids that exhibit a Pattern A astogeny. Hence, character polarity was determined using the Pattern A orthograptids, Hustedograptus uplandicus and H. teretiusculus, as the outgroup. Our analysis produced a large number of equally parsimonious trees, which differed mostly in the internal relationships of species within well-supported genera. In a strict consensus tree, Orthograptus is a paraphyletic taxon from which the amplexograptids, rectograptids, and geniculograptids are derived. The Orthograptus calcaratus species group is the primitive sister group to the rest of the derived orthograptids, and the Orthograptus quadrimucronatus group is sister to the Rectograptus - Amplexograptus - Geniculograptus subclade. Rectograptus is the sister group to Amplexograptus and Geniculograptus. The relationships within the O. quadrimucronatus species group and Amplexograptus are unresolved. Overall, our analyses suggest that Amplexograptus and Rectograptus are more closely related to each other than either is to Orthograptus, and that the O. calcaratus species group represents the first branch of the orthograptine radiation.