PHYLOGENY, CHARACTER EVOLUTION, AND UPDATED TAXONOMY OF EARLY ORDOVICIAN GRAPTOLITES

The early astogenetic patterns and sequence of budding are considered the most relevant for phylogenetics and taxonomy in graptolites. The morphological diversity in Early Ordovician graptoloids is typically considered very conservative, making the identification of monophyletic groups for reliable taxonomy problematic. An updated cladistical analysis solves this problem by utilizing new characters involving proximal end development of the graptolite colony, as well as continuous characters showing proximal geometry. The resulting cladogram reveals 5 major clades relevant for taxonomy: the Anisograptidae, Dichograptidae, Sigmagraptidae, Glossograptidae, and Arienigraptidae. The Anisograptidae are here defined as the stem group of the Graptoloidea, with the dichograptids and sigmagraptids considered sister groups, and the glossograptids and arienigraptids sister groups. These clades are defined by synapomorphies such as the loss or retention of bithecae, asymmetrical proximal branching, proximal symmetry, sicula size and shape, and origin of the first theca. Quantitative analysis of proximal developments using a landmark-based method to capture proximal end geometries illustrates further disparity between clades, specifically with regards to sicula length and width, and position of important landmarks in the proximal end structure. These variations in proximal development of the colony appear to demonstrate macroevolutionary trends as the Ordovician progressed. In addition to the results of the analysis, new terms are introduced to better explain variation within these groups, such as sigmagraptine type vs. pseudisograptid type prothecal folds, parallel vs. expanding conus shape, paired rhabdosomal spines, and unconnected vs. monopleural/dipleural development.