PHYLOGENETICS OF THE EARLY ORDOVICIAN PLIOMERID TRILOBITES PROTOPLIOMERELLA, PSEUDOCYBELE, AND RELATED TAXA: UNRAVELLING THE BASE OF THE CHEIRUROIDEAN RADIATION

The Great Basin of the western United States is a source of abundant, well-preserved, stratigraphically early cheiruroidean species, which have historically been poorly described or overlooked, despite informing unclear and unexplored relationships within and between the families Cheiruridae and Pliomeridae. Such taxa include groups of species classified as Protopliomerella and Pseudocybele, and species assignable to a new pliomerid genus. A phylogenetic hypothesis of this group is necessary to understand the relationships of numerous new pliomerid taxa, and provide a basis for further work reconstructing pliomerid relationships and searching for the basal node of Cheiruridae.

Parsimony analysis of this group included 22 species coded for 40 characters and 106 character states. All characters were unordered. A species of Hintzeia served as the outgroup. An implicit enumeration search resulted in 5058 trees of length 108, consistency index .61, and retention index .80.

Prior to this analysis, Pseudocybele included only three species. Membership of Pseudocybele is increased by seven namable species, and the analysis reveals that a species previously assigned to Pseudocybele forms a new genus with six new species. The new genus is robustly supported by CI-one synapomorphies and a high bootstrap value. Results also indicate that Pseudocybele and the new genus are sister taxa, and Protopliomerella forms a grade at the base of the tree. Pseudocybele may not be monophyletic, as several members resolve outside the clade containing P. nasuta.

This explicit phylogenetic hypothesis is a starting point for further phylogeny reconstruction including related pliomerid taxa such as Cybelopsis, Strotactinus, and Hintzeia. This work will build toward a modern understanding of the inclusivity of both Pliomeridae and Cheiruridae, resolving relationships between these complex clades.