PHYLOGENY OF THE EARLY ORDOVICIAN TRILOBITES IBEXASPIS, PSEUDOMERA, AND RELATED TAXA: RECONSTRUCTING PLIOMERIDAE FROM THE GROUND UP

Silicified faunas from Lower Ordovician strata of the Great Basin contain numerous species of pliomerid trilobites. Many of these taxa have been overlooked or poorly described in the past, despite the fact that their abundance and excellent preservation makes them essential for phylogeny reconstruction. The lack of an explicit phylogenetic hypothesis for these groups obscures the structure of Pliomeridae and inhibits exploration of broader relationships between Pliomeridae and Cheiruridae.

Data from new and previously established species show that cranidia, pygidia, and hypostomes of Pseudomera have often been incorrectly associated in the literature, resulting in chimaera reconstructions and confusing phylogenies. This information also demonstrates that Kanoshia Harrington, 1957, is a junior synonym of Pseudomera Holliday, 1942.

Parsimony analysis includes some 30 species, among them members of Pseudomera, nine new species of Ibexaspis, and a new genus of 10 species including one formerly assigned to Hintzeia. The results indicate that Ibexaspis and the new genus are sister taxa, but members of Pseudomera may create paraphyly in this group. This clade is potentially sister to a group including Protopliomerella, Pseudocybele, Lemureops, and Hintzeia, and this hypothesis will be explored in future work.

Establishing phylogenies for Lower Ordovician pliomerids forms an essential basis for incorporating Middle Ordovician and younger taxa, such as Ectenonotus, and eventually building a modern understanding of this group that will in turn form the basis for investigating higher level phylogenies.