NEW GENERA OF DIMEROPYGID TRILOBITES FROM THE EARLY ORDOVICIAN OF THE WESTERN UNITED STATES AND THE PHYLOGENETIC STRUCTURE OF THE FAMILY

Dimeropygidae Hupé, 1953, is a clade of generally small, vaulted, and tuberculate trilobites presently assigned to 19 genera and 113 species. The group ranged in time from latest Cambrian to end-Ordovician. Although globally distributed at lower latitudes during the Middle and Late Ordovician, about 60% of its named species diversity is Laurentian, as is almost all of its known Early Ordovician distribution. Several parsimony analyses of limited components of the family have been published, but there has been no formal hypothesis of its overall phylogenetic structure.

Field based revision of rich secondarily silicified faunas spanning the Early Ordovician and early Middle Ordovician of the Great Basin (Nevada, Utah, and Idaho) has yielded a wealth of undescribed new species diversity. Silicification permits the recovery of individual sclerites, completely freed from the matrix via acid digestion. Hence the species may be documented on the basis of multiple examples of most sclerites, including in many cases early growth stages. New finds include species which should be assigned to currently named generic clades and many that represent entirely new components. Some of the latter have been described and new genera proposed (Tulepyge Adrain and Westrop, 2006; Bearriverops Adrain and Westrop, 2007; Heckethornia McAdams and Adrain, 2009). However, current collections include at least 40 well represented new dimeropygid species. Preliminary cladistic analysis suggests that these should be assigned to nine genera, of which four are new.

Further, new early (Skullrockian Stage; early Tremadocian) species representing the genus Hystricurus sensu stricto, reveal that this taxon is unambiguously a dimeropygid. Resemblance of later species of Hystricurus to the plethora of other Early Ordovician aulacopleuride species traditionally assigned to Hystricuridae Hupé, 1953, is superficial and Hystricuridae is a junior subjective synonym of Dimeropygidae.

Species diversity and synapomorphies of the new genera are summarized, and the morphology of stratigraphically early species of Hystricurus is documented, emphasizing synapomorphies with Dimeropygidae. The new data are incorporated into species-level analyses of the new Early Ordovician genera and a preliminary analysis of the overall structure of the family.