Bearriverops, a new Lower Ordovician trilobite genus from the Great Basin, western USA, and classification of the family Dimeropygidae

Abstract

Bearriverops n. gen. is a distinctive clade of small, vaulted trilobites from the Lower Ordovician (Ibexian Series; Stairsian Stage) of Utah and Idaho. The genus includes at least seven new species known from silicified material of which five are well enough known to name: B. alsacharovi, B. borderinnensis, B. deltaensis, B. ibexensis, and B. loganensis (the type species). All are known from the lower Fillmore Formation in western Utah; B. alsacharovi and B. loganensis are also known from the Garden City Formation of southeastern Idaho. Both units record deposition in shallow subtidal environments above storm wave base. Bearriverops is characterized particularly by a suite of pygidial apomorphies apparently related to spiral enrollment. Its close relatives include a large group of mostly undescribed Skullrockian and Stairsian species with more conventional spinose pygidia. Together, the taxa are referred to Dimeropygidae, which is considered a senior synonym of Toernquistiidae. Cladistic parsimony analysis of Bearriverops indicates that B. alsacharovi and B. borderinnensis are sister taxa, and that B. loganensis, B. deltaensis, and the plesiomorphic B. ibexensis are successive sister taxa to this clade.