The Early Ordovician (late Tremadocian; Stairsian) dimeropygid trilobite Pseudohystricurus Ross JONATHAN M. ADRAIN, TALIA S. KARIM & STEPHEN R. WESTROP ADRAIN, J.M., KARIM, T.S. & WESTROP, S.R., 2014:04:25. The Early Ordovician (late Tremadocian; Stairsian) dimeropygid trilobite Pseudohystricurus Ross. Memoirs of the Association of Australasian Palaeontologists 45, 215-232. ISSN 0810-8889. The Early Ordovician genus Pseudohystricurus Ross has been treated as a minor taxonomic wastebasket for poorly known species featuring small, dorsally convex cranidia and tuberculate sculpture. No previously assigned species apart from the type species appear to belong to the genus. The type species, P. obesus Ross, has been known in its type area from only a single cranidium. New collections demonstrate that it is confined to the recently proposed P. obesus Zone, the uppermost trilobite zone of the Stairsian Stage. The species is revised on the basis of new collections from the type locality in the Garden City Formation, southeastern Idaho, and from the Fillmore Formation, western Utah. The genus is known from the underlying Pseudoclelandia cornupsittaca Zone from a species described in open nomenclature from the Fillmore Formation. The oldest known species is P. wigglesorum sp. nov., from the Bearriverops alsacharovi Zone of the Garden City Formation. Details of developmental morphology revealed by silicified specimens offer compelling evidence thatPseudohystricurus is closely related to the Middle–Late Ordovician Dimeropyge Öpik. Jonathan M. Adrain (
[email protected]), Department of Earth and Environmental Sciences, University of Iowa, 121 Trowbridge Hall, Iowa City, Iowa 52242, USA; Talia S. Karim, University of Colorado Museum of Natural History, 265 UCB, University of Colorado, Boulder, CO 80309, USA; Stephen R.