Trilobites from the base of the type Whiterockian (Middle Ordovician) in Nevada

No trilobite species, and very few genera, pass from the Lower (Ibexian) in to the Middle Ordovician (Whiterockian), which is a turning point in Lauren tian trilobite history. Trilobites from three sections exposing the base of the Whiterock Series basal Middle Ordovician) in Nevada are described and illustrated. They are attributable to different. and generally more open-sh elf, biofacies from the Bathyurid biofacies trilobites described from the I bex area, Utah, by Fortey and Droser (1996), but include species in common, which allow correlation into the Ibexian type section. At Little Rawhide M ountain the basal Middle Ordovician is developed in Olenid biofacies, descr ibed for the first time in western North America from rocks of this age. Co rrelation based on species-level similarity shows that the "spike" that has been used to define the type base of the Whiterockian land hence the Middl e Ordovician) at Whiterock Canyon is at a level younger than the base of th e Whiterockian assumed in recent discussions of its international correlati on. The type Whiterock base correlates with the Psephosthenaspis glabrior t rilobite Subzone at Ibex, well above the major change in trilobite faunas a t the base of the Psephosthenaspis Zone (P. microspinosa Subzone). In all s tudy sections there is an abrupt change of facies after trilobite Zone J, p ossibly associated with regression. Four new species are described: Cloacas pis tesselata; Harpillaenus rossi, Acidiphorus? lineotuberculatus. and Bent hamaspis serus.