Early Silurian radiolaria from northern Nevada, USA

Abstract

Radiolarians recovered from three sites in the early Llandoverian Cherry Spring chert, north-central Nevada, provide the first Early Silurian radiolarian data from the conterminous United States. Two assemblages were recovered that contain abundant pylomate sphaerellarians, rotasphaerids, inaniguttids, and possible palaeoactinosphaerids. The pylomate taxa have an intermediate spine morphology with Cessipylorum and Aciferopylorum , bringing into question the present criteria for distinguishing these two genera. Rotasphaerids include both species of Rotasphaera and Secuicollacta , that have five primary rods per spine unit, similar to those present in Ordovician assemblages (Caradocian-Ashgillian) reported from Nevada and Australia. Oriundogutta is another common component shared between the Cherry Spring and Caradocian-Ashgillian age faunas. To a lesser extent, the Cherry Spring fauna resembles late Early-Late Silurian (Wenlockian-Ludlovian) assemblages that contain abundant inaniguttids and rotasphaerids. None of the younger species of inaniguttids have been recognized, however, and younger species of rotasphaerids differ in that they have six primary rods per spine unit and a more diverse spine morphology. These preliminary data indicate a stronger similarity between Late Ordovician and early Llandoverian assemblages than between early Llandoverian and Wenlockian-Ludlovian assemblages. The Cherry Spring chert faunas contain several distinct forms, such as Cessipylorum (?) sp. A and B, that may prove useful for biostratigraphic correlation.