OLENELLOID TRILOBITES OF THE SOUTHERN GREAT BASIN, U.S.A., AND A REFINEMENT OF UPPERMOST DYERAN BIOSTRATIGRAPHY

Uppermost Dyeran strata of the southern Great Basin record the final 2-3 million years of the evolutionary history of the Olenelloidea, members of the stratigraphically oldest and morphologically most primitive trilobite group. New fossil collections from over 20 localities spanning the Lower Cambrian shelf and made at high stratigraphic resolution offer the opportunity to study this period of olenelloid evolution in unparalleled detail. Olenelloid standing diversity was generally high, with up to eight species temporally co-occurring at any given locality. The collections also demonstrate that olenelloids underwent a high rate of morphological change throughout this interval, such that a total of at least 40 species (14 new) in 11 genera (three new) are represented in strata younger than the Zabriskie Quartzite. Specimens representing early ontogenetic stages are known for over half of these species. All but one of the species (and five of the genera) were apparently endemic to the southern Great Basin. Several well-supported cases of anagenetic morphological change (evolutionary or ecophenotypic) are apparent, often involving two or more morphotypes previously treated as distinct species. Upper Dyeran biostratigraphy is also revised in light of the new data. A total of eight zonules are proposed, refining the traditional three-zonule subdivision of the upper Olenellus Biozone. From oldest to youngest, the new zonules are named for and defined on the first occurrence of Arcuolenellus arcuatus, Bristolia mohavensis, Mesonacis n. sp., Br. insolens, Peachella iddingsi, Bolbolenellus euryparia, Br. brachyomma, and Nephrolenellus multinodus. Lower and upper intervals can be informally recognized in each of the Br. insolens and N. multinodus Subzones based on the (anagenetic?) appearance of Br. anteros and N. geniculatus, respectively. Assemblages of similar morphotypes are known from Canada, the Argentine Precordillera, the Appalachians, and Greenland, offering some potential for broad correlation around Laurentia despite the high provinciality of Early Cambrian trilobites.