Uppermost Ordovician (upper Katian-Hirnantian) graptolites of north-central Nevada, U.S.A.

摘要

Forty-three graptolite species belonging to fifteen genera are described from the upper Katian ornatus and pacificus biozones and Hirnantian extraordinarius and persculptus biozones of Vinini Creek and Martin Ridge reference sections of north-central Nevada. Approximately half of the species described have not been previously recorded from Nevada, six species are left in open nomenclature. Infraorder Neograptina and Styracograptus gen. nov. are erected. The maximum graptolite diversity is in organic-rich black shale in the lower part of the pacificus Biozone in the Vinini Formation. Species diversity decreased abruptly at the top of the Diceratograptus mirus Subzone, recognized herein in the upper part of the pacificus Biozone. Faunal turnover reached a peak in the lower part of early Hirnantian extraordinarius Biozone where long-dominant Ordovician clades (diplograptines) are rapidly replaced by normalograptids (Neograptina), presumably evolved in, and invading from, a less-temperate higher latitude. Eight late Katian diplograptine species recur in the upper part of the extraordinarius Biozone but, in contrast to their former abundance, are present there only as very rare individuals. Even more unusually, eight diplograptine species (members of Dicellograptus, Anticostia, Rectograptus, Paraorthograptus, Phormograptus, Styracograptus and Appendispinograptus ) also reappear in the uppermost part of the Vinini Creek section, well into the persculptus Biozone (which is topped by a prominent stratigraphic unconformity). These occurrences record a complex extinction pattern among graptolites that involved a radical but extended ecological reorganization rather than a synchronous global collapse of the pre-glacial ecosystem. The biozonation applied in the Nevadan sections correlates well with those established in the Yangtze Platform of China, southern Kazakhstan, north-eastern Siberia and Northern Canada.