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

Abstract

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. •

The classification of the Pterobranchia (Cephalodiscida and Graptolithina)

“…Therefore, cladistic approaches are still limited to a few well-known groups, such as the retiolitids (Lenz & Melchin 1997, Bates et al 2005 and the Ordovician to lower Silurian biserial axonophorans (Mitchell 1987, Mitchell et al 2007, Štorch et al 2011. In these groups, enough taxa are available as isolated or relief specimens, yielding the morphological details necessary for a reasonable cladistic analysis.…”

“…Especially in the western hemisphere, the taxonomy and evolution of the Graptolithina has been hotly debated in recent decades with the increasing popularity of cladistics and the availability of cladistic analyses for a number of groups within the Graptolithina (e.g. Fortey & Cooper 1986, Mitchell 1987, Bates et al 2005, Mitchell et al 2007, Štorch et al 2011. This approach has helped us to understand the general relationships of certain groups better, but a complete analysis of all graptolite taxa has not been attempted.…”

“…- (Melchin et al 2011, p. 296) The partial clade that includes the most recent common ancestor of Metabolograptus ojsuensis (Koren' & Mikhaylova) and Retiolites geinitzianus (Barrande) but excluding those taxa included in the Family Retiolitidae.…”

“…Akidograptinae Li & Ge 1981, p. 227) Definition. - (Melchin et al 2011, p. 295) The partial clade that includes the common ancestor of Avitograptus avitus and Monograptus priodon and all of its descendents, including Dimorphograptus elongatus, but excluding those taxa included in the Family Monograptidae (i.e. excluding the uniserial monograptids).…”

Graptoloid diversity and disparity became decoupled during the Ordovician mass extinction

“…The graptoloids experienced some of the highest extinction intensities among marine invertebrates during the HME (12), and the surviving taxa were a limited sample of pre-HME lineages. Before the mass extinction event, graptoloid lineages can be divided into two monophyletic clades that diverged more than 20 million years before the HME: the Neograptina and the Diplograptina (13,14). Although the Diplograptina were much more species-rich and morphologically disparate before the HME (Fig.…”