http://worldcat.org/entity/work/id/1640940

Siluro-Devonian microfaunal biostratigraphy in Nevada

From a regional microfossil study of Silurian and Lower Devonian strata in central Nevada, a large and diversified fauna of Foraminiferida and Ostracoda is described in detail for the first time. The foraminifers consist of 22 genera and 45 species. Two of the genera, Anictosphaera and Atelikamara, and the following species are new: Anictosphaera progressa, Aschemonella bastillensis, Atelikamara incomposita, Haplophragmoides antiquus, Hemisphaerammina bipatella, Sorosphaera compacta, Sorosphaera inflata, Storthosphaera malloryi. Recognition of Haplophragmoides antiquus in the Lower Devonian of central Nevada is the earliest record of the family Lituolidae. The ostracode fauna contains 29 genera and 20 identifiable species. Of these, Longiscapha is a new genus, and Aechmina cortezensis, Longiscapha nevadensis, Rudderina mutaspina, Thilpsura bispinosa, Ulrichia obesita, and Winchellatia fragilis are new species. The Foraminiferida recorded here have been combined with previously known Silurian species from the Midwest to establish a tentative series of assemblage zones for the Silurian and Lower Devonian of North America, which are based on dominance of the different subfamilies. The Nevada fauna represents the Upper Silurian, characterized by the Psammosphaerinae, and the Lower Devonian, characterized by the Astrohizinae and Hemisphaerammininae. The Lower and Middle Silurian assemblages, best known in the Midwest, have been divided into the following three zones: 1) Albion -- Hippocrepininae, Rhizammininae, Ammodiscinae; 2) lower Niagaran -- Saccammininae, Ammodiscinae; 3) middle Niagaran -- Hippocrepininae, Psammosphaerinae, Hemisphaerammininae. In general, there is not sufficient evidence for widespread correlations on the basis of the ostracodes recognized here, though several genera and species of the Nevada fauna compare closely with those known from Appalachian strata of uppermost Silurian and Lower Devonian age. The physical environment shows local variances with time which are recognized regionally as lithosomes. Association of the foraminifers with several lithosomes suggests overall similarity to associations in other parts of North America. Associations between ostracodes and the various lithosomes remain uncertain due to their sporadic and limited occurrence.

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