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Continuous sections of fossiliferous Lower Devonian (Siegenian and Emsian) strata comprise five major faunal assemblage zones based primarily on brachiopods. The faunal succession which furnishes the basis for zonation is documented by description and illustration of the brachiopod species obtained from relatively large collections made in sequence at critical localities.

The Siegenian begins with the Quadrithyris Zone which has its type occurrence in the upper Windmill Limestone of Coal Canyon, from 10 to 36 feet above beds with Monograptus hercynicus nevadensis. The Quadrithyris Zone brachiopod fauna most closely resembles faunas from western and Arctic Canada and from the USSR (Old World Province) and is less like faunas of the Appalachian Province of eastern North America.

The Spinoplasia Zone succeeds the Quadrithyris Zone and carries a large brachiopod fauna whose affinity is almost exclusively with the Appalachian Province of eastern North America. The Spinoplasia Zone brachiopod fauna indicates a late Helderbergian age and a position within the Siegenian.

The Trematospira Zone succeeds the Spinoplasia Zone and is characterized by an Appalachian Province fauna of Oriskany age and a position at or near the top of the Siegenian.

The Acrospirifer kobehana Zone succeeds the Trematospira Zone and contains a few newly-appearing brachiopod genera that indicate Old World faunal affinity together with a few Appalachian Province genera that carry over from the Trematospira Zone below. By position, the A. kobehana Zone appears to be correlative with the Esopus Formation (Etymothyris Zone) of eastern North America and is assigned to the Lower Emsian.

The Eurekaspirifer pinyonensis Zone succeeds the A. kobehana Zone and carries a rich and well-preserved brachiopod fauna of Old World aspect. The zone is assigned to the Emsian and probably includes beds correlative with parts of both the Lower and Upper Emsian. The oldest productids and the oldest goniatites in the western United States occur in this zone.

The E. pinyonensis Zone is succeeded by beds formerly included in the zone, but which have been separately designated Elythyna beds. The latter are probably of Late Emsian age and appear to represent the highest fossiliferous Lower Devonian strata in the Great Basin. The brachiopods of the Elythyna beds are most closely related to those of the E. pinyonensis Zone below and not to faunas from succeeding beds that can be confidently identified with the Middle Devonian.

The Siegenian and Emsian zonal sequence of central Nevada serves as a standard for the western United States and in some respects for all of western North America because Siegenian-Emsian brachiopod faunas from western Canada are wholly of Old World aspect and, considering the megafauna, can only be related to Appalachian Province faunas by way of an intermediate and transitional faunal succession.

All brachiopods known from the five major assemblage zones are listed according to their stratigraphic occurrence; 111 are described and illustrated. New subfamilies are Parachonetinae and Spinellinae. New genera are Leptocoelina, Pseudoparazyga, and Discomyorthis. New species are Schizophoria parafragilis, "Brachyprion" mirabilis, Aesopomum varistriatus, Atrypina simpsoni, Cyrtinaella causa, Orthostrophella monitorensis, Leptostrophia inequieostella, Leptocoe-lina squamosa, Spinoplasia roeni, Leptocoelia murphyi, Pleiopleurina anticlastica, Megakozlowskiella magnapleura, Levenea navicula, Stropheodonta filicosta, Stropheodonta magnacosta, Megastrophia transitians, Sieberella pyriforma, Ancillotoechia aptata, Coelospira pseudocamilla, Gypidula praeloweryi, Mclearnites invasor, Anoplia elongata, Nucleospira subsphaerica, Levenea fagerholmi, Brachyspirifer pinyonoides, and Spinella talenti.

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