Eridostraca Adamczak, 1976

Casier, Jean-Georges, Berra, Ivan, Olempska, Ewa, Sandberg, Charles & Préat, Alain, 2006, Ostracods and facies of the Early and Middle Frasnian at Devils Gate in Nevada: Relationship to the Alamo Event, Acta Palaeontologica Polonica 51 (4), pp. 813-828 : 824

Order Eridostraca Adamczak, 1976 Genus Cryptophyllus Levinson, 1951

Cryptophyllus sp. A aff. sp. 11 sensu Braun (1967) ( Fig. 4P).—This species is represented by a single poorly preserved valve in sample DVG19a.

Palaeoecology of ostracods

(J.−G. Casier and E. Olempska)

Ostracods are generally rare or absent in the lower member of the Devils Gate Limestone ( Table 1), unlike the large numbers present in the upper member investigated previously for the study of the Late Frasnian mass extinction ( Casier et al. 1996; Casier and Lethiers 1997, 1998a, b). Ostracods of the Devils Gate section belong exclusively to the Eifelian Mega−Assemblage, a substitute for the “Eifelian ecotype” of Becker (in Bandel and Becker 1975), since the term “ecotype” has been improperly used in this case ( Casier 2004; Casier et al. 2005). The Eifelian Mega−Assemblage, characterized by the presence of palaeocopid, platycopid, metacopid, podocopid, and more rarely of leperditicopid ostracods and Eridostraca , is indicative of shallow, and generally well−oxygenated environments. The relative proportions of these ostracod groups permit recognition of several assemblages from lagoonal environments to marine environments below storm wave base, which are controlled principally by the energy, the salinity, and the nature of the substrate. Neither ostracods indicative of poorly oxygenated environments (Myodocopid Mega−Assemblage) nor of deep and (or) cold environments (Thuringian Mega−Assemblage) have been observed until now in the Devils Gate section.

At the base of the lower member of the Devils Gate Limestone, and below the 29−cm thick Alamo Event Bed ( Sandberg et al. 1997, 2003; Morrow and Sandberg 2006) ostracods are present in 30% of samples with a relative abundance of platycopids that are indicative of very shallow water conditions. Ostracods are abundant only in sample DVG−03−4 collected in the upper part of the late Mesotaxis fasiovalis Zone and in sample DVG−03−6 collected probably in the Palmatolepis transitans Zone. Voronina? eureka sp. nov. is virtually the sole species present in the first one. This mono−specificity is indicative of semi−restricted water conditions. Sample DVG−03−6, contains five species of podocopid, three of platycopid and one of palaeocopid ostracods; it is indicative of very shallow marine water conditions. The rarity of ostracods in all other samples indicates shallow semi−restricted water conditions; their absence may indicate very stressful lagoonal conditions. Similarly, conodonts are rare to absent in these beds. Close to the Alamo Event Bed, the rarity of ostracods is probably linked to the high energy of the environment as displayed by the sedimentological analysis.

In the upper part of the lower member of the Devils Gate Limestone, and above the 29−cm−thick Alamo Event Bed, ostracods are present in all the collected samples except for DVG20a. The platycopids, indicative of shallow environments, are relatively abundant and diversified except for sample DVG19. In this last sample, two species, Serenida dorsoplicata sp. nov. and Youngiella cf. mica Rozhdestvenskaja, 1972 , dominate the ostracod fauna. However, the presence of several species of podocopid ostracods in all these samples is indicative of strong marine influence. The carbonate platform was always very shallow, becoming progressively more marine.