Published online by Cambridge University Press: 26 July 2017
The term dysaerobic was first coined by Rhoads and Morse (1971) to define conditions in which oxygen levels ranged between 1.0 and 0.1 mL of oxygen/litre of water. Such low oxygen tensions were thought to exclude all but soft-bodied invertebrates with the absence of shelly fossils being one of the diagnostic features of the dysaerobic zone. Subsequent observations of recent dysaerobic environments has revealed that shelly invertebrates can survive in oxygen values as low as 0.3 mL/L (e.g. Savrda, Bottjer and Gorsline, 1984). Consequently the dysaerobic zone is now used in the more qualitative sense to denote conditions in which oxygen levels are sufficiently low to cause a notable change in the fauna (Kammer and others, 1986; Wignall and Myers, 1988). Faunal abundance and diversity is reduced under low oxygen conditions compared to more normally oxygenated (aerobic) biofacies.