Control mechanisms of Carboniferous brachiopod zones in eastern Australia



Analysis of the Carboniferous brachiopod zones of eastern Australia shows that they were affected by two major controlling factors - eustatic changes of sea level, and a deterioration (cooling) of climate. Eustatic lowering of sea level caused the removal of the sea from narrow shelf areas and a loss of habitat, and was responsible for the first (late early to middle Visean) of two major episodes of faunal extinction. Subsequent transgression restored a modified warm-water cosmopolitan fauna to the shelf regions. Smaller changes in sea levels were probably responsible for the abrupt disappearance of many species or genera at zonal boundaries and their replacement by a new set of species and genera in succeeding zones. The second major episode of extinction was caused by the lowering of temperature in the latest Visean to early Namurian because of the rapid southerly movement of Australia. The warm water cosmopolitan fauna was eliminated and replaced by the low-diversity Gondwana fauna. Both mechanisms produced particular faunal signatures. The diversity of faunas on either side of the hiatus produced by eustatic lowering of sea level is constant in areas with uniform climatic conditions, and in warm to temperate regions there are low levels of endemism. Faunas associated with a sudden lowering of temperature suffer a significant drop in diversity but are characterized by a high level of endemism.