ENVIRONMENTALLY-INDUCED SUCCESSION WITHIN SHALLOW MARINE BIVALVE-BRACHIOPOD COMMUNITIES FROM REGRESSIVE CARBONATE STRATA OF THE UPPER ORDOVICIAN OF MANITOULIN ISLAND, ONTARIO, CANADA
The relative dominance of bivalves (Pelecypoda) versus brachiopods in ancient marine environments is a classic case in paleoecology, often envisioned in terms of competition between two ecologically similar groups for the same resources. Although much research has focussed on this problem within the context of the Late Permian mass extinction, a more generalized picture of the influences of biological crises on shelly faunal dominance is largely lacking.
Carbonates of the upper Georgian Bay Formation (mid to late Richmondian) of Manitoulin Island, Ontario host a diverse macrofauna with numerous co-occurring brachiopod and bivalve species. Early silicification of calcareous skeletons through much of this interval has allowed identification of a local-scale succession of bivalve-brachiopod communities through a probable 1-2 million year span preceding the first phase of the Late Ordovician mass extinction.
Faunal richness among articulate brachiopods decreases upward from a high in the upper Meaford Submember (10-11 gen. & sp.), and markedly declines above the lower Kagawong Submember (6-7 gen. & sp.), only one species (Hebertella occidentalis) commonly occurring above this level. In the same interval, richness among bivalves increases from upper Meaford and lower Kagawong biostromes (4-6 gen. & 5-7 sp.) through the middle Kagawong (8 gen. & 12 sp.) to a maximum in the upper Kagawong (9-10 gen. & 14-15 sp.). Evenness of bivalve-brachiopod assemblages generally increases upward due to the development of more diverse bivalve communities with declining levels of single-species dominance, though minimum evenness values are observed for the basal Kagawong Submember as a result of low species richness and the high numerical dominance of Zygospira kentuckyensis. Faunal restriction due to shoaling within normal wave base is likely to account for the latter, greater diversity and evenness among articulate brachiopods below, and bivalves above this level, being attributable to calmer conditions below wave base, and leeward of wave-baffling biostromes, respectively. Abrupt decline in brachiopod diversity seemingly relates to the deterioration of normal marine conditions attendant with declining relative sea level, increasing bivalve diversity perhaps reflecting eutrophication of the shrinking epeiric sea.