Approaches to Palaeoecology: A Case Study from the Late Cretaceous (Campanian-Maastrichtian) Benthic Bivalves of Ariyalur, Tamil Nadu

Palaeoecology has now become an imperative source for the palaeontologists in basin studies. The different concepts/Principles and practices till recently used in palaeoecological investigations are reviewed and the organism-community concept, due to its simplicity and more practicalibilty than holistic concept, is followed here. About 1,300 bivalves have been collected from the Campanian-Maastrichtian (Late Cretaceous) sediments exposed in and around Ariyalur in Tamil Nadu. These benthic bivalves belong to four associations (determined by Q-mode cluster analysis). The trophic behaviour of these bivalves depicts that epifaunal suspension-Feeders more or less dominated throughout the Campanian-Maastrichtian times and preferred firm fine-to medium-sandy carbonate substrates. Rarefaction, richness and evenness have been used to determine the diversity values of different associations, which vary from high to low.

The different palaeosynecological approaches indicate that the Campanian Sillakkudi Formation reflects relatively low energy conditions in comparison to younger horizons (Maastrichtian) of the sub-basin where bivalves apparently colonized a shifting high-energy substrate. The palaeosalinity from the Sillakkudi Fromation up to the Ottakkovil Formation ranged between brachyhaline to euryhaline with temporary phases of hypersaline conditions. The uppermost Kallamedu Formation was deposited in an oligohaline salinity regime or even in a fresh water environment. The worldwide Late Ctretaceous rise in the sea level is evidenced in the Cauvery basin by maximum flooding during the Middle Maastrichtian. Sedimentation took place in a high energy, neritic carbonate ramp setting. The carbonate ramp was replaced by a siliciclastic ramp in the late Middle Maastrichtian (Ottakkovil Fonnation).