Occurrence of Arc t ostrea: Existence of high energy Paleoenvironment in Cauvery basin, India during Late Cretaceous .
The distribution of ammonites in Arogyapuram/Dalmiapuram, Ariyalur and Senthurai areas show a maximum abundance at Ariyalur and a decrease in its abundance from Ariyalur around 30-35 Km. An uplift which caused marine regression throughout Cauvery basin was proposed by earlier workers. In the present study author propose a terrain style at Cauvery basin with respect to Senthurai, Ariyalur and Dalmiapuram in early Cretaceous and after uplift in late Cretaceous. Hence the possibility of a shallow water basin in Dalmiapuram and senthurai regions cannot be ruled out. If it happens , there is enough chance to create a very deep canyon like terrain and water from both sides of Ariyalur accumulated in that region with a depth of more than 100 m. This favors the migration of ammonites from Dalmiapuram and Senthurai regions to Ariyalur region.
More than 30 samples of Arctostrea were collected by the author, The Rastellum (Arctostrea) genus in cavery basin may have originated in late Jurassic with adaptive modifications like high toothed commissure. The reason for such adaptation is the existence of high energy environment with heavy sedimentation. Arctostrea, comprising strongly elongated, slender, arcuate forms with high zigzag commissure were dominant during this time. These genera differ in their morphology of features externally and internally, but some representatives of genus Alectryonia (e.g., Al. rastellaris or Al. gregarea) are elongated and arched. Probably the Arctostrea originated from such forms.
Key Words : Cauvery Basin, Arcostrea , Cretaceous, Alectryonia, Ammonite, Al. rastellaris, ,Arogyapuram, Ariyalur
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