Early Neogene Radiolarian faunal turnover in the northern Indian Ocean: Evidence from Andaman-Nicobar

Two intervals of faunal turnover are revealed by the study of radiolarians from the Early to Middle Miocene sequence of Andaman-Nicobar belonging to Stichocorys wolffii-Calocycletta costata-Dorcadospyris alata zones. These faunal changes are reflected in the values of species diversity, change in abundance of taxa, origination and extinction events and change in radiolarian assemblages. One such faunal change is identified in the latest Early Miocene. The time of this faunal change is marked by the extinction of species like Carpocanopsis cingulata and appearance of Calocycletta costata, Giraffospyris toxaria, Acrocubus octopylus and Liriospyris parkerae, an increasing trend in percentage of cold water species and a decreasing trend in species diversity upwards. The interval coincides with the time of initiation of cooling of sea surface water. Another, and the most prominent faunal turnover of radiolarians is recognized in the Middle Miocene Dorcadospyris alata Zone at about 14.8–12.7 Ma and is characterized by almost complete disappearance of an earlier dominant assemblage and an increase in abundance of an assemblage that was practically absent in the older sequence. The time of this turnover can be correlated with the time of Middle Miocene cooling identified in the examined sequence.