Phyletic evolution of the mid-Cretaceous radiolarian genus Turbocapsula from southern Tibet and its applications in zonation

A radiolarian biostratigraphic study ranging from the latest Barremian to the Aptian of a pelagic sequence (section BB1) near Babazhadong in southern Tibet revealed phyletic relationships of species belonging to the genus Turbocapsula. Three species and two subspecies are included in this genus: Turbocapsula tetras LI & MATSUOKA, n. sp., T. fugitiva O'DOGHERTY, and T. costata (T. costata (WU) multicostata LI & MATSUOKA, n. ssp. and T. costata (WU) costata). Turbocapsula fugitiva appears to have arisen from T. tetras by losing the septa between the third and the fourth segments. Turbocapsula fugitiva seems to have given rise to T. costata multicostata following the development of thicker costae between its two to five rows of pores. With the decrease in the number of the costae and the development in the costae, T. costata multicostata evolved into T. costata costata. The phyletic evolution of the genus Turbocapsula has a high stratigraphic value. Based on the phyletic evolution of Aurisaturnalis and Turbocapsula, two radiolarian zones have been defined: the Aurisaturnalis carinatus Zone and the Turbocapsula costata Zone. These zones are defined by evolutionary first appearance bio-horizons (EFABs), which are more reliable than the first and the last occurrence bio-horizons over wide areas. The Turbocapsula costata Zone is subdivided into the Turbocapsula costata multicostata Subzone and the Turbocapsula costata costata Subzone. Based on comparisons with radiolarian biostratigraphic research conducted in other areas, in which age-diagnostic calcareous nannofossils and planktonic foraminifera occur alongside radiolarians, the Aurisaturnalis carinatus Zone and the Turbocapsula costata Zone are assigned to the Lower Hauterivian-Lower Aptian and the Lower Aptian-Lower Albian, respectively. This newly proposed zonation is useful for comparing the stratigraphic relationships of Turbocapsula-bearing horizons in other areas. A zonal scheme like this scheme can be used for identifying certain episodes such as mid-Cretaceous oceanic anoxic events (OAEs) in the pelagic realms. The Early Aptian OAE 1a is located between the first appearance bio-horizon of the genus Turbocapsula and the EFAB of T. costata multicostata.