EXTINCTION PATTERN OF MAOKOUAN FUSULINACEANS IN SOUTHWEST CHINA

Previous studies have demonstrated that large fusulinacean species (shell length > 6mm) and species with distinctive keriotheca in the wall structure became extinct during the Guadalupian mass extinction. Survivors were small species with simple wall structures. Analysis of fusulinacean data from Guizhou, southwest China reveals that diversity change in large and small species (shell length < 6mm) groups were very similar throughout the Maokouan when the fusulinaceans are divided arbitrarily into large and small species groups. However, significant differences in the extinction processes, such as the timing of extinction pulses, have been found between different clades with different wall structures, i.e., nakinellids, schwagerinids, verbeekinids and neoschwagerinids, and between large and small species groups within schwagerinids and verbeekinids. In the early stage of the extinction, nakinellids and neoschwagerinids, small species group of verbeekinids and large species group of schwagerinids were affected dramatically. However, small species group of schwagerinids and large species group of verbeekinids were not affected until the Late Maokouan when extinction pressure increased greatly. Except nakinellids, both small and large species groups in other clades were all hit severely near the end of Maokouan. Survivors of the Maokouan extinction all belong to the nakinellids and could be further classified into two types: (1) old species which had first appeared in the underlying Chihsian, such as Nakinella compacta and N. inflata, which were able to adapt to restricted, low energy marine environment. (2) new species appearing first in the Late Maokouan, such as Codonofusiella schubertelloides and Reichelina cribroseptata. They all share such peculiar features as possessing a very thin wall test and an uncoiled last volution which could be an adaptation to a floating habit. The results suggest that biological characters of such as test size and structures could have certain effects on survivorship of species in the early stage of mass extinction but were ineffectual during the extinction climax. The relatively higher survivorship in nakinellids indicates that the Maokouan extinction of fusulinaceans could be caused by the deterioration of marine benthic environment associated with the global regression.