BIOSYSTEMATICS OF GASTROPODA FOSSIL FROM PERMIAN TAK FA FORMATION, AMPHOE TAK FA AND AMPHOE TAKHLI, CHANGWAT NAKHON SAWAN

The Permian carbonate rocks of the study area from Amphoe Tak Fa and Amphoe Takhli, Changwat Nakhon Sawan belong to the Tak Fa Formation, Saraburi Group. The exposures have been measured that totally reach a thickness of 50 meters and samples were collected for both petrographic study and paleontological investigation. The rock consists of bedded limestones, mudstones and dolomites, together with nodular cherts and intercalation of dark-grey shale, which comprise many marine invertebrate fossils such as fusulinids, pelecypods, corals, spongs, calcareous algae and bryozoa, especially a diversity of gastropod. Forty gastropod species are reported, among them 17 nominate species and 23 species in open nomenclature. Thus, this fauna represents one of the richest Permian gastropod faunas known from Southeast Asia. Twelve species and one genus are new to science. The new genus is Takfaia. The new species are Pharkidonotus khaonoiensis, Khumerspira thailandensis, Baylea? umbilicata, Takfaia kuesi, Glabrocingulum magnum, Knightinella ornata, Anomphalus lateumbilicatus, Yunnania pulchra, Microdoma carinata, Trachydomia takhliensis, Goniasma tricarinata and Cambodgia acuminata. The gastropod fauna is dominated by typical Late Palaeozoic cosmopolitan genera with bellerophontoids and pleurotomariines being most abundant. They were subdivided into 3 assemblages, including Bellerophon sp. – Glabrocingulum sp. zone, Stegocoelia sp. and Gastropod barren zone. The study of carbonate facies and fauna indicates that the depositional environment was on shelf lagoon within the carbonate platform varying from shallow to deep marine.