Palaeontology of the early cretaceous non-marine molluscan assemblages from the Sao Khua formation in Nong Bua Lamphu, Phu Wiang and Sahat Sakhan areas, Northeastern Thailand

Abstract

Sixteen taxa of freshwater mollusan assemblages from 16 shell beds at 12 localities in Nong Bua Lamphu, Phu Wiang and Sahat Sakha Areas were studied. They were subdivided into 4 assemblages, including, Koreanaia (Eokoreanaia) sp. - Yunnanoconha sp. Assemblage, Yunnanoconcha sp. cf. Y. khoratensis Assemblage, Pseudohyria (Matsumotoing) somanai - Trigonioidoidea fam. indet. Assemblage and incertae cedis (4) Asseblage. Koreanaia (Eokoreanaia) sp. - Yunnanoconcha sp. Assemblage was discovered only in Huai Lao Yang Fossil Locality, near the boundary between the Sao Khua Formation and the underlying Phra Wihan Formation. The assemblage is related to mudstone and siltstone, and can be related to Hauterivian - Barremian Nippononaia (Eonippononaia) tetoriensis - Koreanaia (Eokoreanaia) cheongi - Danlengiconcha elongata Assemblage of East Asia. Yunnanoconcha sp. cf. Y. khoratensis Assemblage is reated to mudstone and siltstone. Yunnanoconcha sp. cf. y. khordtensis possesses their morphlology and size as intermediate form between Yunnanoconcha sp. and Yunnanoconha khoratensis of the khok Kruat Formation. Pseudohyria (Matsumotoina) somanai - Trigonioidoidea fam. indet. Assemblage is widely distriuted in sandstone and conglomeratic sandstone of the Sao Khua Formation. Pseudohyria (Matsumotoina) somanai Tumpeesuwan, Sato & Nakhapadungrat, 2010 was proposed as new species and firstly reported of the subgenus Pseudohyria (Matsumotoina) in Thailand. This subgenus is restricted to Late Barremian of Japan and Korea. This evidence might be suggested Late Barremain age for the Upper part of the Sao Khua Formation. incertae cedis (4) Assemblage was discovered only in 3 shell beds of Phu Wiang Areas. This assemblage is related to conglomeratic sandstone. Hinge teeth and muscle scars of incertae cedis (4) are not preserved on the molds; therefore, exact identification is impossible. Based on the evidences of molluscan assemblages, Hauterivian - Early Barremian and Late Barremian Ages were assumed for the lower and upper parts of the Sao Khua Formation, respectively.