Discovery of fossil animals from the basal clay of Permian-Triassic boundary in the Meishan area of Changxing, Zhejiang and its significance
Rut Lin, He Jin-wen, Chen Chu-zhen and Wang Yi-gang
The marine continuous strata across the Permian-Triassic boundary are well exposed in the Meishan area of Changxing, Zhejiang. The lowermost Triassi known as the mixed fauna] horizon can be ascen-dingly subdivided into Mixed beds 1,2 and 3- Mixed bed 1 is composed of basal clay below and dark grey mudstone above. Before 1986, most Chinese scholars regarded the base of the basal clay as the Permian-Triassic boundary, because no fossils have been found in it, however, based on the characters of clay minerals and the change of sedimentary environment, He Jinwen (1981) held that the basal clay should be referred to the uppermost Changhsing and the Permian-Triassic boundary may be placed at the top of the basal clay. Since then, a few conodont specimens (four specimens per kilogram) obtained from the basal clay bave been identified by Clark and others as follows: Neogondolella changxingensis, N. de-flecta, N. carinata and Hindeodella minutus. Recently, a great number of marine fossils were discovered in the basal clay by writers. They comprise non-fusulinacean foraminifers Bradyina sp., Globivalvulina sp., Gcinitzina sp., Hemigordius spp., Giomospira sp., Tetrataxis sp., Nodosaria sp., Textularia sp.; conodonts Neogondolclla changxingcnsis, N. deflecta, N. subcannata, Hindeodclla sp.; ostracodes Basslcrella obesa and brachiopod Crvrithyris sp.. Among them, most are the typical forms of late Late Permian age in South China, while the index fossils of Triassic have not yet been seen. Apparently, this fauna exhibits a distinct aspect of late Late Permian age. As a whole, the palaeontological data available in the present paper may prove that the basal clay is formed in the restricted shallow sea in which the sea water is quiet and stagnant, belonging to a slightly reductive and low energy environment and is late Late Permian in age. Accordingly, the Permian-Triassic boundary should be drawn at the top of the basal clay.