Early Devonian conodonts from the Kuan Tung Formation, Thailand: systematics and biogeographic considerations

Abstract

A small conodont assemblage, obtained from a limestone marking the boundary between the middle and upper units of the Kuan Tung Formation, Satun Province, South Thailand, contains inter alia Pandorinellina steinhornensis steinhornensis, Polygnathus labiosus mawsonae n.subsp., and Pseudooneotodus kuangtungensis n.sp., indicating an Emsian age (perbonus-serotinus Zones). Studies on ontogenetic variation in Pa elements of Polygnathus labiosus mawsonae n.subsp. show that smaller Pa elements have fewer transverse ridges on the posterior third of the platform, more distinct marginal denticulation on the platform and a proportionately smaller basal cavity. The basal cavity lips may break off in mature specimens; this gives an appearance of a small basal cavity. The distribution of P. labiosus, P. pseudoserotinus, P. perbonus and Pseudooneotodus kuantungensis n.sp., in the Shan-Thai and South China Terranes, together with supporting evidence from thelodontid and placoderm fishes, suggests that these terranes were in close proximity to the eastern margin of Gondwana at the end of the Early Devonian.