A comparison of recent and fossil large, high-spired gastropods and their environments: the Nopparat Thara tidal flat in Krabi, South Thailand, versus the Swiss Kimmeridgian carbonate platform

摘要

High-spired nerineoid gastropods are abundant in the tropical Kimmeridgian carbonate deposits of the Swiss Jura Mountains. Understanding the mode of life of this extinct group of gastropods is crucial for palaeoenvironmental reconstructions. The knowledge on their ecology is, however, limited by the near absence of comparable large high-spired gastropods in Recent carbonate systems. Large high-spired turritellid gastropods are, however, abundant in siliciclastic-dominated environments in the tropical Indian Ocean. In order to compare the Recent and fossil environments of these two morphologically similar groups, a Recent tidal flat, where abundant turritellids occur, was studied in the National Park of Nopparat Thara in the district of Krabi, South Thailand. Ninety specimens of Turritella duplicata were measured and mapped, revealing zonations in population distribution, both parallel and perpendicular to the beach line. Attention was also paid to other faunal elements and sedimentary features. The thus-gained information on the depositional environment was then compared to a section of shallow-water carbonate deposits from the Kimmeridgian of the Swiss Jura Mountains. These are characterised by the occurrence of tidal laminites, dinosaur tracks, beach deposits and nerineoid gastropods. Especially for one succession, the faunal composition and sedimentary structures in the fossil record show intriguing similarities to those found today on the Nopparat Thara tidal flat, indicating that it formed in a comparable setting. It is therefore suggested that for some Kimmeridgian carbonate deposits, the Nopparat Thara tidal flat, although dominantly siliciclastic, represents an environmental analogue.