Permian Brachiopods from Southern Thailand

Abstract

The Rat Buri Limestone was sampled for silicified brachiopods at 7 localities along the southern peninsula of Thailand. From north to south these localities are: Ban Kao, Khao Phrik, Khao Tok Nam, Khao Chang, Phangnga, Ko Muk NE, and Ko Muk NW. This limestone forms steep monadnocks that project above the alluvium (or the sea at Muk Island), and lacks any clear stratigraphic succession. Fossils indicate that Permian limestones identified throughout Thailand as the Rat Buri range in age from Sakmarian through Kazanian. The brachiopods from the peninsular localities indicate a late Artinskian (Baigendzinian) age and are correlated with the lower Byro Group of Western Australia, the Bitauni fauna of Timor, the upper Amb Formation in the Salt Range of Pakistan, the Lower Permian in the Karakorum Range, and the Trogkofel Limestone of the Karawanken Range in Yugoslavia. Western Hemisphere correlations are with the Copacabana Group of Peru and Bolivia and, very tenuously, with the topmost Cathedral Mountain or the lower Road Canyon Formations in West Texas.Analyses of life habits of the brachiopods indicate the following: Ban Kao lay nearest the Permian shore; the Rat Buri region was under shallow and fairly clear water, perhaps offshore from a reef; Phangnga was a muddy environment with many spiny and attached forms; Ko Muk was also fairly clear, and an especially favorable place for the growth of brachiopods. Sampling efficiency ranges from rather poor (Index .30) to very good (Index .75) with an overall index of .85 for the entire fauna. The Permian Index indicates that these faunas lived under tropical conditions, but the presence of certain genera suggests that seaways were open to Boreal regions.The brachiopod fauna consists of 109 species and 81 genera, of which one family, 15 genera and 71 species are new; 78 of these genera are considered here. The new genera (with family position in parentheses) are:Nematocrania(Craniidae);Demonedys(Chonetidae);Stictozoster(Productellidae);Comuquia, Dyschrestia(Overtoniidae);Incisius(Incisiidae, new family);Caricula, Gratiosina(Marginiferidae);Bibatiola, Celebetes(Chonetellidae);Stereochia(Dictyoclostidae);Litocothia(Lyttoniidae);Goleomyxa(Atriboniidae);Cruricella(Ambocoeliidae);Tipispirifer(Cyrtospiriferidae).Certain genera and species were selected for functional analyses. The lophophore ofIncisiusis interpreted as a filled-in schizolophe. The muscles of the Ambocoeliidae are reconstructed with a set of adjustor muscles designed to raise the shell to allow it to gape. Life position and muscle arrangement ofParalyttonia(and by analogy,Rigbyella)are reconstructed. The mode of growth and possible function of the stegidial plates ofTipispiriferare presented and, in the same vein, previous interpretations are the sequence of growth in the stegidium of the Devonian genusSphenospiraare criticized and analyzed. The cardinalia ofCleiothyridinaare interpreted with regard to muscle attachment, and the apical perforation is compared to the cardinal process of other brachiopods. The lophophore ofChonetinais reconstructed as a ptycholophe whose direction of growth is determined by the position and configuration of the anderidia.Derbyiaand other Orthotetacea are depicted as having attached to the substrate by byssus-like pedicular fibers, and thus were able to cling to loose sediment.