Fish from the Upper Devonian of the Shan-Thai terrane indicate proximity to east Gondwana and south China terranes John Long, Clive Burrett Geology Department, University of Tasmania, P.O. Box 252 C, Hobart, Tasmania 7001, Australia ABSTRACT We report the first discovery of abundant Devonian vertebrates from Thailand. An unusual form of coronodontid shark tooth with six equal-sized cusps occurs only in the Upper Devonian of Thailand (Shan-Thai terrane) and south China, and a new species of Phoebodus is reported that occurs elsewhere only in south China and Australia (east Gondwana terrane). The occurrence of the chondrichthyan Harpagodens (Thrinacodus) ferox Turner 1982 in the Late Devonian of Thailand, Australia, and south China predates the appearance of this species in Euramerica (early Carboniferous). These data suggest Late Devonian proximity of the Shan-Thai, south China, and east Gondwana terranes, and is in accord with recent paleomag- netic data. INTRODUCTION Burrett and Stait (1985, 1986) and Shergold et al. (1988) have shown that the Shan-Thai terrane (western Thailand, eastern Burma, west- ern Yunnan, and part of peninsular Malaysia; Fig. 1) was adjacent to the Australian sector of Gondwana during the Cambrian and Ordovi- cian, on the basis of similar shared genera and species of nautiloids, brachiopods, gastropods, trilobites, stromatoporoids, and even a chiton. There are no available data for the Silurian, but the Devonian is well represented in Thailand and is important, in that Bunopas (1981) sug- gested Shan-Thai rifted apart from east Gond- wana in the Late Devonian-Early Carbonifer- ous to collide with other Asian terranes during the Triassic (Burrett, 1974; Metcalfe, 1988). Other workers have postulated that this rift oc- curred in the Ordovician (Burrett and Stait, 1985) or much later, even Jurassic time (Audley-Charles, 1988). Limestone collected in early 1988 from along the road south of Mae Sariang to Ban Mae Sam Lap (Fig. 1) has produced abundant microver- tebrates and conodonts; the latter indicate age ranges for the section from Emsian to latest Famennian (doV/VI). These represent the first assemblage of vertebrates known from the Devonian of Thailand. The youngest samples have the most abundant vertebrate and cono- dont assemblages, suggesting a possible slowing down of sedimentation rates toward the end of the Devonian. The limestones are from thin se- quences and lenses within a strongly deformed region of apparently unfossiliferous siliciclastics that includes many graywackes (Fig. 1). MICROVERTEBRATE FAUNA The only Devonian vertebrates known pre- viously from Thailand are two scales (one the- lodontid, one acanthodian) from an unknown locality somewhere in the Lower Devonian north of Mae Sariang (Blieck and Goujet, 1978; Blieck et al., 1984). Recent studies have demon- strated the value of Devonian vertebrates in bio- geography (Young, 1981,1987a, 1987b) and in refining paleogeographic base maps (Young, 1987a). The Mae Sam Lap fish assemblage is a pelagic fauna dominated by chondrichthyan (shark) remains with rare acanthodian and ac- tinopterygian remains. The vertebrates are de- scribed by Long (1989). The assemblage is dominated by teeth of a new species of Phoebodus characterized by few ornamental ridges on the lingual face of the cusps, subrectangular root outline, and slight asymmetry of the three main cusps (Fig. 2A). This new species is also found in Australia (Turner, 1982; Phoebodus cf. P. politus) and south China (Wang and Turner, 1985; P. poli- tus). P. politus is known from the Late Devo- nian (Famennian) of the United States and Germany (Newberry, 1889; Gross, 1973), and is distinguished by finely striated lingual faces on the main cusps and additional small internal lat- eral cusps (Gross, 1973). A record from the Himalayas is probably erroneous (Gupta and Janvier, 1979). An unusual tooth form that has five or six Figure 1. Locality maps showing terrane nomenclature and sample locations. Geology based on field work by Long and Burrett. GEOLOGY, v. 17, p. 811-813, September 1989 811