贵州关岭上三叠统的楯齿龙类化石

PLACODONT (REPTILIA: PLACODONTIA) FROM UPPER TRIASSIC OF GUIZHOU, SOUTHWEST CHINA

记述的新铺中国豆齿龙（Sinocyamodus xinpuensis gen. et sp. nov.）代表了齿龙目的一新属。这是齿龙类化石在我国的首次发现，也是这类特殊的海生爬行动物在欧洲、北非和中东以外地区的首次记录。标本产自贵州关岭晚三叠世瓦窑组一段的泥晶灰岩中，与鱼龙类、幻龙类及海龙类等多种海生爬行类同时产出。这一化石组合表明，贵州关岭三叠纪海生爬行动物群的性质与西特提斯动物群（wstern-Tethyan fauna）的性质极为相似，这两个地区在晚三叠世早期有广泛的动物交流。

Based on an articulated skeleton, a new genus and species of cyamodontoid placodont Sinocyamodus xinpuensis is erected. As the first placodont discovered in China (Guanling, Guizhou Province), S. xinpuensis was collected from the first member of the Wayao Formation, which was considered to be the early Late Triassic. The diagnosis of the new genus and species could be generalized as following: Relatively small cyamodontoid placodont; orbital distinctly elongated (the ratio of the longitudinal diameter of the upper temporal fossa to that of the orbit is 1.23); rostrum short and rounded; premaxilla with 3 bulbous teeth; carapace does not cover pectoral and pelvic girdiles; limbs and tail covered with osteoderms; humerus with a distinct and distally notched ectepicondylar groove; entepicondylar foramen absent; phalangeal formula in manus 2*3*4*4*3(2*3*4*4*?2 in pes). For a long time the distribution of placodonts has been restricted to coastal stretches of western periphery of the Triassic Tethys and epicontinental seas of central Europe. Traditionally this group was considered to have originated from the Germanic Basin at the end of the Early Triassic, and then spread to western periphery of the Triassic Tethys. The early existence of cyamodontoid in the lower Anisian of Israel raises the question of whether placodonts reached Europe from the east or outspread from Europe toward the east. Rieppel (1995) proposed an eastern rather than a western Tethyan center of the origin of the Sauropterygia, including the placodonts and the eosauropterygia and further explained their fossil distribution by an eastward expansion into the eastern Pacific Province and a westward expansion into the western Tethyan Province, The discovery of S. xinpuensis in southwestern China represents the first occurrence of placodonts in eastern Tethyan Province, where more forms are expected to be discovered from older deposits. Until now, all major groups of Triassic marine reptiles including nothosaurs, ichthyosaurs (Li, 1999) and thalattosaurs (Rieppel et al., 2000) in western Tethys are mirror-imaged in southwestern China. The South China block, which is geographically situated between the western Tethyan and the eastern Pacific domains may put up mixed faunal affinities of both trans-Pacific and western-Tethyan. Based on vertebrate fossils the relationship to trans-Pacific fauna was weakly supported by some fragmentary materials from southern China and Wyoming (Rieppel, 1999), and the western-Tethyan relationship appeared to be much better supported by this new fauna, especially the cyamodontoid placodont described above, which has not been found in northern America by far. This comparability indicates that there was a lot faunal interchange between the west Tethy and southwest China in the early Late Triassic.