| 论文摘要 |
Galeaspida is an endemic clade of jawless stem-gnathostomes (‘ostracoderms’), known exclusively from the Silurian and Devonian of China and Vietnam. Among galeaspids, the family Sanqiaspidae is of particular interest for providing the first information on the galeaspid trunk and caudal fin; however, the cranial morphology of sanqiaspids remains poorly known. Here we report the first occurrence of Sanqiaspis in the Xujiachong Formation of Qujing, East Yunnan, and describe new material of Sanqiaspis from two other sites in South China. The new data corroborate the presence of the two median transverse canals in Sanqiaspis as a possible synapomorphy of the Sanqiaspidae. As a key member of the Xujiachong Assemblage, Sanqiaspis is known from the Xujiachong, Guanshanpo, Posongchong, and Si Ka formations. Stratigraphic correlation of eight Early Devonian sections in South China and North Vietnam indicates that the Sanqiaspis-bearing layers can be correlated with the transitional beds between the Lianhuashan and Nahkaoling formations in Liujing, Guangxi. Therefore, the middle-upper part of the Si Ka Formation in northern Vietnam and the upper part of the Lianhuashan Formation in Guangxi should be dated as Pragian, rather than Lochkovian. The fish assemblage from the Pragian of the South China Block documents the diversification of galeaspids, notably Huananaspidiformes, and exhibits much the same endemic pattern as that of early vascular plants and brachiopods. This endemism might be linked with the semi-closed epicontinental nature of the South China Sea during the Early Devonian, which formed a biodiversity hotspot as in the modern Red Sea. |
