A redescription of the Silurian Sinogaleaspis shankouensis (Galeaspida, stem-Gnathostomata) from Jiangxi, China
GAI Zhi-Kun, SHAN Xian-Ren, SUN Zhi-Xin, ZHAO Wen-Jin, PAN Zhao-Hui, ZHU Min
AbstractSinogaleaspis shankouensis is redescribed based on 11 new specimens collected from the type locality of the Xikeng Formation in Xiushui county, Jiangxi Province. The in-depth morphological study indicates that the sensory canal system of S. shankouensis exhibits a mélange of characters of plesiomorphic galeaspid taxa, Eugaleaspiformes, Polybranchiaspiformes and Huananaspiformes. The grid distribution of the sensory canal system on the dorsal side of the head-shield, which comprises four longitudinal canals intercrossed with six pairs of transverse canals in S. shankouensis, probably represents a plesiomorphic condition of vertebrates. S. shankouensis belongs to the Sinogaleaspis-Xiushuiaspis Fauna or the Maoshan Assemblage which represents the first diversification of galeaspids in the Telychian, Llandovery of the Silurian period. The sedimentary paleoenvironment of the Xikeng Formation in Xiushui, Jiangxi Province is suggestive of a brackish water environment, whereas a large sum of muddy gravels in fish-bearing sandstone beds point to a short distance of potamic transportation. This indicates that the Sinogaleaspis-Xiushuiaspis Fauna may live in a fresh water environment in the river rather than their burial area in the sea.
Key words Xiushui, Jiangxi, Silurian, eugaleaspids, Sinogaleaspis, sensory canal system