A quantitative study of the Ordovician cephalopod species Sinoceras chinense (Foord) and its palaeobiogeographic implications

Sinoceras chinense (Foord) is one of the most important and dominant cephalopods in lower Upper Ordovician deposits of China and has been used traditionally as an index species for the lower Katian of South China. Eight published subspecies, including some morphologically diverse forms, have been assigned to the species, and a taxonomic revision of Sinoceras chinense is required. Based on an examination of the 33 published specimens and 12 new specimens, the relationships of Sinoceras chinense and its allies are analysed using cluster analysis and non-metric multi-dimensional scaling. The results segregate these specimens into three discrete groups: (1) the Sinoceras chinense (Foord) Group; (2) the Sinoceras eccentrica (Yu) Group; and (3) non-Sinoceras chinense Group. Accordingly, we emend the diagnosis of Sinoceras chinense and exclude the specimens of the non-Sinoceras chinense Group. Based largely on this taxonomic redefinition and previously published occurrences of Sinoceras chinense, the palaeobiogeographical distribution of Sinoceras is reconstructed in the South China, North China, Tarim and Tibet. Based on the occurrences, ranges and phylogeny of this genus and its included species, we conclude that Sinoceras originated in South China in late Darriwilian, subsequently dispersed into neighbouring blocks in the Sandbian to the early Katian, and became entirely extinct abruptly in the early late Katian.