EARLY EVOLUTION OF CEPHALOPODS IN CAMBRIAN AND ORDOVICIAN: EVIDENCE FROM CHINA AND THE ADJACENT REGIONS

Since the early twentieth century, when the first undisputed fossil cephalopods were discovered from late Cambrian, over 160 species of cephalopods have been described from North, South China, North America, and possibly Siberia and Kazakhstan, from the latest Cambrian. We compiled all the published Cambrian cephalopod occurrences, and the results show that the oldest undisputed cephalopods are from the Jiangshanian Stage of North China. After their origination, cephalopods reached their first diversity peak in the early Stage 10 of Cambrian. This initial diversity peak was followed by the “late Trempealeauan Eclipse”, which eradicated nearly 95% of late Cambrian cephalopod genera. The extinction event coincides with similar extinctions of trilobites and some other marine groups. The rapid subsequent diversification of cephalopods during the Tremadocian of Early Ordovician was paralleled by a diversification of graptoloids and radiolarians.

The Middle to Late Ordovician is a critical period for the evolution of cephalopods. Two cephalopod diversity peaks were recognized in South China, one in the early Dapingian and the other in the early–mid Katian, separated by a decline in the Middle–Late Ordovician transition. Among the most significant evolutions is the replacement of the Middle Ordovician ellesmerocerids and endocerids faunas by the Late Ordovician lituitids, oncocerids and tarphycerids faunas. Along with the replacement, the conducted paleogeographic analysis indicates that during the Middle Ordovician three biogeographic provinces are recognized on northeastern peri-Gondwanan region, i.e., the Australia, the North China–Tibet–Sibumasu (NTS), and the South China–Altun (SA) provinces. This provincialism lasted through the time, and retained largely a similar pattern in the Late Ordovician, with only the merging of Tibet and Sibumasu regions with the SA Province to form a South China–Tarim–Tibet–Sibumasu (STTS) Province. Based on the analysis of biofacies of the different faunas in Middle and Late Ordovician and published paleogeographic reconstruction, it is proposed that the dynamic variation of cephalopod provincialisms in these regions is probably related to the differentiated paleoenvironments caused by changing paleolatitudes and locations of these terranes.