Integrated Lower-Middle Ordovician graptolite and chitinozoan biostratigraphy of the Jiangnan Slope Region, South China

摘要

An integrated Early-Middle Ordovician graptolite and chitinozoan biostratigraphical study was carried out to improve the correlations between the two fossil groups in two sections in the Jiangnan Slope Region (deeper outer-shelf mud-sand-carbonate belt), South China. Six graptolite biozones and two chitinozoan biozones (including an undefined interval) were recognized. The chitinozoan Lagenochitina praepirum appears to be a stratigraphically useful species, despite being geographically restricted to South China, appearing at a level coincident with the first appearance of the index graptolite Levisograptus austrodentatus. Thus, the base of the Lagenochitina praepirum Biozone can also be regarded as the base of the Darriwilian Stage. The chitinozoan abundance in both sections is exceptionally low, and the assemblages do not contain the biostratigraphically important Belonechitina species. The distributions of both chitinozoans and graptolites are eco-dependent but show different onshore-offshore diversity trends. Chitinozoans are more diverse nearer to shore whereas graptolites are more diverse offshore, preferring slope facies. This may be caused by the different ecological preferences of chitinozoans and graptolites. (C) 2018 Elsevier Ireland Ltd Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. Published by Elsevier B.V. All rights reserved.