| Popis: |
Traditionally, the lower boundary of the Emsian (Lower Devonian) has been defined as the base of the dehiscens Zone marked by the first entry of the conodont Polygnathus dehiscens. However, following taxonomic revision of P. dehiscens, the current practice is to use the first appearance of P. kitabicus to define the base of the Emsian. As a result, specimens previously identified as P. dehiscens from South China need to be thoroughly reviewed. Careful examination of the oral and aboral features of the platform reveal that most specimens previously described as P. dehiscens from nine sections in Guangxi and Yunnan actually refer to P. excavatus excavatus or P. excavatus ssp. 114. In the South China Block, widespread and massive deposition of Devonian carbonates began during the time of the excavatus Zone, whereas the kitabicus Zone is probably located in the underlying strata, which mainly comprise marine siliciclastic rocks. Thus, the precise level of the lower boundary of the Emsian in South China is still unknown. |
Full Text from ScienceDirect