Conodonts across the Lochkovian/Pragian boundary in central Guangxi, South China

Abstract

The Lochkovian and Pragian (Lower Devonian) in the South China Block are mainly represented by siliciclastic rocks that were deposited soon after the end of the Kwangsian Orogeny during the Silurian–Devonian transition. The Pragian conodonts in the South China Block have been poorly documented, and the Lochkovian/Pragian boundary, defined by the first appearance of the conodont Eognathodus irregularis, has never been successfully demonstrated. The present study provides new evidence by restudying conodonts from the Gaoling Member of the Nahkaoling Formation at the Lingli section, central Guangxi. In addition to the previously reported Zieglerodina? tuojiangensis Lu in Lu et al., Pandorinellina exigua lingliensis Lu in Lu et al., and Pandorinellina exigua exigua (Philip) by Lu et al. (2023), the conodont record is expanded to include Pseudooneotodus beckmanni (Bischoff and Sannemann), E. irregularis (Druce), E. grandis n. sp., and Eognathodus sp. A. With the lowest occurrence of E. irregularis in sample 21LL-21, the Lochkovian/Pragian boundary at the Lingli section is located at a level approximately 17.29 m above the base of the Nahkaoling Formation. This is for the first time that the Lochkovian/Pragian boundary is precisely documented in the South China Block. Above this level the strata from sample 21LL-21 to sample 21LL-46 can be correlated with the lower Pragian irregularis-profunda Zone. Moreover, the appearance of the newly described species, E. grandis n. sp., from the irregularis-profunda Zone in the Nanning-Liujing district suggests that the development of a well-arranged median longitudinal row of nodes, previously treated as the most important morphological change in the evolution from Eognathodus Philip to Polygnathus Hinde during the middle Pragian, in fact, can be dated back much earlier to the early Pragian. Accordingly, the derivation of Polygnathus from Eognathodus during the Pragian needs further investigation.