Discovery of the earliest Devonian conodonts from Xinjiang, Northwest China

The base of the Devonian is defined at the first appearance of graptolite Uncinatograptus uniformis uniformis, with the occurrence of trilobite Warburgella rugulosa rugosa and conodont Caudicriodus woschmidti as the auxiliary indicators. Recently some researchers questioned whether Caudicriodus woschmidti can be an indicator, and proposed Caudicriodus hesperius to mark the base of the Devonian in carbonate successions. However, the paleogeographic distribution of this species is mainly in the southern hemisphere or near the equator, and there was no definite record of it in the northern hemisphere during the Early Devonian. Due to lithological restrictions, conodont data of the Silruain–Devonian Boundary (SDB) are relatively rare in China, and Caudicriodus hesperius has not been identified. In this study, specimens of Caudicriodus hesperius were obtained from the base of the Kaokesaiergai Formation in the eastern Junggar, northern Xinjiang, which represents the first record of this taxon in China, suggesting that the age of the formation is Early Devonian, rather than late Silurian as previously believed based on the benthic faunas. The encrinurid trilobites, which commonly occur in the Silurian, were also recorded in the same horizon with C. hesperius, indicating that encrinurids may cross the SDB and range into the earliest Devonian. The discovery of Caudicriodus hesperius in northern Xinjiang significantly expands the paleogeographic range of this index conodont to the Central Asian Orogenic Belt in the northern hemisphere, and suggests that Caudicriodus hesperius may be used as a global index for the base of the Devonian and for possible stratigraphic correlation of the SDB in different paleogeographic and paleotectonic settings.