A new fossil record of Adiantum (Pteridaceae) from Oligocene of Qaidam Basin, Northwest China and its palaeoenvironmental implications

Abstract

Adiantum, a genus of leptosporangiate ferns, belonging to the Pteridaceae, is characterised by marginal sori, reflexed pseudoindusia, and dichotomous venation. As a large and widespread genus, Adiantum includes about 225 extant species and a nearly worldwide distribution, but its fossil record is rather scarce. Here, we describe a new fossil species of Adiantum as A. qaidamense Chen and Yan, n. sp. based on fossil fertile and sterile pinnules from the Oligocene Shangganchaigou Formation in the western Qaidam Basin, Qinghai Province, China. These fossils are mainly characterised by three clearly marginal sori covered by orbicular or oblong pseudoindusia. Both morphological comparisons and cluster analysis indicate that A. qaidamense can be distinguished from previously known species, and A. capillus-junonis seems to be morphologically the most similar extant species. The discovery of A. qaidamense represents an unequivocal macrofossil record of Adiantum in China, and improves our understanding of the palaeogeographic distribution of the genus and the species richness of the Oligocene flora in the western Qaidam Basin. According to the habitats of extant Adiantum, the existence of A. qaidamense probably indicates that there was a flourishing forest growing on probable calcareous soil with abundant water availability during the Oligocene in the western Qaidam Basin.