Fossil pinnae, sporangia, and spores of Osmunda from the Eocene of South China and their implications for biogeography and paleoecology

The genus Osmunda L. contains approximately 10 extant species widely distributed in tropical and temperate regions, with the greatest concentration of species in East and Southeast Asia. Osmunda is characterized by dimorphic or commonly hemidimorphic fronds with dimorphic pinnae. Its geological history has been traced back to the Triassic. Most records of the genus are based on rhizomes and rarely on pinnae bearing sporangia and spores. Here, we describe fossil pinnae, sporangia, and spores of Osmunda lignitum (Giebel) Stur recovered from the middle Eocene of the Changchang Formation in the Changchang Basin, Hainan Island and the Youganwo Formation in the Maoming Basin, Guangdong, South China. The fossils closely resemble the extant Osmunda banksiifolia (C. Presl) Kuhn of the subgenus Plenasium on the basis of their morphological and anatomical structures. The present occurrence of O. lignitum indicates subg. Plenasium flourished and extended from the high latitude regions such as Northeast China to the low latitude areas of South China during the middle Eocene. Large numbers of specimens described here also indicate that Osmunda was the dominant understory fern element beneath mixed evergreen broad‐leaf angiosperm and gymnosperm forests living in a warm and humid environment. The lowest latitudinal occurrence of the genus Osmunda, O. lignitum (Giebel) Stur, is reported to closely resemble the extant O. banksiifolia (C. Presl) Kuhn of the subgenus Plenasium based on fossil pinnae, sporangia, and spores recovered from the middle Eocene of South China. This occurrence indicates subg. Plenasium flourished and extended from the high latitude regions to the low latitude areas of South China during the middle Eocene. Large numbers of specimens suggest that Osmunda was the dominant understory fern element beneath mixed evergreen broad‐leaf angiosperm and gymnosperm forests living in a warm and humid environment.