In situ spores of Asplenium and their implications for the evolution of the Aspleniaceae: a case study from the Lower Cretaceous Changcai Formation in eastern Jilin Province, China

Based on the gross morphology of leaves and fertile organs, including in situ spores, three species of Asplenium Linnaeus (Asplenium changcaium Sun et Liu, Asplenium popovii Samylina and Asplenium tiefanum Deng) have been recognized for the first time in the Lower Cretaceous Changcai Formation in the Helong area (Yanbian), eastern Jilin Province, China. These are the only records of fossil Asplenium so far of an age that has been constrained between 125.1–116.8 Ma by isotopic dating, and might among the oldest in the world. This occurrence and earlier findings indicate that the divergence age of Asplenium from the other eupolypods was earlier than the Aptian (Early Cretaceous), and that the fern phylogeny based on the living molecular data and a few fossil records needs to be refined. Systematic comparisons between fossil A. changcaium, A. popovii, A. tiefanum and living Asplenium species in respect of the size and shape of in situ spores and the gross morphology of fronds indicate that the living section Asplenium may be closely related to fossil Asplenium.