Re-evaluation of the systematic position of the Jurassic-Early Cretaceous fern genus Coniopteris

The extinct fern genus Coniopteris was a typical component in the Mesozoic flora with a wide distribution in both Northern and Southern Hemispheres from Early Jurassic to Early Cretaceous, and it played a very important role in stratigraphic correlation, while its character evolution and systematic position have both been unsolved mysteries. In this study, we analyzed the combined morphological and molecular data available to re-evaluate the systematic position of Coniopteris. Our main results include: (1) all extant genera in Dicksoniaceae have bivalvate indusia, while most species of Coniopteris have cup-shaped indusia; (2) most Dicksoniaceae occur in the tropical zone, different from the temperate habitats for most Coniopteris species; (3) Coniopteris may be herbaceous, whereas extant Dicksoniaceae are mostly tree-like ferns; and (4) Dicksoniaceae and Thyrsopteridaceae all have oblique annuli, while most species of Coniopteris have vertical ones. Based on these differences, Coniopteris should not be treated in Dicksoniaceae. Its systematic position was further evaluated by a cladistic analysis using morphological characters of spore-producing organs and spores and integrated with studies of fern molecular phylogeny as a constraint (molecular backbone). The results show that Coniopteris is clustered with Dennstaedtia, Lindsaea, and Odontosoria, not with Calochlaena, Dicksonia, and Lophosoria in Dicksoniaceae. We therefore propose that Coniopteris may be more closely related with Polypodiales than with Dicksoniaceae, and Coniopteris is probably a stem group of Polypodiales. This interpretation is consistent with the vertical annuli of Coniopteris, an apparent synapomorphy of Polypodiales.