Stem diversity of the marattialean tree fern family Psaroniaceae from the earliest Permian Wuda Tuff Flora

This paper investigates the taxonomic diversity of marattialean stems from the family Psaroniaceae preserved in-situ within the Wuda Tuff Flora. Three species of the permineralized stem Psaronius and eight species of the compression/impression stem Caulopteris are documented, but it has not been possible to associate the anatomically preserved species with the compression/impression ones due to preservational limitations. The higher diversity of Caulopteris species may relate to anatomical features that could distinguish additional species of Psaronius not being preserved, or it may reflect the rarity of anatomical preservation in the flora limiting the available dataset and it is also possible that a “generalized” Psaronius anatomy was common to several stem species showing distinct leaf scars (Caulopteris) and distinct leaves (Pecopteris). The eight species of Caulopteris recorded are very close to the previously reported diversity of nine species of the vegetative leaf Pecopteris from the Wuda flora, which also indicates a high diversity within the family. However, intraspecific variation makes the identification of species of Pecopteris and Caulopteris rather difficult, potentially inflating the taxonomic diversity of these genera. We show that Psaronius stems from the Wuda flora include one species from the Psaronius blicklei-group, one species that shares characters with both Psaronius blicklei and Psaronius panxianensis groups, and one species that shares characters with Psaronius blicklei-group but possesses a unique one different from both Psaronius blicklei and Psaronius panxianensis groups. These species appear to represent transitional forms evolved from the Carboniferous–Cisuralian (early Permian) Psaronius blicklei-group that are on the evolutionary lineage leading to the Lopingian (late Permian) Psaronius panxianensis-group. In addition, a single occurrence of a stem of Psaronius in organic attachment to a tri-pinnate frond of Pecopteris orientalis (Schenk) Pot. is documented, representing an important starting point for future Psaroniaceae whole-plant reconstructions within the Cathaysian flora.