A new species of Acitheca (Psaroniaceae, Marattiales) with exceptionally and three-dimensionally preserved sporangia from the Buçaco Carboniferous Basin, western central Portugal

A new fern fossil-species Acitheca machadoi sp. nov. is erected under the family Psaroniaceae (Marattiales), based on adpression fossil remains of fertile foliage from the Monsarros Formation (upper Stephanian C, upper Gzhelian) of the Buçaco Carboniferous Basin, in western central Portugal. Acitheca machadoi comprises relatively narrow and long fertile pinnules bearing exceptionally and three-dimensionally preserved sporangia with in situ spores. These sporangia are elongate (fusiform) and one of the smallest documented for the genus, with a triangular pyramid-like shape in the upper part and rounded to hexagonal-like attachment base. Its occurrence within an assemblage dominated by mesophytes to hygrophytes, with fewer drought-tolerant forms, suggests a wetland environment with a seasonal climate, framed in the transition from humid to dry conditions in western Iberia during the Late Pennsylvanian–early Permian. This palaeobotanical finding provides new insights on the palaeoecology and species diversity of Acitheca , whose currently known fossil record is certainly underestimated. • A new species of the extinct marattialean fern fossil-genus Acitheca is described from the Upper Pennsylvanian of Portugal. • The new species is erected based on elongate and very small sporangia, comprising one of smallest sporangia known for genus. • Acitheca machadoi sp. nov. shows exceptionally and three-dimensionally preserved sporangia with in situ spores. • New insights on the palaeoecology and species diversity of the genus Acitheca .