New information and family relationship (Hymenophyllaceae) for the fossil fern genus Acrostichopteris Fontaine and a new species from the Lower Cretaceous (Albian) of Spain

Premise Reinvestigation of previously described fossil plants using improved techniques can provide insights for additional characters resulting in better family assignments and relationships to modern plants, as in this new study of the fossil fern Acrostichopteris from the mid-Cretaceous. The discovery of new species within genera provides additional characteristics for comparisons and support for family assignments. Methods Fossil material was uncovered from the matrix (degagement) for the specimens described here. Material was removed from the matrix and mounted for examination by scanning electron microscopy (SEM). Spores were extracted and prepared for examination with light, fluorescence, and SEM. Characters for the previously described material from the Potomac Group in the USA and the new material from Spain were compared with those of other fossils assigned to the genus. Results The genus, described by Fontaine in 1889, reexamined by Berry in 1911, and assigned to the Schizaeaceae by Reed in 1947, is reassigned to the family Hymenophyllaceae on the basis of characteristics of the pinnules, sori, sporangia, and spores. We describe a new species in the genus from the Lower Cretaceous (Albian) of Spain that most likely belongs in this family, based on characteristics of the pinnules and the sori. A summary of all the species previously assigned to the genus is included. Conclusions Assignment of this fossil to the family Hymenophyllaceae is based on all characters known and comparison to modern species, provides the time for appearance of characteristics within the family, and indicates diversification of Hymenophyllum species in the mid-Cretaceous.