Culcita remberi sp. nov., an Understory Fern of Cyatheales from the Miocene of Northern Idaho

Abstract

Premise of research. Excavations at the Clarkia fossil beds in northern Idaho have recovered numerous exceptionally well-preserved plant fossils over the past 45 years. We report on a compression fossil of a fertile frond from a tree fern found at the Emerald Creek locality.Methodology. Intact leaf material was liberated from the substrate by wetting with hydrofluoric acid followed by submersion in a water bath. The lamina was preserved between sheets of acetate and photographed, and fertile pinnules with sporangia were mounted on aluminum stubs and imaged with scanning electron microscopy. Spores were released from sporangia after incubation in Schulz’s solution and treatment with KOH.Pivotal results. The specimen is similar to members of several extant fern lineages, including Hymenophyllaceae, Dennstaedtiaceae, and the tree fern order Cyatheales. It shares with these groups the possession of oblong, marginal sori, although the sori of the specimen are most similar to the cup-shaped sori of Dennstaedtiaceae and the bivalvate sori of Culcitaceae. The fossil also has sporangia, each with a complete, uninterrupted, oblique annulus, which is strongly associated with Culcitaceae.Conclusions. On the basis of its overall lamina morphology and details of the sori and especially the sporangia, we conclude that this specimen belongs to an extinct species of the monogeneric tree fern family Culcitaceae. The occurrence of a tree fern in the temperate plant community of the Miocene Clarkia locality is unexpected but not incompatible with the habitat preferences of this genus, as the two extant species of Culcita are found in montane altitudes and temperate forests elsewhere in the world.