The filmy ferns (Hymenophyllaceae) are an ancient fern lineage best known today for their fragile, translucent foliage. As their name suggests, these ferns have exceptionally delicate fronds, which rarely fossilize. Nevertheless, their fossil record stretches back to the Triassic, with Hopetedia praetermissa showing that the family has retained its beautiful membranous fronds for over 200 million years. Today, filmy ferns are confined to damp, sheltered habitats, where they are protected from desiccation.

A particularly well-known fossil fern sometimes placed in this family is Aculea bifida. Like modern Hymenophyllum, it grew from creeping rhizomes with upright fronds, but it appears to have been more tolerant of open conditions. Its narrow, needle-like leaves were delicate but likely not truly membranous. Fossils from an in-situ site on Alexander Island in Antarctica show Aculea forming dense thickets up to 80 cm tall under a patchy canopy of araucarians and podocarps. Its presence in the Early Cretaceous of Australia, southern Patagonia, and Antarctica reveals a high-latitude distribution around the ancient southern polar region.