Cretaceous moss-cyanobacteria association reveals trophic interactions in polar forests of Antarctica

Through symbiotic associations with microbes, mosses play many important ecological roles in terrestrial ecosystems, such as facilitating trophic interactions and nitrogen cycling. However, little is known about interactions between mosses and microbes in past ecosystems. Here, we characterize a permineralized moss with epiphytic cyanobacteria and tubular hyphae from the Cretaceous of Antarctica. The fossil was identified in a calcium carbonate concretion from the Beta member of the Santa Marta Formation Cretaceous (Campanian) from James Ross Island. The fossil was studied using acetate peels and light microscopy. The fossil moss is assignable to Bryopsida. Cyanobacteria trichomes with heterocyst-like cells are present in leaf axils and the perichaetium. In some leaf axils, epiphytic fungi are preserved. The apical perichaetium contains several archegonia; one contains an egg cell while another contains a possible embryo. Fossil epiphytic cyanobacteria demonstrate that moss associations were part of Antarctic polar forests during the Cretaceous. Recognizing their importance in modern ecosystems, moss-cyanobacteria associations likely contributed bioavailable nitrogen to the nutrient cycle of ancient polar forests, which would benefit organisms in different trophic levels. This work reveals long-term stasis in commensal relationships between cyanobacteria and mosses and provides a minimum age for these relationships.