In situ volcanically baked killarney fern fossils (Hymenophyllaceae) from the late Holocene of the Azores archipelago (Portugal)

Among the most delicate native vascular plants present in the Azores archipelago (central Atlantic Ocean, Portugal) are the Hymenophyllaceae, known as the filmy ferns, due to their characteristic translucent one-cell thick lamina lacking stomata. Importantly, these characters impose a restriction to shaded and high humidity habitats, promoting rapid decay, leading to a low fossilization potential, as revealed by a worldwide scant macrofossil record of this family. Here we describe the first macrofossils of Hymenophyllaceae from Macaronesia, found on Faial Island, Azores archipelago. The fossils were preserved in situ within a 1000-1200 yr. BP vesicular ash-fall tuff overlain by an ignimbrite deposit. The specimens are three-dimensionally incorporated within the matrix, presenting overlapping, and are preserved as impressions with exceptional cuticular preservation. Morphological and anatomical characters match the sterile fronds of Vandenboschia speciosa, a native fern of the Azores Islands. Taphonomically, these fossils reveal that ash-fall released by sub-Plinian eruptions in oceanic islands can promote the burial and preservation of delicate plants, even when overlaid by hot pyroclastic density currents. Given the medieval age of the fossils, the associated autochthonous palaeoflora (Laurisilva), and their finding in a currently highly anthropically disturbed area, are especially important to reconstruct the palaeoecosystem baseline and inform possible future ecosystem restoration.