Testate Amoebae in the 407 Million-Year-Old Rhynie Chert

The Lower Devonian Rhynie Chert is justly famous for the clear glimpse it offers of early terrestrial ecosystems. Seven species of stem and crown group vascular plants have been described from Rhynie, many preserved in growth position, as well as 14 species of invertebrate animals, all arthropods save for a single nematode population. While these shed welcome light on early tracheophytes and land animals, modern terrestrial ecosystems additionally contain a diversity of microscopic organisms that are key to ecosystem function, including fungi, protists and bacteria. Fungi ranging from mycorrhizae to saprophytes are well preserved in Rhynie rocks (and references therein), and oomycetes are also present. Both green algae (charophytes) and cyanobacteria have also been locally documented locally. To date, however, heterotrophic protists have not been observed in Rhynie cherts, even though such organisms contribute importantly to carbon, nitrogen and silica cycling in modern terrestrial communities. Here we report the first occurrence of heterotrophic protists in the Rhynie ecosystem, specifically a population of testate microfossils interpreted as arcellinid amoebozoans. These new fossils expand the ecological dimensions of the Rhynie biota and support the hypothesis that arcellinids transitioned from marine through freshwater environments to colonize soil ecosystems in synchrony with early vascular plants.