A Triassic semi-arid upland community of herbaceous ferns and rhizophagous arthropods evidenced by trace fossils in rhizogenic calcrete pedorelicts from SW England

We present rare evidence of a Triassic upland biome. Pedorelicts (soil clasts) in continental sediments in SW England derived from a Middle-Upper Triassic semi-arid upland palaeoenvironment were studied using micro-CT scans and microscopy. They feature rhizogenic calcrete containing networks or mats of very fine roots that have been fossilized as rhizotubules, mostly 0.09–0.4 mm but up to 1 mm in diameter, with up to 4.7 m of rhizotubule length per cm³. Calcite dissolution and/or precipitation textures occur around the rhizotubules, and within them, calcite linings of 5–100 μm thick include calcified epidermal cells and in some of the thickest cases, cortical cells. We interpret these as evidence of a plant physiological process facilitating water and nutrient absorption in a semi-arid palaeoclimate. The size and both dichotomous and lateral branching of the rhizotubules, along with the interpreted palaeoecology, support identification of the producers as herbaceous calcicolous leptosporangiate ferns (Polypodiidae). Larger unlined tunnels of irregular 0.3–2 mm cross-section, some containing ovoid, spheroidal, clustered or irregular limonite grains, from 20 μm to over 100 μm in size, interpreted as coprolites, traverse the rhizotubule networks. These are attributed to burrowing rhizophagous arthropods with mandibles and/or other means sufficient to breach some calcified rhizotubules. They also provided sites for later root growth. These pedorelicts host rare trace fossil evidence of an upland community of plants and animals with roles in pedogenesis during significant intervals of geomorphic stability between major pluvial reworking events.