Exceptional ecological resilience of nematode-like animals after fluvial events: insights from complex burrow networks from the Carboniferous Intrasudetic Basin, Czech Republic

Numerous trace fossils of Cochlichnus anguineus, including their earliest regularly branched or reticular-networked variants, accompanied by tetrapod traces, were found in over 10 horizons within siltstone layers overlying a coal seam in the Upper Carboniferous Intrasudetic Basin, Czech Republic. Horizons with trace fossils are typically separated by centimetre-scale of weakly-bioturbated siltstone. All bioturbated layers belong to a stratum marked by in-situ vertically oriented stems of the tree lycopod Lepidodendron sp. and horsetail Calamites sp. This indicates a high rate of continental sedimentation, as the trunks would otherwise have decayed in the humid tropical climate. Moreover, the abundance of colonisation horizons indicates that the siltstones did not originate from a single rapid event but resulted from multiple depositional pulses, likely floods, over several years. Consequently, each colonisation window may have been brief, lasting weeks to months. Ichnological and biomechanical analyses suggest that the producer of these unprecedentedly complex sinusoidal trace systems was a nematode-like organism with a rigid, slender body, surficial mucus, and the ability to reverse muscular waves to retreat and redirect. Enhanced manoeuverability, klinotaxis, and burrow-construction capacity facilitated efficient substrate exploitation and ecological resilience after fluvial events, likely facilitating expansion into fluvial and terrestrial environments, where they persist today.