Late Silurian and Early Devonian terrestrialisation: Ichnological insights from the Lower Old Red Sandstone of the Anglo-Welsh Basin, U.K.

Late Silurian to Early Devonian deposits from the Lower Old Red Sandstone Anglo-Welsh Basin preserve a diverse assemblage of trace fossils that collectively comprise an important record of early terrestrialisation. Five main palaeoenvironmental categories are represented: (1) perennial rivers, (2) ephemeral rivers, (3) floodplain lakes, (4) alluvial fan, and (5) marginal marine. Ichnocoenoses mainly reflect the activity of aquatic and/or semi-aquatic animals. Fluvial deposits preserve evidence for colonisation by eurypterids, fish and myriapods. Ephemeral river channel margins/splays preserve the most diverse ichnofauna composed of deposit feedersʹ burrows and trackways produced by arthropods feeding and foraging in channel-margin soft grounds that were colonised after fluvial flood events. High densities of deposit-feeding structures may indicate of explosive population growth in response to periodic, acyclical replenishment of nutrients in these aquatic settings. Ichnotaxa common to both perennial and ephemeral fluvial environments suggest that there might have been only one aquatic ecosystem. The degree to which the channel-margin ichnofaunas record a truly subaerial terrestrial arthropod signature is difficult to ascertain and most trace makers were probably semi-aquatic. Some terrestrial animals invaded channel-margin areas to feed and were also capable of inhabiting high-energy, alluvial fan environments. However the fans contain low diversity ichnological assemblages and were likely to have been inhabited by an opportunistic fauna. Possibly the only truly subaerial terrestrial ichnocoenosis is that of pedogenically modified mudrock with Scoyenia, which represents an early soil ichnofauna. The majority of ichnocoenoses described from continental palaeoenvironments could be classified under the Scoyenia ichnofacies. Marginal marine deposits contain an impoverished Skolithos–Cruziana ichnofacies typical of Silurian–Devonian brackish water assemblages.