Trace fossil assemblages in Upper Silurian tuff beds: Evidence of biodiversity in the Old Red Sandstone of southwest Wales, UK

The Upper Silurian/Lower Devonian Old Red Sandstone of the southwest Wales is dominated by fluvial sediments deposited in a mud-rich, low gradient, dryland setting that, apart from rare finds of fish head shields and spines, is lacking in fossil fauna. The Moor Cliffs Formation contains several fine-grained tuff horizons that act as regional markers for correlation, three of these are substantial in thickness (0.5–4 m), laterally persistent and contain abundant trace fossils. The tuffs record sudden volcanic events associated with the convergence of Avalonia with Laurentia, possibly accompanied by tsunami. The trace fossils preserved on the tops of individual falls include the locally profuse development of ovoid faecal pellets in close association with the tops of trumpet-shaped burrows. Surfaces also preserve arthropod locomotion and foraging traces (Palmichnium antarcticum, Diplopodichnus biformis, rare Cruziana sp., and bilobed trails). Vertical and horizontal burrows (Beaconites antarcticus) are common. Other traces found are microbially generated wrinkle marks (mat grounds) and “cauliflower” structures. Traces in the tuffs record a greater diversity of faunal activity than that observed in the encasing dryland environment sediments and indicate a possible preservational bias provided by deposition of the air fall tuffs or colonization of the tuff deposits by opportunist populations following phytoplankton blooms associated with the volcanic events. They remain the main indicators of biodiversity in this relatively fossil-poor continental stratigraphic interval.