THE CREATURES OF THE SILURIAN TUFFS: EVIDENCE OF SURPRISING BIODIVERSITY IN THE ‘BARREN' OLD RED SANDSTONE OF SW WALES, UK

The Upper Silurian Old Red Sandstone of the Anglo-Welsh Basin is dominated by fluvial sediments deposited in a mud-rich, low gradient, dryland setting that, apart from rare finds of fish spines and head shields, is lacking in fossil fauna. The Moor Cliffs Formation in southwest Wales contains several fine-grained tuff horizons that act as regional correlatable markers, three of which are substantial in thickness, laterally persistent and contain abundant trace fossils. The tuffs record sudden volcanic events associated with the convergence of Avalonia with Laurentia, and were possibly accompanied by tsunami. The trace fossils preserved on the tops of individual falls include the local profuse development of ovoid faecal pellets in close association with trumpet-shaped burrow tops. Surfaces also preserve arthropod locomotion and foraging traces (Palmichnium antarcticum, Diplopodichnus biformis, rare Cruziana sp., and bilobed trails). Possible anachronistic “disaster” faunas associated with the volcanic events are microbially-generated wrinkle marks (mat grounds), blistered surfaces and “cauliflower” structures. Vertical and horizontal burrows (Beaconites antarcticus) are commonly observed. Traces in the tuffs record faunal activity observed in the encasing dryland environment sediments as well as colonization of the tuff deposits by opportunist populations. They remain the main indicators of biodiversity in this relatively barren continental stratigraphic interval.