Fish trails from the Lower Old Red Sandstone of Tredomen Quarry, Powys, southeast Wales

摘要

Alluvial deposits of the St Maughans Formation (Lower Old Red Sandstone, Early Devonian) of Tredomen Quarry, near Brecon, southeast Wales, have yielded the oldest known trails of swimming fish as well as body fossils of heterostracans and osteostracans, their inferred producers. Undichna unisulca comprises a single sinusoidal wave (of varying amplitude and wavelength) and is attributed to the caudal lobe or fin of a swimming heterostracan or osteostracan. Variation in the dimensions of U. unisulca trails, together with functional analysis of their inferred producers, suggests different fish sizes and swimming speeds. Undichna cf. simplicitas shows a more complex arrangement of intertwined waves and is interpreted as being produced by a combination of the caudal fin, anal spine and paired pelvic spines of an acanthodian. A new ichnotaxon, Undichna trisulcata isp. nov., consists of three isolated furrows arranged in parallel with associated pectoral fin imprints, and is interpreted as either the trail left by a 'cruising' cephalaspid, intermittently pushing off the substrate with its pectoral fins, or a trail made by a cephalaspid landing on the substrate. The presence of Undichna within these relatively proximal fluvial sediments (displaying no evidence of marine influence) is indicative of an in situ vertebrate freshwater community. Taphonomic constraints on Undichna preservation, in combination with sedimentological analysis, suggest weak bottom currents and/or rapid burial, post-trail formation. Arthropod trackways (Diplichnites gouldi Types A and B, Protichnites isp., and Palmichnium pottsae), a bilobed trail (cf. Cruziana), worm burrows (cf. Cochlichnus) and large meniscate backfilled burrows (Beaconites barretti) contribute invertebrate components to this ichnofauna. The association of Diplichnites trackways on the same surfaces as Undichna supports a semi-aquatic habit of their inferred myriapod producers. Copyright (C) 2004 John Wiley Sons, Ltd.