Trace fossils in the lower Beacon sediments (Devonian), Darwin mountains, southern Victoria Land, Antarctica

Recent proposals for a systematic nomenclature of trace fossils are briefly reviewed and adopted in the naming of new forms: Arthropodichnus darwinum Gevers; Arthropodichnus gouldi Gevers; and Arthropodichnus antarcticum Gevers. The new genus Arthropodichnus is designated to indicate trace fossils produced by arthropods. The host rocks are fineto medium-grained sandstones (orthoquartzites), of probable Devonian age. It is demonstrated that the numerous worm "trails" are endogene tubular feeding burrows (fodinichnia Seilacher), intermittently filled with the products of rather rhythmic defecation. Two types occur: narrow sinuous forms on relatively smooth bedding surfaces and giant forms, up to 130 mm wide and over 1 m long, crowding highly bioturbated layers. The annelid worms responsible probably were marine polychaeta. Associated with the narrow, near-surface worm burrows are numerous walking trails (repichnia Seilacher) of arthropods. Three types are found: narrow parallel grooves, 9 to 18 mm apart and which sometimes disappear in elliptical pits, are attributed to epistrate-shovelling and burrowing arthropods on the strength of small footprints in the wider of these trails. Other trails, 22 to 43 mm in width, exhibit very clear, closely spaced footprints in shallow grooves. The arthropods responsible for these must have had numerous walking limbs attached to a subrectangular thorax. They may have been formed by trilobites. The third type is of large size (trail widths 290 to 300 mm) and shows continuous median telson drag marks from which diverge at a marked angle short rows of three, occasionally four, footprint pits. Minor structures within the latter indicate spines on the distal joints of the walking limbs. The detailed shape of the pits suggests that the footprint diverge rearwards against the direction of travel. The producers could thus have been large eurypterids; but there is no indication of a flattish or paddle-shaped posterior pair of appendages. The origin of these trails is thus in doubt. All relevant sedimentary and biologic features suggest a shallow-water marine environment.