HORSESHOE CRABS LIVED IN PERMO-TRIASSIC ANTARCTIC FRESHWATER RIVERS AND LAKES: TRACE FOSSIL EVIDENCE FROM THE BUCKLEY AND FREMOUW FORMATIONS, BEARDMORE GLACIER AREA, CENTRAL TRANSANTARCTIC MOUNTAINS (CTAM), ANTARCTICA

The Upper Permian Buckley Formation and Lower Triassic Fremouw Formation represent braided river, coal-forming swamp, and lacustrine environments and braided river-floodplain environments, respectively. These formations in the Beardmore Glacier area were deposited from about 70 to 75 degrees and 65 to 70 degrees South latitude, respectively, which is currently within the present position of these outcrops today. Fieldwork during the 2010-2011 CTAM field season discovered nearly 20 trackways attributed to horseshoe crabs in shallow-water lacustrine deposits of the Buckley Formation at Lamping Peak and in abandoned channel deposits of the Fremouw Formation at Wycoff Glacier. Trackways are preserved in concave epirelief and convex hyporelief in the form of true tracks and undertracks of various depth. Trackways are preserved as two chevronlike series, each of four oval to circular, or bifid V-shaped impressions, which rarely exhibit scratches with the digits, that are forwardly directed. Tracks are also preserved as a pair of digitate or flabellar, toe-shaped variable imprints that produce a birdfoot-like impression. None of the trackways recovered contained a median longitudinal groove. The trackways are 4 to 6 cm wide, with the longest trackways approximately 40 cm. These trackways are assigned to the ichnotaxon Kouphichnium natalensis and are nearly identical to marine and other freshwater horseshoe crab trackways from the late Paleozoic to recent and late Paleozoic to Jurassic, respectively.

These trackways provide definitive evidence that horseshoe crabs occupied freshwater environments in high latitude settings by the Late Permian. This occurrence also demonstrates that the Antarctic climate overall was much warmer than today, and that the winter months where not cold enough to freeze water bodies solid, allowing the horseshoe crabs to likely fall into a state of suspended animation until warmer weather returned. Horseshoe crab trackways in the Lower Triassic freshwater environments also demonstrate that they survived the greatest mass extinction of all time and thrived in braided river environments. The Antarctic evidence also suggests that horseshoe crabs were globally distributed in freshwater environments by the Permo-Triassic, and likely spurred landward by the assembly of Pangea.