Cambrian–Ordovician non-marine fossils from South Australia

Abstract

Newly discovered trace and body fossils from the Grindstone Range Sandstone of South Australia reveal evidence of megascopic life on land during the Cambrian–Ordovician. Arthropod trackways (Diplichnites gouldi) are interpreted here to have formed on land. The most persuasive evidence for this view is that footprints vary in clarity along the length of the trackway as it traversed moist then dry silt, then biological soil crust. Compatible, though not diagnostic of walking on land is trackway symmetry, without one side buoyed up by current. The footprints bulge outward and are partially filled with miniature talus cones. Footprints also are alternate as in walking, rather than opposite as in sculling. Arthropod resting traces (Selenichnites sp. indet.) have 11 lateral furrows, and footprints are bundled into sets of 8–11, most like euthycarcinoids. No arthropod dwelling burrows were found in associated palaeosols, so the track maker was more likely amphibious than fully terrestrial. Associated trace fossils include a new ichnotaxon of burrow, Myrowichnus arenaceus gen. et sp. nov. Thallose impressions (Farghera robusta gen. et sp. nov.) have the radiating dichotomous form of lichens, algae and liverworts. All these trace and body fossils were found in weakly developed palaeosols. Other palaeosols in the same formation are evidence of terrestrial ecosystems of modest biomass, weathering, carbon sequestration and stability in dry tropical regions.

Key words:

• Ordovician
• South Australia
• euthycarcinoid
• track
• trace fossil
• lichen

Acknowledgements

Pauline Coulthard offered advice on aboriginal sacred sites in the field area, and Barbara and Warren Fargher graciously gave permissions for fieldwork on Wirrealpa Station. Fieldwork was funded by the Petroleum Research Fund of the American Chemical Society, and aided by Christine Metzger. This paper was greatly improved by technical advice from Nigel Trewin, Bruce Runnegar and Dmitry Grazhdankin, and detailed reviews by Stephen McLoughlin, Mihail Tomescu, Nigel Trewin, Patrick Orr and several anonymous reviewers.