Arthropods invade the land: trace fossils and palaeoenvironments of the Tumblagooda Sandstone (?late Silurian) of Kalbarri, Western Australia

The trace fossils of the Tumblagooda Sandstone (?late Silurian) of Kalbarri, Western Australia are spectacular in their variety and preservation. They provide a unique insight into the activities of the early invaders of terrestrial environments, and reveal the presence of a diverse fauna dominated by arthropods. Within the Formation trace fossil assemblages can be related to fluvial, aeolian and marine sand-dominated environments. Two distinct and diverse ichnofaunas are recognised.

The Heimdallia–Diplichnites Ichnofauna occurs in sandstones deposited in broad low sinuosity braided fluvial channels, between which were mixed aeolian and waterlain sandsheets, small aeolian dunes and flooded interdune and deflation hollows. Heimdallia is the major bioturbator, favouring shallow pools. Other burrows include Tumblagoodichnus (gen. nov.), Diplocraterion, Skolithos, Beaconites and Didymaulyponomos. Arthropod trackways (Diplichnites) occur on surfaces of waterlain sands and on foreset bedding of aeolian dunes, and represent some of the earliest reported terrestrial trackways. Other trackways include Paleohelcura and Protichnites, and the digging traces Selenichnites and Rusophycus are also present. At least ten types of arthropods are required to produce the observed traces. Myriapods, eurypterids, euthycarcinoids, xiphosurids and scorpionids are considered responsible for the trackway assemblage.

The Skolithos–Diplocraterion Ichnofauna occurs at the top of the exposed section in sandstones that overlie a thick fluvial sequence containing few traces. The strata are considered to represent marine influence at a fluvial/marine transition. They show variable trough cross-bedding, complex planar cross-bedding with down-climbing sets, ripple lamination, and fining-up sequences with bioturbated tops. Traces are dominated by crowded Skolithos up to 1 m long, together with two forms of Diplocraterion. Daedalus and Lunatubichnus (gen. nov.) burrows occur in a few beds and Aulichnites trails cover some foreset surfaces of cross-bedding.

The trace fossils and the sedimentology of the Tumblagooda Sandstone bear a remarkable similarity to those of the lower part of the Taylor Group of Antarctica, which is probably Devonian in age. It is suggested that the two represent a similar age, stratigraphy, and range of environments on the margins of Gondwana. Large unvegetated fluvial outwash plains with variable aeolian influence were essentially coastal in character and fluvial/marine transitions occur in sand-rich environments. The animals responsible for the traces inhabited coastal areas but many could survive outwith marine influence, and arthropods responsible for some types of Diplichnites trackways walked out of water.

The rich diversity of trackways attributable to arthropods illustrate that the invasion of terrestrial environments by arthropods, particularly large forms, was well-established by the beginning of the Devonian. The basis of the food chain was algal and bacterial films which bound the surface sediment in freshwater pools.