Early Palaeozoic colonisation of land: evidence form the Tumblagooda Sandstone, Southern Cararvon basin, Western Australia

The early Paleozoic Tumblagooda Sandstone outcrops principally in the vicinity of the Murchison River in Kalbarri National Park, Western Australia. It contains a great variety of trace fossils that provide a unique insight into the activities of early invaders of the terrestrial environment, and may record one of the earliest known freshwater and terrestrial ecosystems. The traces reveal that this nascent terrestrial fauna was dominated by arthropods. In outcrop the sandstones are more than 1 km thick and comprise predominantly mixed fluvial and eolian deposits. The age of the Tumblagooda Sandstone has been the subject of much debate. Initial analysis of the trace-fossil assemblage suggested a Late Silurian age. Preliminary work on conodont faunas in sediments of the conformably overlying Dirk Hartog Group also indicated a Silurian age. However, arguments have been made for an older, Ordovician age based on paleomagnetic and pedostratigraphic studies. In this review it is argued that deposition is linked to the known ages of regional uplift of the hinterland, and thus inferred to be Early to mid-Silurian. A previous study recognised two distinct trace-fossil assemblages. One, comprising a mixture of burrows and arthropod trackways, represents a freshwater/terrestrial ecosystem that inhabited sands interpreted as having been deposited in broad, low sinuosity, braided fluvial channels, between mixed eolian and water-lain sandsheets, small eolian dunes and flooded interdune, and deflation hollows. The major bioturbator was Heimdallia. Other burrows include Tumblagoodichnus, Beaconites and Diplocraterion. A variety of arthropod trackways, predominantly Diplichnites, formed on water-lain sands and foreset beds of eolian dunes. Other tracks include Siskemia and possible examples of Paleohelcura and Protichnites. Other arthropod traces include Rusophycus and Cruziana. Likely arthropod track makers include myriapods, eurypterids, euthycarcinoids and xiphosurids. A single trackway is interpreted as having been made by a tetrapod and as such pushes back the record of this group from the mid-Devonian to the Early–mid-Silurian. This trace fossil assemblage can be assigned to the Scoyenia ichnofacies. A second trace fossil assemblage, assignable to the Skolithos ichnofacies occurs higher in the section, in strata traditionally interpreted as having been deposited in a marginal fluvial-marine environment. The ichnofacies is dominated by burrows, especially Skolithos, but also Diplocraterion, Daedalus and Lunatubichnus. Rare locomotory traces are assignable to Diplichnites and Aulichnites. Preservation of the arthropod trackways in the Scoyenia ichnofacies was facilitated by the nature of the fluvial/eolian environment. Many of the tracks show indication of having been created subaerially on wet sand surfaces, and preserved by a covering of fine, eolian sand. The presence of extensive dwelling burrows and terrestrial trackways in the Scoyenia ichnofacies represents arguably the earliest known freshwater/terrestrial ecosystem. Moreover, it supports the view that one of the major steps in evolution, the colonisation of land by animals, may have been from rivers, rather than directly from the sea.