Western Australia is well known for its very old rocks, but it also has many significant localities of younger rocks (Phanerozoic, less than 541 million years). Geologists working on trace fossils in the Tumblagooda Sandstone have described what may be some of the earliest steps of life on land. The following describes the Tumblagooda Sandstone and its formation and landscape, as well as the aquatic and terrestrial traces of life it preserves.
The Tumblagooda Sandstone outcrop in the Kalbarri National Park, 600 kilometres north of Perth, and features red and white rocks. The Tumblagooda Sandstone is a sedimentary rock, deposited as successive layers of sand. These layers form the colourful horizontal banding, or bedding, seen along the coast’s gorges and cliff sections.
The red and white striping and blotching of the sandstone is caused by iron oxides staining the rocks. The colouring probably formed shortly after the rock was deposited, in an oxygen-rich (rather than stagnant or swampy) environment. The colouring often highlights sedimentary features such as burrows and ripples.
Early Tumblagooda Sandstone landscape
When the Tumblagooda Sandstone was deposited, there would have been a high mountain range to the east of Kalbarri, extending north–south for several hundred kilometres. At the foot of the mountains, a broad (more than 100 kilometre wide) plain would have been covered by a continuous sheet of constantly shifting river channels, without any land vegetation to bind the channel banks.
This environment is unusual today, only comparable to the world’s driest deserts. The river courses met the sea at large, extensive tidal flats. The tidal range during the time the Tumblagooda was deposited may have been larger than today, and the boundary between the river plain and the tidal flats would have been very irregular and constantly changing.
There would have been abundant sea life, but the land would have contained only simple vegetation, including mosses, liverworts and other simple plants. We see evidence of the organisms living in the shoreline sediment in the rocks of Tumblagooda today.
Abundant trackways indicate arthropods such as eurypterids and trilobites had ventured from coastal waters and colonised the tidal flats. They had just begun to find their way temporarily onto the land, preparing to evolve to fully terrestrial arthropods. Intensely burrowed layers of sedimentary rocks indicate worms mined the sediments just beneath the flats in search of nourishment, as occurs in coastal flats today.
Ichnology is the study of trace fossils (ichnofossils), which indicate an organism’s behaviour, such as locomotion, feeding, burrowing, escaping and even defecating. Trace fossils can be used by paleontologists to understand the environment of deposition of sedimentary rocks, and sometimes their age.
An ichnofauna is a collective of traces left by organisms. Ichnofaunas, especially those formed in marine environments, can be restricted to very specific conditions or water depths. These types of studies can be used by petroleum companies to understand hydrocarbon resources.
Signs of life
Only one body fossil has been found so far in the Tumblagooda Sandstone — an impression of a strange arthropod called Kalbarria, which belongs to a now-extinct group called the euthycarcinoids. Body fossils are rare in the sandstone because the depositional environment was not favourable for the preservation of organic material. This is because the active, high-energy river system would have quickly broken down any carcasses, and the water’s high oxygen level would have rapidly decayed flesh. Despite this, the sandstones are rich in signs of life, preserving abundant tracks, traces and burrows of organisms that lived while the sandstone was being deposited. These ichnofossils give us an insight into not only what organisms were present while the sandstone was forming, but also what the environment was like.
Ichnofossils of the Tumblagooda Sandstone are comparable to those of the Devonian Taylor Group in Antarctica and the Ordovician–Silurian Mereenie Sandstone at Kings Canyon in central Australia. The majority of traces in the Tumblagooda Sandstone were formed by animals living under water, called aquatic or subaqueous. However, the form of some trackways seem to indicate that they were formed above the water. These may be evidence of some animals' first steps on the land.
The terrestrial traces seen in the Tumblagooda Sandstone consist of many types of Diplichnites, a walking trace. Diplichnites are trackways, made up of a pair of parallel rows of ‘pits’ or paddle-like impressions, which can extend over surfaces for many metres. Tracks made in sands that were either very dry or very wet often lack definition, but those made in just the right conditions have exceptional detail preserved.
Diplichnites traces are interpreted to be formed by arthropods walking over a sediment surface, so the range of walking trails indicates the area was populated by a diverse fauna of arthropods during deposition. When the trails are well preserved, scientists can count how many legs the trace-maker had, helping to identify the arthropods that made which traces. Trilobites, euthycarcinoids and eurypterids are all thought to have been present when these Sandstones were deposited.
There are many examples of aquatic or subaqueous burrows in the Tumblagooda Sandstone. A few of the more common types include:
- Skolithos: a simple vertical or near vertical burrow that can be tens of centimetres in length, with some examples up to 2 metres long
- Heimdallia: forms intensely burrowed beds with a ‘can of worms’ appearance in tidal sandflat deposits
- Aulichnites: a horizontal, gently meandering trail, possibly created by a marine snail crawling or grazing
- Tumblagoodichnus hockingi: a trace unique to the Tumblagooda Sandstone. A probable arthropod hunting burrow, named after GSWA geologist Roger Hocking — who discovered it during the first detailed study of the Tumblagooda Sandstone.
Age of the Tumblagooda Sandstone
The age of the Tumblagooda Sandstone is currently poorly constrained, with previously documented ages ranging from the Cambrian to the Cretaceous. At present, the most likely age is thought to be Ordovician or Silurian. Scientists from GSWA and elsewhere are working on new techniques to find out exactly when this formation was deposited. Some of the trace fossils discussed here — and a model of an animal that might have created them — can be seen in the Western Australian Museum’s Diamonds to Dinosaurs gallery.
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