Australia: The Land Where Time Began

A biography of the Australian continent 

Talbragar Fish Beds - Jurassic - 175 Ma

Located in New South Wales near the town of Gulgong, the fish beds are found in loose blocks of chert in the soil from the weathering of the Jurassic Purlawaugh sediments that is overlain by the Pillaga Sandstone, and underlain by the Narrabeen Sandstone of Triassic age. The rock of this deposit are fine-grained, ochre-coloured silicified shale. The blocks containing the fossils are believed to be what remains of a stratigraphic layer, an isolated lake-bed deposit, that has been completely eroded away. A freshwater vertebrate fauna is found at this site. The chert bed is believed to have accumulated over a very short period, possibly as short of 250 years, and may even be as little as several seasons. The fine iron-rich silt of the lake bed was apparently anaerobic, ideal conditions for the preservation of the animals and plants that settled on it.

At the time this deposit was being laid down Australia was within, or very near, the Antarctic Circle, but there is no indication of glaciation at this site. The plant fossils at the site include kauri pine, other conifers and cycads are a similar vegetation type to that found in the Warm Temperate Rainforests on the Atherton Tableland in north Queensland at the present. This site shows a rare glimpse of the fish of the Australian Jurassic. All the fish at the site are actinopterygians, ray-finned fish. This fish group first appears in the Triassic and at the present are the most common type of fish in both oceans and freshwater. Leptolepis is the commonest teleost in the Talbragar beds.

There are 2 other divisions of teleosts in this deposit, though few species of each. Coccolepis australis is the only palaeoniscoid, and 3 holosteans, a macrosemionotid (Uabryichthys), and 2 genera of the Australian endemic family Archaeomenedae (Archaeomene and Madariscus). The skeletons of these primitive fish are less ossified and their scales are thicker than modern teleosts.

The fish at this site appear to have been killed, probably all at the same time, and buried over a very short period, possibly by a single cataclysmic event, resulting in good preservation and concentration of fossils.

Talbragar Fish Beds²

According to Kear & Hamilton-Bruce the Talbragar Fish Beds are now believed to be of  Upper Jurassic age, though they have traditionally been believed to be of Lower Jurassic age. They also suggest that the deposits were formed in an extensive system of freshwater lakes that were smothered by a volcanic eruption. These deposits have proven to be relatively rich in fossils, though they are geographically restricted. In central northern New South Wales this site has been exposed in the Talbragar River valley. A diverse range of fish, invertebrates and plants have been found in the Talbragar Beds that the suthors² believe lived in and around a shallow lake about 160 Ma.

The main horizon in which fossils have been found consists of an iron-rich cherry-shale lens that is ochre-coloured, that has been weathered to the point where it is in the form of loose blocks floating in soil. The authors² suggest the individual slabs of rock could possibly be erosional remnants of the original lake bed deposit, that is regarded as the underlying Purlawaugh Formation, though fresh exposures have been uncovered more recently.

The main outcrop is in northern New South Wales northeast of Gulgong on the eastern slope of Farrs Hill. Support for the volcanic suggestion is that the maximum of these beds is 600 mm, suggesting the original layer was very thin, possibly accumulating over a very short period, as would occur in the volcanic scenario.

Lacking definitive microfossil dating the proposed Late Jurassic age of the Talbragar Beds is based on macrofossil assemblages and  zircon crystal isotopic dating.

The evidence of vegetation in this deposit suggest it was diverse, including such taxa as equicetaleans, ferns, bennettitaleans, that included Otozamites, Taeniopteris, a pentoxylacean, and a variety of podocarps that included Rissikia talbragarensis, Elatocladus australis, and araucarian conifers including Podozamites jurassica, Brachyphyllum. According to Kear & Hamilton-Bruce the description that has been used describe these elements is a 'Jurassic kauri pine forest with an araucarian-dominated canopy interspersed with a pentoxylacean-rich lower tree zone, scattered podocarps and podocarps and fern-cycad heath zones surrounding lake margins' Kear & Hamilton-Bruce²).

The Talbragar Fish Beds Flora

Sources & Further reading

1. Mary E. White, The Greening of Gondwana, the 400 Million Year story of Australian Plants, Reed, 1994
2. Kear, B.P. & Hamilton-Bruce, R.J., 2011, Dinosaurs in Australia, Mesozoic life from the southern continent, CSIRO Publishing.