Australia: The Land Where Time Began
Talbragar Fish Beds - Late Jurassic - 175 Ma
The Talbragar site covers an are of about 4 hectares. The site was discovered by Mr. A. Lowe of Wilbertree in 1889 and publicised in the same year by William Anderson, Geological Surveyor, New South Wales Mines Department (White, 1981).
The best fossil fish site in New South Wales, there are many fish fossils and plant fossils, both of which are well documented (e.g. Bean, 2006; Dulhunty & Eadie, 1969; Wade, 1941, 1953; White, 1981, 1994; Woodward et al., 1895). The potential for the discovery of new species is considered to be high, in particular insects and other invertebrates, as well as the possibility of finding new reptiles and birds.
Significance of the fossils4
According to the authors4 a very complete picture, in particular of the flora of the environs of a small lake, is provided by the fossil assemblages. .
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 depositis 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 talbragarensis (Woodward, 1895) is the commonest teleost in the Talbragar beds. This species has since been reassigned to a new genus Cavenderichthys (Arratia, 1997)³, the amendment being later confirmed in a review (Bean, 2006a), of the actinopterygian taxa from Talbragar³.
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 thickness 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 equisetaleans, 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 to 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).
According to the authors³ the conclusion by Grant-Mackie et al., (2000, p. 344-345) "All marine faunas show clear Tethyan or eastern Tethyan affinities throughout the Jurassic along the northwestern and northern Australian margins as far east as New Guinea and the Pahu Terrane"; and that "Whilst there is clear and continuing Tethyan influence, there is in the Middle Jurassic a strong south Andean connection and a weaker Mexican link which continue into the Late Jurassic." The authors³ suggest further work is required to determine if this conclusion applies to the biota on land.
The eastern basins of Australia are the sites where the terrestrial biotas of the Jurassic are best represented. In these basins forests were dominated successfully by cheirolepidiacean, araucariacean and podocarpacean conifers as well as a range of seed-ferns. The floras of the Early Jurassic displayed moderate differences between those in western parts of Australia and those in the eastern part, expressed especially in the Exesipollenites association distribution that was located along the margin of the Tethys Ocean (Grant-Mackie et al., 2000). Assemblages of the Later Jurassic are essentially pan-Australasian, that the authors³ suggest may reflect the higher latitude and more insular setting of Australia-New Zealand in the later part of the Jurassic.
Because of the sparseness of vertebrate fossils known from the Jurassic in Australia there are not many definitive palaeogeographic conclusions that can be drawn, only the possibility of a Pacific geographic province in the Middle Jurassic. Fish of the semionotidae were widespread in the Jurassic, with several taxa known from the Southern Hemisphere, providing global biostratigraphic, geochronological and palaeobiogeographic data. Detailed work on scales and teeth are suggested by the authors³ to possibly assist in improved zonation of continental rocks and better marine and non-marine correlation as this group of fish appear to have been capable of diadromy (migration of fish between freshwater and marine water in either direction). Potential for a refined stratigraphy may result based on the teeth of hybodonts as a good global record of non-marine sharks that is emerging (Thies et al., 2007).
Better correlation of marine/non-marine sequences on the margin of Western Australia requires more work. In the basins of eastern Australia there are shortcomings in the correlation of latest Middle to Late Jurassic units. With no marine microfossils available for these basins, and as palynozones in this interval are relatively long-ranging, more details are required of spore-pollen taxa and their ranges for better biostratigraphic resolution.
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