Australia: The Land Where Time Began

A biography of the Australian continent 

Talbragar Fish Fauna of the Late Jurassic³

Jurassic age fish beds in New South Wales were first recognised as a result of an assessment of fish from Talbragar, near Gulgong (Woodward, 1895). More information on the faunal composition and palaeoenvironmental setting of the fossil fish beds has been provided by more recent studies (Bean, 2006a, 2006b). In the Talbragar fauna the most common species of fish is "Leptolepis" talbragarensis, Woodward, 1895, the species being suggested to be the first teleostean to appear in the fossil record of Australia (Long, 1991). The species was later assigned to a new genus, Cavenderichthys, (Arratia, 1997). In a later review of the actinopterygian taxa in the Talbragar deposits this taxonomic reappraisal and amendment was supported (Bean, 2006a). Specimens had originally been referred to 3 species, "Leptolepis" talbragarensis, "L." gregarious and "L." loweii (Woodward, 1995). It was later suggested that the 3 species were actually synonyms (Wade, 1941), and later still this suggestion was supported and they were all assigned to C. talbragarensis (Arratia, 1997). Lynne Bean has more recently carried out a detailed comparison of  the species with taxa that are at present referred to Leptolepis, as well as with other forms from the Late Jurassic, Tharsis dubious, and Leptolepides sprattiformis, indicated that C. talbragarensis was most closely related to members of the Leptolepididae from the Late Jurassic.

The Talbragar fauna also includes 1 palaeoniscid, Coccolepis  australis and members of the "holostean" family Archaeomenidae, such as Archaeomaene tenuis, Aphnelepis australis and Aetheolepis mirabilis (Bean, 2006b). A member of the Archaeomenidae, Madariscus (Wade, 1941) is based on a series of specimens, all of which are imperfect. The specimens in the Australian Museum, Sydney, and the Natural History Museum, London, have insufficient detail to determine if they should be in a new genus, though according to the authors³ they generally appear to be like a mature form of Archaeomene. Uarbryichthys latus, Wade, 1941, and is not now considered to be a macrosemiid, though its affinities are uncertain, and its affinities are believed to lie with the Macrosemiiformes (Bean, work in progress). It has been found that the Talbragar Fish Fauna. display the strongest faunal similarities with the Koonwarra biota from the Early Cretaceous (Aptian) of Victoria (e.g. Long, 1991), as both faunas have a Coccolepis species, a "leptolepid", and Archaeomenids, rather than the taxa present in older sites in the Clarence-Moreton Basin.

Several specimens of a larger amiiform caturidid fish was found in 2006-2007, that was similar to Furo, Gistl, 1848, that had been found in deposits of Upper Jurassic age from Europe and Asia (see Frickhinger, 1996). This fish from the Talbragar deposits grew to more than 30 cm long and was covered with heavy, enameled scales, and had a single row of teeth on both jaws (to be described elsewhere by Lynne Bean). A recent find by S. Avery from the Talbragar fossil lagerstätten was the first chondrichthyan from the Australian Jurassic. It is a hybodontiform shark, as indicated by its squamation (ST pers. obs.). It is suggested by this find that there is potential for further finds of freshwater or euryhaline forms and for possible correlation with rich lagerstätten in Europe as well s other parts of the world (Thies et al., 2007; Turner, 2008).

A Late Jurassic age is indicated by the fossil-fish assemblage in Talbragar deposits (Bean, 2006a, 2006b). The contrasting dates for the Purlawaugh Formation, formerly associated with the Talbragar Fish Beds, of Early to Middle Jurassic were reported by most previous workers. One example is palynofloras were recorded that are referable to the J2 palynozone of Evans (1966), with an age of from Pliensbachian-early Toarcian, from near the base of the Purlawaugh Formation at another site close to Gulgong (Hind & Helby, 1969). They record assemblages referable to palynozones J3-J4 of Evans (1966), about middle Toarcian-early Callovian, from higher in the Purlawaugh Formation. According to the authors³ it is apparent that the Talbragar Fish Beds are somewhat younger than the parts of the Purlawaugh Formation deposits that contained the palynofloras,  the Talbragar Fish Beds containing no palynofloras, based on the age of the Beds obtained from faunal assemblages and SHRIMP (Sensitive High Resolution Ion Microprobe). The Research School of Earth Sciences at the ANU carried out SHRIMP analysis of samples from the fossil fish bed, indicating an age of 151.55 ± 4.27 Ma, the Oxfordian-latest Kimmeridgian-Tithonian in terms of GTS (2004) for the youngest population. The affinities of Cavenderichthys with forms from the Kimmeridgian of Europe accords with the isotopic date. A maximum age for the strata is provided from this, as there are no signs of transport abrasion on the zircon crystals, indicating that the sediment was deposited at about this time. According to the authors this idea, outlined further below, is supported by the presence of euhedral zircon crystals and the nature of the fossil fish beds, that are strongly silicified, the mass fish kill being the result of felsic volcanic ash from a volcanic eruption falling on the Talbragar lake. This view, as well as an anaerobic environment, is supported by the fine detail in which both the fish and plants were preserved, which would explain the lack of decay in an anaerobic environment.

Several beds, totaling 60-100 cm in thickness, comprise the Talbragar Fish Beds, the upper layers, 30-40 cm, composed of very fine material and lower layers are composed of material that is relatively massive, though still fine-grained, and all layers are heavily fossiliferous (Lynne Bean, pers. obs.). The outcrop, that extends at least 200 m along strike, has been suggested to represent the erosional remnant of the margin of a freshwater lake-bed deposit (Percival, 1979). Several anoxic events are now believed to be indicated by the evidence to be responsible for the mass fish death, that includes occasional volcanic ash eruptions (Bean, 2006a). There is a high concentration of small fish 4-12 cm long that are extremely well preserved in the upper layers, and in the lower layers the fish are scattered throughout the deposit indicating it was a more normal lacustrine environmental deposit. The authors³ say the evidence indicates that this part of the fish bed is not a mound spring deposit as there is little sign of desiccation in the form of mud cracks or surfaces that are exposed  aerially. The evidence also indicates they are not typical overbank deposits as they lack coarse-grained beds with bases that are erosive (crevasse splay sediments) usually associated with such a succession, interspersed with palaeosols that usually develop during periodic exposure. The capping of the most fossiliferous beds is in the form of a very fine-grained uppermost layer deposited during a interval of slow deposition in still water that became anoxic suddenly.

The rock composition has been described as a hard, fine, limonitic cherty shale (Dulhunty & Eadie, 1969), though more recently it is indicated by thin section and chemical analysis of the clastic components of the sediments, they are largely reworked from the underlying sandstone, and with thin tuff interbedding that represent 1 or more ash falls that were very fine (Bean, 2006a). No evidence has been found of carbonate nor calcium ions by the use of EDXA (Energy Dispersive X-ray Analysis). The presence of Liesegang rings that are associated with jointing indicated that the red-brown colour of the fossiliferous beds was added post-depositional. In-filling the fossil fish cavities it is common to find magnesium dioxide, as well as forming dendrites near the block boundaries. Kaolinite and opaline quartz have replaced the organic material in many of the impressions of both fish and plants (Dr. A. Christy, pers. comm. to LB, see also White, 1981b, 1986).

A continuation of the fish beds has been found to the west of the best-known fish beds, that was not stained with iron oxide and is a very fine, soft, white mudstone that is fossiliferous. The authors³ suggest the rock staining and subsequent infilling of fish impressions are artefacts of groundwater movement (Bean, work in progress).

Sources & Further reading

  1. Turner, S., Bean, L.B., Dettmann, M., McKellar, J. L., McLoughlin, S. & Thulborn, 2009; Australian Jurassic sedimentary and fossil successions: current work and future prospects for marine and non-marine correlation, GFF, Vol. 31, (Pt 1-2, June), pp 49-70. Stockholm, ISSN 1103-5897
  2. Cook, Alexi et al., 2012, Australia's Fossil Heritage: A Catalogue of Important Australian Fossil Sites, The Australian Heritage Council, CSIRO Publishing.

 

 

Author: M. H. Monroe
Email:  admin@austhrutime.com
Last updated 14/08/2012

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading