Nature of the fossil vertebrate deposits in Victoria Fossil Cave:

    The Fossil Bed and Ossuaries of Victoria Fossil Cave contain the remains of tens of thousands of vertebrate animals, making it one of the richest deposits of Pleistocene vertebrate fossils in the world (Wells 1975).  Represented are over 93 species of frogs, turtles, snakes, lizards, birds, monotremes, marsupials and placental mammals.  The remains include skulls, jaws and complete skeletal remains which are often associated and, in some cases, articulated.

    The Fossil Bed is located in the Fossil Chamber, a cavern some 60 m long and 20 m wide.  The Bed covers an area of more than 70 sq m, is at least 4 m deep, and is estimated to contain more than 5000 tonnes of bone-laden silt.  The accessible portion of the deposit consists of a cone and fan deposit .  The fossil-bearing beds are believed (Wells et al., 1984) to have originated in a series of depositional stages comprising two events: first, clay soil formation with bone and rock accumulating with the soil products on the cone; and second, sandy soil formation with fast sedimentation on the cone and periodic water flows onto the fan.  The lower part of this sequence is exposed in the 3 m deep stratigraphic pit and consists of a sequence of poorly-sorted to partly-sorted quartz sands containing variable quantities of clay, bone, charcoal and organic remains.  The bone in the deposit is bleached and mineralised and appears to have been exposed sub-aerially for some time in a dry environment.  Bone orientation studies by Wells et al. (1984) show a predominent alignment of large, long bones with the long axis of the cave, with large end downslope, indicating low energy transport of bone and sediment.  Partially articulated material is found at the distal end of the fan or against the wall of the cave.  From the Fossil Bed, the remains of the complete range of animals, from very small to very large, have been retrieved.  The Upper and Lower Ossuaries are located several hundred metres beyond the Fossil Bed and are connected to each other by a short, very low crawlway.

bone bed inside Victoria Fossil Cave
The bone bed inside Victoria Fossil Cave.  The sheer amount of fossils contained within this deposit will provide hundreds of years of excavation work for palaeontologists.  Courtesy: Sonny Vandevelde.

    The deposit here consists of a thin veneer of sediment and bone spread over the cave floor rather than a deep bed as in the Fossil Chamber.  The deposit is also notable for the apparent absence of small vertebrates, suggesting either that the bone accumulated differently to that in the Fossil Bed or alternatively, that water carried smaller material deeper into the cave leaving behind only the larger bones (Wells et al. 1984).  The Ossuaries deposit appears to be of approximately the same age as the Fossil Bed.  In the Lower Ossuary, a low roof conceals the lateral extent of the deposit while in the Upper Ossuary it covers an area of more than 40 sq m.

i) Depositional interpretation:

    For reasons outlined by Wells et al. (1984), a pitfall hypothesis for the accumulation of the fossils is suggested.  Studies of the age-frequency distributions of one of the commonest animals in the fauna, the Red-necked Wallaby (Macropus rufogriseus), suggest that individuals were trapped at random.  The depositional hypothesis proposed is that: 1, large animals fell into the cave via a pitfall; 2, skeletons of those that perished on the cone were buried by incoming sediment, redistributed and disarticulated either by mass movement or gentle rafting when water washed down the cone, or a combination of both; 3, skeletons of animals that perished on the distal part of the fan were subject to even less water movement and hence partial articulation was maintained; and 4, that in the absence of other evidence the proportion of particular skeletal elements within a local area of any sedimentary horizon is correlated with the ease of transport of those elements.

    The entrance through which the animals and sediment entered the Fossil Chamber is now completely sealed but has been determined (by pneumatic and hand augering) to have been a section of chamber exposed by partial roof collapse (Wells et al. 1984).  This was at the base of a doline some 150 m wide, possibly caused by erosion of the limestone by water flowing towards the entrance.  Laterally, the entrance was steep to vertical with an average width of 10 m, finishing in a drop of approximately 11 m to cave floor.

    Although this was probably the mode of accumulation of the larger vertebrates in the deposit, Smith (1971, 1972) suggests that many of the small animals may have been brought into the cave by a predator, probably an owl.  It has been suggested that the entrance of the cave was possibly a roost for predatory birds, though Van Tets & Smith (1974) believe that many of the birds represented in the deposit were washed into the cave.  At least some of the animals (for example, some of the frogs, bats and elapid snakes) probably lived in the cave, much as they do today.

ii) Age of the deposits:

    The bone-bearing sediments in Victoria Fossil Cave are estimated to have accumulated between 15,000 and greater than 280,000 years before present (BP).  Preliminary racemisation of bone and uranium series dating of bone from the upper levels of the Fossil Bed are, for uranium series, 125,000 years BP U/Th and 150,000 years BP U/Pa (Ayliffe and Veeh, 1988; Wells et al., 1984) and, for racemisation, 50,000 years BP and 70,000 years BP (plus or minus 20%) for the same levels (J. Bada pers. comm. in Wells et al., 1984).  Charcoal associated with fossil bone from the top 1.5 m of the Fossil Bed (Levels 1-10) has been dated at between 16,700 years BP for Level 1 and 35,000 for Level 10.  For the Upper Ossuary, carbon dating of bone gave a date of 36,500 years BP, while racemisation of bone gave a date of 20,000 years BP for the same deposit.  Preliminary dating of apatite and collagen is equivocable, and Wells et al. (1984) suggest that the deposit may be outside the range of this dating technique.  More accurate estimations of the deposits' ages are emerging from the dating work using a variety of methods.  These include uranium series analyses in collaboration with Linda Ayliffe at the Australian National University (ANU).  TIMS dates on flowstones capping and/or sandwiching bone bearing sediments are extending the age of these deposits to beyond 280,000 years BP.  Based on the dates from speleothems in the caves, it is possible that some deposits may be found to be in excess of 500,000 years old.  Electron Spin Resonance (ESR) dating on tooth enamel in collaboration with Rainer Grun of the Quaternary Dating Laboratory has confirmed a Middle Pleistocene age for the Fossil Bed and Grant Hall.  Dates range back to 350,000 years BP, confirming results from the U-series dating.  Optically Stimulated Luminescence (OSL) dating of the associated quartz in the bone beds by Richard Roberts of LaTrobe University is at a preliminary stage.  His dates are suggesting some sands entered the caves between 200,000 to 300,000 years BP.

Acknowledgement: This subsection of Thylacoleo Revealed is based upon the following: HAND, S. 1988. Draft Nomination of the Pleistocene Fossil Vertebrate Deposits of Victoria Fossil Cave Naracoorte, South Australia for inclusion on the World Heritage List. Unpub. report to SA National Parks and Wildlife Service.
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