A diverse Pleistocene marsupial trackway assemblage from the Victorian Volcanic Plains, Australia
Introduction
Fossil trackways commonly preserve characteristics of organisms not available from their skeletal fossil record. Trackways can provide insight into the biomechanics and palaeobiology of a species not evident from the skeleton (Lockley, 1998). It may be possible to determine the gait, foot morphology, size and speed of travel of individuals, as well as factors of wider ecological importance, such as community composition and species diversity (Camens and Wells, 2009). In addition, the rheology of the sediments preserving the prints can reveal aspects of the environment at the time of deposition (Allen, 1989, Allen, 1997).
Mammalian trackways have been widely studied in many parts of the world (e.g. Leakey and Hay, 1979, Belperio and Fotheringham, 1990, Cohen et al., 1993, Lea, 1996, Allen, 1997, Lockley et al., 1999, Ataabadi and Sarjeant, 2000, Mietto et al., 2003, Triggs, 2004, Webb et al., 2006), as have those of dinosaurs (e.g. Alexander, 1976, Thulborn and Wade, 1984, Lockley, 1986a, Thulborn, 1989, Thulborn, 1990, Farlow et al., 2000). However, mammalian fossil trackways are rare in Australia, the five described sites being restricted to the Pliocene and Pleistocene. These sites include late Pleistocene human footprints at the Willandra Lakes in New South Wales (Webb et al., 2006) and Holocene human footprints at Clare Bay in South Australia (Belperio and Fotheringham, 1990). Marsupial traces include macropodid (possibly Protemnodon) footprints, from near Clare Bay (Belperio and Fotheringham, 1990), Pleistocene Diprotodon footprints at Lake Callabonna (Tedford, 1973, Wells and Tedford, 1995) and Pliocene Euowenia grata footprints from the Warburton River (Camens and Wells, 2009), all in South Australia, and kangaroo tracks from the Willandra Lakes, New South Wales (Webb et al., 2006). The site described below thus contains more fossil marsupial footprints than have previously been described for the whole of the Australian Quaternary.
Most fossil mammal trackways previously described were created by hooved ungulates (e.g. Laporte and Behrensmeyer, 1980, Loope, 1986, Allen, 1989, Allen, 1997, Lea, 1996, Lockley et al., 1999, Fornós et al., 2002), their hard hooves having the potential to leave tracks both deeper and in a wider range of substrates than the soft-footed marsupials. This may help to explain the paucity of fossil mammal footprints in Australia, as the soft feet of all members of the Australian fauna would leave shallower tracks that are less clearly defined and less likely to be preserved.
Here we describe the most diverse assemblage of marsupial trackways yet discovered in Australia. Their superb preservation allows us to determine the trackmakers at familial, generic or even species level. In the case of the most spectacular trackway, we are able to estimate the animal’s speed and suggest its sex. Our geological analysis provides temporal context for the trackways together with an understanding of the environment in which they were active. Accordingly, we aim to shine a light on the late Pleistocene ecosystem of the fertile and well watered volcanic plains of south-eastern Australia.
Section snippets
Regional setting
The megafaunal marsupial trackways occur on a lake margin within the Victorian Volcanic Plains (VVP) of south-eastern Australia (Fig. 1). Details of the location are not provided here because a plan of management to protect the locality from human interference and the elements has not yet been developed. Those details are, however, held at Museum Victoria, Melbourne, in the documentation of six moulds and casts taken from the trackways (MV P230853 to P230858). The unnamed lithostratigraphic
Trackway mapping
The position, length and width of each footprint were recorded on a gridded plan using tape measure and compass. A more or less continuous series of footprints made by one individual as it traversed the site was defined as a trackway. For the diprotodontid trackways the length, width and depth of each print were measured and its distance from an arbitrary datum recorded. Stride length was measured as the distance between corresponding points on successive footprints. Trackway width was measured
Geological description
The trackways occur in volcaniclastic rocks, while associated vertebrate skeletal fossils were found in pockets of sandy mud and ferruginised sandstone that exist within the volcaniclastics. The thickness of the exposed strata is less than 0.5 m. Two contrasting units are present, the lower one being volcaniclastic and the upper one a micritic carbonate. Eight short cores (8–29 cm long) were extracted from the lower unit within the confines of the grid.
Palaeoenvironment
It has been suggested that the environments most conducive to footprint preservation are lake and river margins (Cohen et al., 1993, Mustoe, 2002). The VVP trackways exposure is at the edge of a modern lake and, although the configuration of the lake is probably vastly changed since the trackways formed, the sedimentological evidence strongly supports a lacustrine setting for the host sediments. First- and second-order bed forms (parallel sand bars and ripples) are aqueous in origin, formed at
Conclusion
The VVP assemblage of Pleistocene trace fossils is the taxonomically most diverse group of marsupial trackways known. In addition to diprotodontid, macropodid, vombatid and possible thylacoleonid trackways, the locality also yields trace fossils of invertebrates and plants. Well preserved sedimentary features show that the trackmakers were attracted to a lake which, at some time after the formation of the trackways, changed from near-fresh to saline. The diprotodontid, if correctly identified
Acknowledgements
We thank an anonymous farming family of the Victorian Volcanic Plains who have protected the trackways for decades; Parks Victoria staff, especially John Clarke, Frank Gleeson, Dave Jenson and Rob Wallis; Peter Swinkels and his crew of preparators from Museum Victoria who moulded major sections of the trackways; students, staff and friends of the University of Ballarat and DAP’s crew of volunteers who helped lay out the grid, measure the tracks and excavate the skeletal fossils; and Matt Gibson
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