Roderick Tucker Wells

Electron spin resonance dating of South Australian megafauna sites

In recent years, there has been much debate about the timing of the Late Pleistocene extinction of the Australian megafauna. Some studies postulated a rapid, continental-wide extinction at around 46 000 years ago and that the arrival of humans in Australia, rather than climatic fluctuations, was the main cause for the demise of the megafauna. We have applied electron spin resonance (ESR) dating to a series of teeth from megafauna sites in South Australia, where young ages were expected. During this study, a number of unexpected problems were encountered. These were mainly related to the fact that ESR age assessments on fossil teeth are critically dependent on a realistic reconstruction of the post-mortem uranium uptake into the dental tissues. At virtually all sites, conventional, routine ESR dating, based on the parametric early and linear U-uptake models, would have led to grossly erroneous results. Most teeth were analysed for U-series isotopes with laser ablation ICP-MS, and the results were used to calculate combined U-series ESR age estimates. Only one of the 24 teeth analysed conformed to the commonly applied early and linear U-uptake models. At one of the sites, we found for the first time clear evidence of uranium leaching from dentine. Detailed laser ablation scans revealed that, in contrast to large mammals outside Australia, marsupial tooth enamel does not seem to contain a barrier layer close to the outer surface that blocks uranium diffusion into the enamel. As a consequence, uranium migrates into the enamel layer from both the outside and the inside via the dentine, which makes marsupial teeth generally less well suited for ESR dating. It was particularly difficult to obtain age estimates for the site of Black Creek Swamp, where the sediments contained extreme U-series disequilibrium, and the teeth had unexpectedly accumulated very high U-concentrations (up to 700 ppm in dentine) within a few thousand years. Although most of the sites contained reworked teeth, none of the samples yielded age estimates that were significantly younger than the proposed extinction window of about 40 000–51 000 years.

Ecological and evolutionary significance of sizes of giant extinct kangaroos

A method, based on femoral circumference, allowed us to develop body mass estimates for 11 extinct Pleistocene megafaunal species of macropodids (Protemnodon anak, P. brehus, P. hopei, P. roechus, Procoptodon goliah, ‘P.’ gilli, Simosthenurus maddocki, S. occidentalis, Sthenurus andersoni, S. stirlingi and S. tindalei) and three fossil populations of the extant eastern grey kangaroo (Macropus giganteus). With the possible exception of P. goliah, the extinct taxa were browsers, among which sympatric, congeneric species sort into size classes separated by body mass increments of 20–75%. None show evidence of size variation through time, and only the smallest (‘P.’ gilli) exhibits evidence suggestive of marked sexual dimorphism. The largest surviving macropodids (five species of Macropus) are grazers which, although sympatric, do not differ greatly in body mass today, but at least one species (M. giganteus) fluctuated markedly in body size over the course of the Pleistocene. Sexual dimorphism in these species is marked, and may have varied through time. There is some mass overlap between the extinct and surviving macropodid taxa. With a mean estimated body mass of 232 kg, Procoptodon goliah was the largest hopping mammal ever to exist.

Palaeoenvironmental reconstruction of the Late Pleistocene to Early Holocene Robertson Cave sedimentary deposit, Naracoorte, South Australia

Sedimentological investigations and geochemical analyses of the Robertson Cave deposit, 7 km south of Naracoorte, South Australia, document a palaeoenvironmental record for the Late Pleistocene and Early Holocene. Three distinct depositional episodes spanning the interval 32 – 8 ka are represented by over 4 m of sediments exposed in the chamber 2 excavation pit. The oldest of the three, the lower unit (>30 ka BP) is dominated by reddish and brown sandy silts. A 20 cm band of red silt horizons that cap this unit contain enriched δ15N values (12 – 15‰), low C/N ratios, significant SO3 (20%) and CaO (18.5%) contents, a high alkyl-C component and phosphorus-rich mineralogy. These unique sediment horizons are interpreted as being derived from the accumulation of bat guano and degraded cave rock fragments. Their presence suggests a period of low sediment input into the cave, possibly reflecting the existence of a stable geomorphic environment prior to 30 ka. The time period 30 – 27 ka saw the rapid deposition of the middle unit, a sequence of coarser homogeneous sands that resulted in the entrance to chamber 2 becoming blocked. These horizon-less sediments, with clear aeolian features, are most likely representative of the drier conditions associated with the Last Glacial Maximum, which peaked at 20 – 17 ka. Deposition into the chamber recommenced at approximately 13 ka and continued until 8 ka, resulting in the accumulation of the upper unit, a sequence of silt, charcoal and organic matter rich horizons. Here, a shift to more 13C-depleted values (−25‰ to −27‰) is evident in the bulk soil organic matter and charcoal between 11 and 10 ka, possibly reflecting a shift from the dry Last Glacial Maximum to the wetter environs of the Holocene. The upper unit may also be a record of burning practices associated with Aboriginal migration to the area.

Pliocene and earlier pleistocene marsupial evolution in southeastern Australia

In recent years investigation of Australian Pliocene and earlier Pleistocene fossil occurrences and new methods for their dating has made it evident that there have been important changes in the mammal fauna during the past 5 million years. We review these data and believe they support the following general conclusions. By early Pliocene time (4–5 Ma) many marsupial genera characteristic of the late Cainozoic had appeared. Some species of extinct genera characteristic of the Pleistocene were also present in the late Pliocene (2 Ma). Megafaunal marsupials, notably Diprotodon, and the beginning of the spectacular radiation of a large sthenurine kangaroos, accompanied remnants of Miocene taxa (e.g. Ektopodontidae, and early macropodines like Prionotemnus and Kurrabi), into the early Pleistocene. This turnover, extending across the Pliocene-Pleistocene boundary, roughly corresponds with the time of northward withdrawal of rainforest from southeastern Australia and its replacement there by open Eucalyptus forest. Dust flux from the continent into the Tasman Sea and Indian Ocean increased in volume and frequency over the last four glacial cycles (400 ka) reflecting the progressive denudation of the Australian inland. Procoptodon sthenurines, the last megafaunal genus to appear, arose early in the Pleistocene near the initiation of 100 ka glacial cycling. During the last glacial cycle (120-20 ka) most of the megafauna including all sthenurine species became extinct.

Late Pleistocene megafauna site at Black Creek Swamp, Flinders Chase National Park, Kangaroo Island, South Australia

The occurrence of fossil vertebrate remains at Black Creek Swamp at the western end of Kangaroo Island, South Australia, along with reports of ‘primitive’ stone implements in the vicinity has, for more than seventy years, fuelled speculation that this site would reveal a definitive relationship between humans and megafauna. Radiocarbon dating in the 1970s and again in 2004 suggested accumulation at around the last glacial maximum, making it potentially the youngest megafaunal deposit in Australia. Our excavations produced no artefacts and no evidence of butchering. Taphonomic evidence indicates three phases of drought accumulation around an ephemeral water source. These droughts may have been induced by climate, sinkhole drainage, or both. The fauna includes 29 species; one third of the species are extinct. This component is represented by browsing herbivores and their putative predator, Thylacoleo carnifex. The extant species indicate a mosaic of habitats including open sclerophyll forest, grassy patches, areas of shrubby understorey and semi-permanent water sources. The occurrence of two dwarfed species is suggestive of isolation and resource depletion. Multiple dating techniques (OSL, ESR, U-series and C) revealed a complex geochemical history for this site. New age estimates place the fossil accumulation between 110 and 45 ka.

Palaeontological excavation and taphonomic investigation of the late Pleistocene fossil deposit in Grant Hall, Victoria Fossil Cave, Naracoorte, South Australia

Grant Hall chamber in Victoria Fossil Cave, Naracoorte, South Australia, contains a late Pleistocene faunal assemblage, dated at between 206 and 76 Ka. Taphonomic and faunal analyses indicate that the predominant mode of accumulation was via a surface exposed pitfall trap. An avian predator, such as Tyto alba, may have been responsible for the accumulation of small mammal remains. The faunal assemblage is taxonomically diverse containing at least 47 taxa. It includes many browsing species such as Wallabia bicolour and the extinct Sthenurine kangaroos and Zygomaturus trilobus, as well as small mammals that require trees and a thick understorey. The Grant Hall fauna thus indicates the presence of densely vegetated woodland, interspersed with small patches of open and thickly grassed areas in the proximal vicinity of the old cave entrance. The relative abundances and species composition of the Macropodidae fauna in Grant Hall are significantly different from other faunal assemblages found at Naracoorte. This study has provided palaeoecological information for a time period not previously investigated at the Naracoorte Caves; detailed surveying of the chamber was undertaken as part of the study.

DIPROTODONTID FOOTPRINTS FROM THE PLIOCENE OF CENTRAL AUSTRALIA

ABSTRACT The first Pliocene marsupial fossil trackways from Australia are described. The trackways, attributed to Euowenia grata (De Vis) (Diprotodontoidea, Marsupialia), occur in the middle Pliocene Tirari Formation, Warburton River, northern South Australia. The trackways were formed as the animals made their way across a soft claypan. Pad impressions, subsequently infilled by a gypsum-cemented clay, indicate how weight was distributed within the pes.

Palaeobiology of Euowenia grata (Marsupialia: Diprotodontinae) and its presence in northern South Australia

Recovery of a specimen of Euowenia grata (De Vis, 1887) from mid Pliocene sediments of the Tirari Formation on the bank of the Warburton River in the Lake Eyre Basin provides the first recorded account of this species in South Australia. The specimen comprises a partial skull including left and right premaxillae, maxillae, and left zygomatic arch, along with an almost complete upper dentition (missing the left I2). An articulated hind leg and pes found downstream at the same stratigraphic level, as well as both fore- and hind-feet of a single individual, are also referred to E. grata and represent the first postcranial material assigned to the species. A reconstruction of the pes indicates that much more of the body weight was borne by the tarsus in this species than in plesiomorphic diprotodontids, such as Nimbadon Hand et al., 1993, or Ngapakaldia Stirton, 1967, although E. grata does not exhibit the more extreme enlargement of the tarsus seen in graviportal Pleistocene diprotodontids. E. grata is found here also to be the only known Australian marsupial, extant or extinct, to exhibit fusion of all three cuneiform bones in the tarsus. We suggest that the diprotodontine hind limb and pes had evolved graviportal adaptations in the Pliocene as well as in the Pleistocene members. We also suggest that E. grata may have been able to rear up against trees while browsing.

Miocene sea cow (Sirenia) from Papua New Guinea sheds light on sirenian evolution in the Indo-Pacific

ABSTRACT A partial postcranial skeleton (vertebrae and ribs) of an indeterminate sirenian is described from Selminum Tem cave in the Hindenburg Range, Western Province of Papua New Guinea. It was derived from a section of the Darai Limestone dating to the Burdigalian-Serravallian (early-middle Miocene) and representing shallow platform carbonates. The thoracic vertebrae are remarkably small, being comparable in size to the vertebrae of Nanosiren garciae and implying small body size, although it is uncertain whether the specimen represents a diminutive adult or juvenile individual. These fossils represent the geologically earliest mammal recorded from the island of New Guinea and the earliest evidence of Sirenia in Australasia. Thus, this fossil evidence provides a minimum date (∼11.8 Ma) for the earliest presence of sirenians in Australasian coastal marine ecosystems, as well as their primary food source, seagrasses.