The impacts of environmental change on late Quaternary fossil fauna at Cathedral Cave, eastern Australia

Author: Diana Fusco

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Fusco, Diana, 2021 The impacts of environmental change on late Quaternary fossil fauna at Cathedral Cave, eastern Australia, Flinders University, College of Science and Engineering

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The late Pleistocene in Australia was marked by the loss of >90% of large terrestrial vertebrate species and the formation of modern faunal assemblages. The relative roles that humans, who arrived on the continent c. 60,000 years ago, and climatic change played in shaping observed patterns have long been debated. Rich empirical datasets that allow for changes in the proportional representation of species to be tracked through time are critical for resolving the relative effects of these two drivers. These also hold great potential for informing on how modern ecosystems may respond in the future to changing climates and intensifying anthropogenic pressures. Such records, however, are rare.

Here I investigate faunal responses to environmental change through the Cathedral Cave deposit in the Wellington Caves system of central-eastern Australia. This is the stratigraphically deepest late Pleistocene vertebrate sequence in Australia. This study is the first to track changes in faunal assemblages through the late Pleistocene in central-eastern Australia, and the highest resolution palaeoecological study of its type yet undertaken in Australia. Excavation of the deposit to a depth of 4.2 m revealed 13 sedimentary layers, the distinctiveness of which were supported by sedimentary and geochemical data. Radiocarbon dating of charcoal clasts and optically stimulated luminescence dating of sediments reveals that the cave was accumulating fossils and sediments between 65.8 and 43.2 thousand years (ka) ago, during the megafaunal extinction interval. A hypothesised roof entrance, now hidden by a large stalagmite, was open during this period, acted as a pitfall trap for larger species and an entry for roosting owls, which contributed most of the remains of smaller species. This was followed by a hiatus of 37 ka during which the roof entrance closed. Deposition reinitiated during the mid-Holocene, at 5.8 ka and continued at least until 0.81 ka when the roof entrance closed again. This chronology refutes earlier published ages that indicated the deposit had accumulated through the Last Glacial Maximum.

Fossils were identified by comparing them to known osteological material and published descriptions. At least 69 species of terrestrial mammals have now been identified from the deposit, 66 of which were collected during this study. Cathedral Cave thus contains the most diverse late Pleistocene mammal deposit in Australia. Species relative abundance, richness, diversity and evenness trajectories revealed a typically temperate faunal community with incursions of some xeric taxa during drier periods. The relative proportions of the temperate and xeric groups fluctuated subtly in response to drier, cooler conditions that were in effect in the middle of MIS 4, 65 ka ago. This was accompanied by reduced richness, diversity and evenness values. During MIS 3, the community underwent greater restructuring, with faunal changes consistent with drying conditions 50.6–44.7 ka ago. This interval is not associated with any identified regional climatic changes but postdates the regional arrival of humans. Faunal turnover among the small-medium bodied mammals remained low during MIS 3, with most displaying changes in their relative abundances, rather than extirpation or extinction.

By contrast, most of the 11 extinct megafaunal mammal species vanished from the record by 50–45 ka ago with just two persisting until at least 43.2 ka ago. This timeline aligns better with human mediated extinction than with known climatic changes and small mammal responses. The timing and nature of observed patterns fits better with that reported from comparable southern Australian records rather than from fossil sites above or near the Tropic of Capricorn, the latter which lack the resolution available in the southern and Cathedral Cave records.

The incongruity of response trends shown by the large and smaller bodied species suggests that each was subject to different drivers, or, that they were affected unequally by the same drivers. These findings point to the complexity that is implicit in natural systems and highlights gaps in our understanding of community responses to environmental change. This study clarifies the temporal range of nine megafaunal taxa and demonstrates for them a probable overlap with humans. For four of these taxa, Cathedral Cave provides their first, and only, dated occurrence. These data exemplify the large gaps that remain in the Australian fossil record and highlights the inadequacy of current extinction chronologies from which extinction causes are being inferred.

Keywords: palaeontology, palaeoecology, pleistocene, wellington caves, palaoeenvironment, marsupials, mammals, late quaternary, holocene, sedimentology, cathedral cave, anthropogenic impacts, climate impacts

Subject: Earth Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2021
School: College of Science and Engineering
Supervisor: Gavin Prideaux