School of BioSciences - Theses
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ItemQuaternary diversity dynamics of Australian reptiles( 2022)Predicting the outcomes of anthropogenic impacts on ecosystems is an essential step to counteract the recent biodiversity crisis. The Quaternary fossil record offers a unique opportunity to formulate such predictions by testing how ecological communities and / or species distributions change through time, e.g., in response to the repeated and intensifying shifts in global climate during the glacial-interglacial cycles. Such paleoecological information is particularly critical for ectothermic vertebrates, such as reptiles and amphibians collectively known as herpetofauna, as these groups comprise a high number of threatened species and are particularly sensitive to changing climates. Yet, in most cases, the investigation of long-term faunal dynamics requires a morphology-based taxonomic or ecological identification of fossilized elements. For herpetofauna this has been notoriously difficult, due to a lack of comparative knowledge about the osteological variation in modern taxa, underdeveloped osteological museum collections, and the prevalence of cryptic diversity. These difficulties pose a major challenge when paleontological data are intended to inform conservation, because applied conservation measures fundamentally rely on (species-level) taxonomy (e.g., the IUCN Red List). In this thesis, I test the recognizability of herpetofaunal species in the Quaternary Australian fossil record and apply alternative methods for inferring climate-related faunal dynamics, through a combination of quantitative paleontological and neontological methods. Australia is ideal for such an analysis as the continent comprises an exceptionally high herpetofaunal diversity as well as numerous Quaternary fossil sites, preserving a relatively continuous temporal sequence of reptile and amphibian fossils. I show in Chapter 1 that faunal change can be detected at higher taxonomic levels (above the species-level) and that changes in relative abundance of different reptile subfamilies over time correspond to changing aridity throughout a fossil deposit in western Victoria. This suggests that historical baselines for evaluating the stability of modern ecosystems may be established even in the absence of species-level taxonomic resolutions. The central aspect of this thesis is addressed in Chapters 2 and 3. Using a quantitative approach based on 3D geometric morphometrics, I leverage digital morphological data (CT scans) to test how reliable individual bones of agamids (Chapter 2) and varanids (Chapter 3) can be assigned to (modern) lower-level taxonomic or ecological categories. My results show that genus- or subgenus-level as well as ecological identifications can be confidently achieved in most cases (> 90%). Thus, these categories constitute appropriate groupings for the investigation of temporal diversity dynamics. In contrast, species-level identifications were generally less reliable and sensitive to incompleteness of the bones or sample size. These results add to the long-standing question of transferability of modern species boundaries to the fossil record and imply that a comparison of modern and past (species-level) biodiversity may be prone to identification errors, at least within these groups. Finally, in Chapter 4, I integrated fossil occurrences, generated through the quantitative identification framework developed in the previous chapters, with (paleo-)species-distribution modelling, population genomics and osteological data of modern specimens to examine the decline of the threatened Mountain Dragon (Rankinia diemensis). This integrative approach revealed a strong link between Quaternary climate change and ongoing habitat loss and fragmentation in this temperate-adapted agamid lizard. My results suggest that increasing temperatures will likely force R. diemensis to further shift its distribution to higher altitudes, leading to a reduction of suitable habitat and increasing fragmentation of populations as global warming proceeds. Overall, my thesis provides new insights into the possibilities and limitations of the Quaternary Australian herpetofaunal fossil record in a conservation-paleobiological context, as well as an extensive resource of virtual morphological data and a quantitative methodological framework for future research.