An early Eocene near-polar flora from eastern Gondwana (Tasmania, Australia) — systematics, adaptations and palaeobiogeographic implications of the non-flowering plants
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Date
2024
Authors
Slodownik, Miriam Andrea
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Hill, Robert S.
Goodfellow, John
Goodfellow, John
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Thesis
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Abstract
The Macquarie Harbour Formation (MHF) of Tasmania, Australia, contains well-preserved fossil plants from a subpolar (~65°S) lowland forest that grew approximately 53–50 million years ago during the Early Eocene Climatic Optimum. This assemblage represents one of the oldest and southernmost post-Cretaceous macrofloras in Australia. These factors render the MHF assemblage as an ideal resource for studying high latitude forests and the origin and evolution of Australia’s floras, which were profoundly influenced by decreasing global temperatures and continental northward drift since the early Eocene. In this thesis, I examined the non-flowering plants of the Macquarie Harbour flora and described them systematically to provide a comprehensive overview of their diversity. Furthermore, I discussed their ecophysiological strategies to life at high latitudes, and their biogeography. This work was based on existing fossil collections from several outcrops, in addition to 419 new specimens that I collected and prepared. I revealed anatomical details using advanced imaging techniques such as scanning electron microscopy (SEM), ultraviolet macro- and fluorescence micro-photography, and X-ray tomography. I systematically described, revised, and phylogenetically analysed the new and existing taxa, and derived biogeographic data by conducting a comprehensive review of the published literature. In the first chapter, I outlined the geological and palaeoenvironmental background for the Macquarie Harbour floras. I summarised the stratigraphy of the Macquarie Harbour Formation and my geological field observations. Furthermore, I outlined the general biogeographic trends of the Southern Hemisphere in the Cenozoic and introduced the concepts of south polar forests and polar refugia. In the second chapter, I have presented new fossils of Araucarioides linearis (Araucariaceae), including leaves and the first reproductive organs of the genus. I revised the generic and specific diagnoses, and designated A. sinuosa as a junior synonym. Phylogenetic analyses revealed that Araucarioides forms the sister to the ‘agathioid clade’, which comprises extant Agathis and Wollemia. This result, in combination with presence of Araucarioides in the Late Cretaceous, provides solid evidence for a pre-Cenozoic divergence age of the agathioid from the Araucaria clade. Araucarioides was restricted to the high palaeolatitudes and showed morphological adaptations to the pronounced seasonality. These factors may have also facilitated its survival during the end-Cretaceous mass extinction (~66 Ma). Its extinction was likely linked to the changes in light regime and climate, resulting from the northward movement of Australia and New Zealand. Next, I focused on the fossil record of Komlopteris in the Southern Hemisphere. I discovered abundant new fossils of Komlopteris cenozoicus. These anatomically preserved fossils confirmand the affiliation of the genus with the extinct gymnosperm order Umkomasiales and thus confirms that these fossils represent the youngest pteridosperms (“seed ferns”) to date. Furthermore, a literature review and study of museum collections revealed ten further Komlopteris species of Jurassic to Eocene age from across Gondwana. The lineage, therefore, survived the end-Triassic and end-Cretaceous biotic crises, probably in climatically buffered habitats at high latitudes. Komlopteris’ ultimate demise was linked to the ongoing diversification of angiosperms and the northward migration of Australia. In the fourth chapter, I have provided an overview of all non-flowering plants from the early Eocene Macquarie Harbour Formation. I confirmed the presence of at least twelve distinct species, encompassing nine conifers (four Araucariaceae, four Podocarpaceae, and one Cupressaceae), a cycad (Zamiaceae), a pteridosperm (Umkomasiaceae), and a fern (Schizaeaceae). A reconstruction of leaf shapes revealed different strategies for the optimisation of light harvesting among the identified taxa. The co-existence typical canopy forming and canopy emergent trees, and the presence of understory elements (such as cycads and ferns) suggests a complex forest structure. Whole flora comparisons with contemporaneous assemblages revealed a striking floristic similarity to the Patagonian floras. This suggests that a near-continuous circum-Antarctic phytogeographic zone of ever-wet rainforests may have existed during some time of the early Cenozoic hothouse period. Lastly, I summarised the results of the data chapters under the overarching goals of this thesis: 1, diversity and abundance; 2, adaptations to life at high latitudes; and 3, palaeobiogeography of the Macquarie Harbour flora.
School/Discipline
School of Biological Sciences
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Biological Sciences, 2024
Provenance
This thesis is currently under embargo and not available.