Understanding the theatre of terrestrialization: Silurian-Devonian sedimentary landscapes and continental ecosystems
The colonization of subaerial and non-marine landscapes (“terrestrialization”) by animals and plants was a defining and irreversible innovation during the co-evolution of Earth and its biosphere. Its timing, estimated through ichnological evidence, suggests Cambro-Ordovician excursions were followed by Siluro-Devonian colonization of fully non-marine habitats. Our ichnological understanding of such events can sometimes be biased to particular geographic regions: for example, the known record of terrestrialization is heavily biased to ichnological and sedimentological data from Europe and North America. This does not only reflect a historical bias in study locations, but also the exposure of rocks of this age: the extensive heterolithic red-bed successions of the ‘Catskill Delta’ and ‘Old Red Sandstone’. These strata archive a huge amount of ichnological information but they may not be globally representative because they were almost unanimously deposited in basins bordering the Caledonide/Acadian mountain belts, under warm climates, near the palaeo-equator; and dominated by fine grained continental deposits, representing muddy coastal plains and alluvial floodplains. Less well studied, a suite of similar aged strata occurs across the modern continent of Australia, within a variety of palaeoenvironmental sedimentary facies. Among these, the heterolithic Grampians Group, Victoria, is arguably most akin to ‘Old Red Sandstone’-type facies, recording a range of offshore to continental depositional settings. However, other locations are dominated by coarser (sandstone-dominated) strata that are rare in Euramerican settings, including estuarine facies of the Tumblagooda Sandstone, Western Australia, and aeolian facies of the Mereenie Sandstone, Northern Territory. This thesis presents new field data comparing the sedimentology and ichnology of these Australian sections with other original data from successions in Europe and North America, to present a more global perspective to the global questions surrounding the terrestrialization process. Alongside original observations and interpretations for each of the case studies, the challenges and potential biases associated with understanding terrestrialization, and with ichnology in general, are explored. This includes the benefits and drawbacks of different ichnological techniques, the difficulty of distinguishing marine from non-marine strata, and the constraints that outcrop geomorphology inherently places on any ichnological analysis. Combining the Australian data with the Euramerican data shows that trends of Silurian-Devonian timing and in the ichnodiversity of the colonization event were globally similar. The greatest ichnodiversity occurs in marine-influenced terrestrial strata, whereas arid continental strata record lower diversity ichnoassemblages. Early terrestrial ichnofaunas appear to be dominated by traces produced by arthropods and vermiform organisms, suggesting these were key constituents of colonizing communities. Numerous motifs and recurring signatures are apparent across the case studies, with several key traces occurring almost universally, and a preponderance of very large traces. Through comparison with a larger secondary dataset, the case studies provide a similar, but more detailed, signature of the terrestrialization process, suggesting that they provide a reliable impression of global events as a whole.
trace fossil, ichnology, marine, non-marine, sedimentology, palaeoecology, Palaeozoic, Tumblagooda, Gondwana
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