Early land plants of Victoria: new taxa, taxonomic revisions and insights into plant evolution

<p>Some of the world's earliest land plants are reputedly from the Silurian-Devonian of Victoria. The adaptations that these plants underwent while colonising the land are partly reflected in the morphology of their fossil remains. It has been over 35 years since research has been done on the Silurian-Devonian flora of Victoria. In the meantime significant progress in describing new taxa has been made in contemporaneous deposits globally, id est South America and China. This has led to the belief that the Australian flora was much less diverse than South China, and was part of the northeastern Gondwana phytogeographic unit. However, in comparison to other contemporaneous sites, work on the Victorian flora is in its infancy and would therefore in comparison appear to have been at a lower level of diversity. There are significant gaps in our knowledge relating to Victoria's flora, such as the nature of Victorian zosterophylls: are the zosterophylls the dominant group?; do they all belong to Zosterophyllum?; are they all radially arranged and are there any novel characters evident? Additionally, is the Victorian flora highly endemic or is there evidence of morphological traits seen in the Laurussia or the South China flora suggesting floristic exchange. Furthermore, we have no evidence of progenitors for the "Lower Plant Assemblage" of the Yea flora, which stands out as an anomaly in comparison to other late Silurian flora, due to its advanced architecture and size.</p> <p>This research provides evidence of new taphofloras, presents taxonomic revisions and examines the relationship of the taphofloras with other coeval floras worldwide, especially from the South China Plate, based on specimens collected during the course of the study and by utilising the comprehensive collections held at Museums Victoria. The research is presented as a series of published papers and unpublished chapters. The research argues that there is currently not enough evidence to consider Victoria part of the "northeastern Gondwana phytogeographic unit" as proposed by Hao & Gensel in 1998, but instead some floristic exchange was likely (Chapters 5, 7). This is done first by examining the most diverse group known to occur in both Victoria and the South China Plate, the zosterophylls (Chapters 3, 4). The positioning of Victoria close to the equator and hence close to South China versus more recent research suggesting Victoria was close to 30°south is examined (Chapter 3). The occurrence of rhyniophytes (Chapter 6), which likely required a seasonal environment, supports the latter interpretation and highlights a noticeable difference with the coeval South China flora. Palaeocurrent data (Chapters 3, 8), which is found throughout the depositional succession in the Melbourne Zone is used to show possible terrestrial sources. These terrestrial sources are predominantly from western and southwestern Victoria up until the Emsian, at which time these are complemented by an easterly source (Chapter 8) - which suggests a terrestrial source was present in eastern Victoria earlier than previously thought. Furthermore, examination of the taphoflora at Alexandra (Chapter 8) found a diverse assemblage, including the first record of Cooksonia Lang, 1937 and the first evidence of sporangia with emergences.</p> <p>Taxonomic revisions are proposed for the Victorian Salopella Edwards & Richardson, 1974 (Chapter 6), with a new species being proposed, and Hedeia Cookson, 1935, is proposed as a junior synonym of Yarravia Lang & Cookson, 1935 (Chapter 7). Finally, an examination of the Yea stratigraphy finds no evidence of strata of early Ludlow age (late Silurian), with a Pragian-Emsian age being more realistic (Chapter 9). Evidence is presented showing there to be no evolutionary change between Baragwanathia Lang & Cookson, 1935 of the "Lower Plant Assemblage" and the "Upper Plant Assemblage". Collectively, the findings from this thesis demonstrate Victoria's flora was far more diverse than had previously been thought.</p> <p>Furthermore, floristic exchanges occurred almost equally with both the Laurussian and South China floras. Significantly, evidence for a late Silurian age for the "Lower Plant Assemblage" Yea flora is not supported, and this will impact our understanding of global diversity dynamics for the late Silurian and when phenotypic characters first arose. Future work should focus on Baragwanathia and determining its relationship to other Baragwanathia species, and investigations of the Boola Formation flora for internal anatomy.</p>