Systematics, biostratigraphy and biogeography of Australian Early Palaeozoic trilobites
thesisposted on 28.03.2022, 03:09 by John Richard Paterson
Early Palaeozoic (Early Cambrian to Early Ordovician) trilobites are documented from various regions of Australia, including South Australia, western New South Wales, western Queensland, Northern Territory and eastern Victoria. The recurring theme of this thesis is systematics, including taxonomy and phylogeny, although biostratigraphy, biogeography and palaeoecology also feature prominently. A summary of each section of this thesis is outlined below. Section 1 - Trilobites from the Lower Cambrian succession at Angorichina in the Flinders Ranges, South Australia are described. Silicified material from the Memmema Formation reveals the presence of a new assemblage from the Pararaia bunyerooensis Zone, including the new species Wutingaspis euryoptilos and Yunnanocephalus macromelos. Trilobites of the Pararaia bunyerooensis Zone show a strong affinity with those from the Chengjiang fauna of southwest China. Australian Early Cambrian trilobite biozonation is reviewed, with discussion of distinct assemblages within the Pararaia janeae Zone that have the potential for zonal subdivision, and evidence to support the placement of the northern Australian Ordian/Early Templetonian Stage within the late Early Cambrian. Section 2 - The taxonomy of Discomesites and Estaingia from the Lower Cambrian Cymbric Vale Formation of western New South Wales is revised. Page tides (Discomesites) fragum is considered a senior subjective synonym of P. (D.) lunatulus. Pagetides (Discomesites) spinosus from the Shackleton Limestone in the Holyoake Range, Transantarctic Mountains, is considered to be a junior subjective synonym of P. (D.) fragum. Estaingia cerastes from the Cymbric Vale Formation is considered to be synonymous with Hsuaspis cf. H. bilobata from the Shackleton Limestone. Absolute ages of recently dated tuffs from the Cymbric Vale and Billy Creek Formations are questioned, based on new information regarding the stratigraphic position of the Cymbric Vale Formation tuff in relation to archaeocyathan and trilobite biostratigraphy. The co-occurrence of Pagetides (Discomesites) fragum and Estaingia cerastes in the upper part of the Cymbric Vale Formation and in the Shackleton Limestone represents the first species-level correlation between the Lower Cambrian of Australia and Antarctica using trilobites. The distribution of these trilobite species, in association with the Syringocnema favus archaeocyathan fauna, provides supporting evidence that Australia and Antarctica were connected by a continuous carbonate-detrital shelf during the late Early Cambrian (mid-late Botoman), allowing faunal exchange between these regions. Section 3 - The family Emuellidae Pocock, 1970, was established for Emuella Pocock, 1970, and Balcoracania Pocock, 1970, from the Early Cambrian of South Australia. Based on their peculiar trunk tagmosis, emuellids have been interpreted as sister group of all other trilobites with dorsal facial sutures, and classified as high as the ordinal level. Cladistic analysis with a range of exemplar taxa of the Olenellina and Redlichiina instead resolves the emuellids within the Redlichiina, with tagmosis into a prothorax and opisthothorax (“telosoma”) non-homologous in olenellines and emuellids. A taxonomic revision of Australian species identifies Balcoracania flindersi as a junior subjective synonym of B. dailyi, whereas the two named species of Emuella are considered to be distinct. Balcoracania dailyi possesses up to 103 thoracic segments, the maximum number recorded in any trilobite. Section 4 - The taxonomy of the Cambrian trilobite family Nepeidae is revised. Morphometric analysis of the genera Nepea and Penarosa demonstrate that use of proportions of linear dimensions of the cranidium to differentiate species is invalid, and that infrageneric variation is continuous. The taxonomy and biostratigraphy of all Australian species of Nepeidae is revised. Species considered valid herein include: Nepea narinosa (type species), N. tonsillata, N. nans, Penarosa retifera (type species), P. elaticeps, P. rhinodelphis, P. netenta, Loxonepea loxophrys (type species), and Ferenepea hispida (type species). Folliceps is considered to be a junior subjective synonym of Nepea, Trinepea is considered to be a junior subjective synonym of Penarosa, and Ascionepea is considered to be a junior subjective synonym of Ferenepea. Section 5 - An early Late Cambrian (late Mindyallan-Idamean) trilobite fauna is described from limestone bodies in the Dolodrook River area, eastern Victoria. Fifteen taxa are recognised, including the new species Mindycrusta oepiki. Two trilobite assemblages are recognised: the Protemnites Assemblage and the Mindycrusta Assemblage. The Protemnites Assemblage is interpreted as representing an autochthonous assemblage inhabiting a moderate to high energy, inner shelf (peritidal) environment. The Mindycrusta Assemblage represents a parautochthonous assemblage inhabiting a low to moderate energy, open oceanfacing, outer shelf to upper slope environment. Section 6 - The genus Prosopiscus is of particular importance to palaeobiogeography due to its widespread geographic distribution in Gondwana and peri-Gondwanan regions during the Ordovician. Prosopiscus appears to have been confined to low palaeolatitudes, representing a characteristic member of the warm water eastern Gondwanan shelf faunas. Given the long stratigraphic range of the genus, trends in its distribution during the Ordovician can be observed. Prosopiscus was restricted to, and may have originated in Australia during, the late Early Ordovician (Bendigonian-Chewtonian). By the Middle Ordovician (Darriwilian), Prosopiscus had dispersed to other parts of Gondwana and peri-Gondwana. Possible explanations for the distribution of Prosopiscus are: (1) that there were no oceanic barriers preventing dispersal of trilobites between different regions of Gondwana, thus permitting Prosopiscus to migrate over vast distances uninhibited; (2) that Prosopiscus was not restricted to a specific biofacies; (3) that a major eustatic transgression during the early Darriwilian may have induced the dispersal of Prosopiscus in allowing further development and expansion of marine environments; and (4) that a prolonged planktonic larval stage may have warranted a wide dispersal. Prosopiscus lauriei sp. nov. is described from the late Early Ordovician (Bendigonian-Chewtonian) Tabita Formation at Mount Arrowsmith, northwestern New South Wales.