Biodiversity, biofacies and biogeography of middle Cambrian (Series 3) arthropods (Trilobita and Agnostida) on the East Gondwana margin

• We analyse Cambrian Series 3 trilobites and agnostids from the East Gondwana margin.
• East Gondwana diversity increases during Cambrian Series 3 and peaks in the Guzhangian.
• Major diversifications most likely correspond to eustatic transgressive phases.
• Outer shelf faunas on the margin reflect deep-water biofacies worldwide.
• East Gondwanan faunas have strongest biogeographic links with China and the Himalaya.

Biomineralised arthropods, notably trilobites and agnostids, represent some of the most abundant and diverse constituents of Cambrian shelly faunas worldwide, and have proven to be excellent tools for biostratigraphy, correlation and biogeography. This study presents the first quantitative biogeographic and diversity analyses of middle Cambrian (Series 3) trilobites and agnostids from the palaeoequatorial East Gondwana margin, comprising mainland Australia, Tasmania, New Zealand and eastern Antarctica. Analysis of 224 genera of trilobites and agnostids from 78 fossil sites across three Cambrian Stage 3 time intervals (Stage 5, Drumian, Guzhangian) is presented. Results of the Stage 5 analyses reveal a major grouping of faunas from the Arafura, Georgina and Warburton basins, plus the Gnalta Shelf in New South Wales, typically represented by deep-water, outer shelf assemblages that commonly contain agnostids, oryctocephalids, Pagetia and Xystridura. Faunal exchange between these depocentres was permitted by a transgression that was associated with tectonically induced subsidence of basement blocks in the Georgina Basin, such as the Mt Isa block, during this stage. Drumian faunas are represented by three distinct site groupings: Group D1 is represented exclusively by several Georgina Basin assemblages that occur in shallow marine (intertidal to subtidal) settings, including the common trilobite genera Asthenopsis, Chondranomocare, Fuchouia and Penarosa, plus a range of agnostids, that inhabited an epeiric sea with connections to the open ocean; Groups D2 and D3 are represented by sites along the entire margin, from northern Australia to the Transantarctic Mountains that characterise a range of shallow to deep marine palaeoenvironments. These complex Drumian groupings most likely reflect long-range faunal exchange along the margin permitted by the eustatic transgression taking place at this time, which particularly influenced the distribution of eurytopic agnostid species that are common in these faunas. Two Drumian faunas from the Hodge Slate and Que River Beds in Tasmania (part of Group D3) exemplify a unique biofacies occurring in a deep-water, outer shelf setting, possibly in the lower photic zone, with the former assemblage containing the blind trilobites Meneviella and Holocephalina, and the latter containing only agnostids. Results of the Guzhangian analyses show four obvious faunal groupings, with Groups G1 and G2 being the largest and representing sites along the entire margin, while Group G3 is restricted to some of the Warburton Basin, Tasmanian and Antarctic sites, and Group G4 comprises only Tasmanian sites. Groups G1 and G2 correspond to Boomerangian and Mindyallan faunas, respectively, representing two temporally separated biofacies situated on the outer shelf to slope: Group G1 assemblages typically contain the trilobites Acontheus, Amphoton, Centropleura, Dorypyge, Fuchouia, Huzhuia, Pianaspis and Solenoparia; and Group G2 faunas often contain the trilobites Blackwelderia, Genevievella, Liostracina, Meteoraspis, Metopotropis, Mindycrusta, Palaeadotes, Rhyssometopus and Townleyella; with both groups containing a considerable number of eurytopic agnostid species. Group G4 assemblages also inhabit outer shelf settings, but have lower diversity, with common taxa including the trilobite Nepea and the agnostids Clavagnostus, Oidalagnostus and Valenagnostus. The considerable number of cosmopolitan Guzhangian agnostid species in association with distinct deep-water trilobite-agnostid assemblages along the entire East Gondwana margin strongly reflects the eustatic transgressive event that reached its pinnacle during this stage of Cambrian Series 3 that allowed for greater faunal exchange between areas on the margin and other palaeocontinents and terranes. The East Gondwana margin represents a biodiversity ‘hot spot’ during Cambrian Series 3, containing almost one-quarter (~ 23%) of the trilobite and agnostid genera known worldwide. Our data support previous interpretations that Cambrian Series 3 trilobites and agnostid faunas from the East Gondwana margin, particularly those from Australia and Antarctica, have strong biogeographic links with those from Chinese terranes (especially North and South China), the Himalaya, and to a lesser extent, Iran, Kazakhstan, Laurentia and Siberia. Our data also reveal an overall increase in generic diversity throughout Cambrian Series 3, reaching a peak in the Guzhangian, with major diversifications most likely corresponding to eustatic transgressive phases, particularly in the Drumian and Guzhangian. This diversity trend for the East Gondwana margin closely matches that observed for contemporaneous faunas in other parts of the world, especially in China, Kazakhstan and West Gondwana, although diversity in the latter region reaches an acme in Drumian times.

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