An Early Jurassic conifer-dominated assemblage of the Clarence-Moreton Basin, eastern Australia

University essay from Lunds universitet/Geologiska institutionen

Abstract: During the early Mesozoic two mass-extinction events profoundly reshaped the world’s biota. The first of these took place at the end of the Triassic and affected both marine organisms and terrestrial floras. The second occurred as an oceanic anoxic event during the early Toarcian. Although the Late Triassic floral assemblages of Australia have been the focus of many studies, there have been few investigations of the recovery succession of the following Jurassic period. This study investigates an Early Jurassic terrestrial fossil assemblage from the Inverleigh quarry, Clarence-Moreton Basin, eastern Australia, deposited between these events. The Inverleigh sediments belong to the Marburg Subgroup, an entirely non-marine succession characterised by a low diversity flora. The assemblage is overwhelmingly dominated by a single conifer taxon, Allocladus helgei sp. nov, but ferns and possible lycophytes occur in low numbers. The macroflora is supplemented by an array of dispersed cuticle fragments with well preserved stomata and charcoalified wood. Other mesofossils include five megaspore species, here described under open nomenclature. Additionally, a small assemblage of clitellate annelid cocoons was recovered from the organic residues. The palaeoenvironment is interpreted as waterlogged or frequently inundated floodplain based on the dominant fine-grained lithology and the presence in the fossil assemblage of many free-sporing ferns and isoetaleans, which are dependent on moisture for reproduction. The frequent exposure to flooding would have imposed environmental stresses on the plants growing at the site. This resulted in a low diversity plant community, dominated by species adapted to moist, high-latitude conditions.

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