Lepidopteris callipteroides, an earliest Triassic seed fern of the Sydney Basin, southeastern Australia

Abstract

Earliest Triassic shales in the Coal Cliff Sandstone, Caley Formation, Widden Brook Conglomerate and Dooralong Shale (all basal Narrabeen Group) of the Sydney Basin contain a low diversity fossil flora that survived the greatest mass extinction of all time at the Permian-Triassic boundary. Only one species of seed fern is known from this flora and its affinities were unclear until discovery of its reproductive organs and complete large leaves. An ovuliferous reproductive organ, Peltaspermum townrovii sp. nov., can be attributed to the same plant as the leaves because of their identical stomatal apparatus, which is cyclocytic with papillae overhanging the stomatal pit. Polleniferous organs, Permotheca helbyi sp. nov., may have belonged to the same plant, but are only linked by evidence of association on the same bedding plane yielding no other gymnosperms. Pollen masses found within the polleniferous organ include grains identified as Falcisporites australis (de Jersey) Stevens (1981) when found dispersed. The leaves of this plant have long been enigmatic and attributed to ‘Thinnfeldia’ callipteroides or ‘Dicroidium’ callipteroides; however, these genera had very different cuticular structure. Reassessment of the frond architecture of this plant, based on a large, nearcomplete specimen together with information from cuticles and ovuliferous organs, allows reassignment to Lepidopteris callipteroides (Carpentier) comb. nov. The remarkable cuticle thickness, small stomatal size and low stomatal index of these leaves reflect a time of unusually high atmospheric concentrations of carbon dioxide. This plant was an invader of the Sydney Basin from northern Gondwana, spreading southward during the post-apocalyptic earliest Triassic greenhouse.