Abstract
Several dispersed reproductive organs of bennettitopsid gymnosperms are described and illustrated from Triassic to Cretaceous strata of Australia: Williamsonia eskensis sp. nov. (Middle Triassic), Williamsonia ipsvicensis sp. nov. (Upper Triassic), Williamsonia durikaiensis sp. nov. (Lower Jurassic), Williamsonia sp. (Lower Jurassic), Williamsonia rugosa sp. nov. (Middle Jurassic), Williamsonia gracilis sp. nov. (Lower Cretaceous), Cycadolepis ferrugineus sp. nov. (Lower Jurassic), Cycadolepis sp. (Lower Cretaceous), and Fredlindia moretonensis Shirley 1898 comb. nov. (Upper Triassic). Among these, W. eskensis appears to represent the oldest bennettitalean reproductive structure yet identified. Although global floras expressed less provincialism during the Mesozoic and many genera are cosmopolitan, Australian bennettopsid species appear to have been endemic based on the morphological characters of the reproductive structures. Bennettopsids have a stratigraphic range of around 210 million years in Australia and are widely and abundantly represented by leaf fossils, but only around 20 specimens of reproductive structures, of which half are attributed to Fredlindia, have been recovered from that continent’s geological archive. The extremely low representation of reproductive organs vis-à-vis foliage is interpreted to reflect a combination of physical disintegration of the seed-bearing units while attached to the host axis and, potentially, extensive vegetative reproduction in bennettopsids growing at high southern latitudes during the Mesozoic.
Highlights
Bennettitales are an extinct group of seed plants with superficially cycad-like foliage
The oldest occurrences of the order based on fossil reproductive structures are from Carnian strata of the Alps and South Africa (Crane 1985, 1988; Anderson and Anderson 2003; Pott 2014b; Pott et al 2010a, 2017), specimens documented in this study indicate an earlier origin
We compared the Australian fossils with bennettitalean fertile organs held in the collections of the Geological Survey of India (Calcutta), the Birbal Sahni Institute of Palaeobotany (Lucknow), the Swedish Museum of Natural History (Stockholm), the Shandong Tianyu Museum of Natural History (Pingyi), the Museum of Natural History (Vienna) and the Geological Survey of Austria (Vienna) in order to evaluate whether taxa were shared across large geographic distances
Summary
Bennettitales are an extinct group of seed plants with superficially cycad-like foliage. They are distinguished from Cycadales by their compact, flower-like, reproductive organs (Taylor et al 2009) and the key apomorphy of syndetocheilic (brachyparacytic) as opposed to haplocheilic stomata (Thomas and Bancroft 1913; Florin 1933). Bennettitales appeared in the early Mesozoic, their origins remain ambiguous. Pinnate leaves with oblong–linear, parallel-veined segments of cycadalean or bennettitalean aspect occur in various late Palaeozoic and Early to Middle Triassic fossil assemblages (Taylor et al 2009). Bennettitalean affinities have not been confirmed for any of these leaves by cuticular details (Pott et al 2010b). Bennettitales died out in most parts of the world by the end of the Cretaceous
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