An earliest Triassic riparian ecosystem from the Bulgo Sandstone (Sydney Basin), Australia: palynofloral evidence of a high-latitude terrestrial vertebrate habitat after the end-Permian mass extinction

Abstract

A landmark change in terrestrial ecosystems took place after the end-Permian mass extinction (EPME). This was marked by disappearance of the iconic latest Permian Glossopteris forests in Australian continental successions dated to 252.10 ± 0.06 Ma (mega-annum). Pioneering earliest Triassic spore-producing plants then spread following catastrophic wildfires, soil erosion and algal blooms. These ruderal plant communities were later replaced by vast Dicroidium ‘seed fern’ forests, which established under warm, seasonally dry conditions. By contrast, evidence of coeval vertebrate assemblages is scarce in Australia. The Bulgo Sandstone (Narrabeen Group) fossil biota in the Sydney Basin is, therefore, important as a rare example of associated plant and animal communities from the Early Triassic. The fossils include imprints and organic remains of lycopsid roots and attributed leaves (microphylls), Dicroidium and Lepidopteris foliage, and both erythrosuchid and smaller-bodied archosauromorphs, capitosaurians and other temnospondyls, actinopterygian fishes and sharks. Here, we undertake a palynofloral analysis of the vertebrate fossil-bearing bed within the Bulgo Sandstone to determine the assemblage age and palaeoenvironment. We identified a low-diversity miospore suite that is overwhelmingly dominated by lycopsids (42%), and especially quillworts (Isoëtales/Pleuromeiales) represented by Densoisporites and Aratrisporites. Bryophyte (moss), sphenopsid (horsetail) and ground fern spores are also present. Pollen (14%) comprises forms mainly associated with voltzialean conifers. The non-taeniate bisaccate Falcisporites (Alisporites) produced by Dicroidium is comparatively rare (3%). Based on this taxic composition, we correlate the vertebrate-bearing layer of the Bulgo Sandstone with the lower part of the Protohaploxypinus samoilovichii Oppel Zone and suggest a late Induan to early Olenekian age. Calculated sedimentation rates indicate that the 400 m of sediments between the top of the Vales Point Coal and the base of the Bulgo Sandstone were laid down within a time span of 40–400 ka (kilo-annum). Furthermore, the high portion of spores (37%) implies a parautochthonous organic accumulation that would have derived from local vegetation. This was likely riparian, with Dicroidium (and associated gymnosperm) cuticles (ca 20%) being over-represented because of a deciduous seasonal cycle. Notably, the Bulgo Sandstone accumulated on a high-palaeolatitude floodplain (ca 66–69°S) with dense vegetation flanking waterways that might have provided both shelter and forage for terrestrial vertebrates living less than 1 Ma after the EPME ecosystem collapse.Vivi Vajda* [[email protected]], Swedish Museum of Natural History, Svante Arrhenius väg 9, Stockholm 11414, Sweden; Benjamin P. Kear [[email protected]], The Museum of Evolution, Uppsala University, Norbyvägen 16, Uppsala 75236, Sweden.

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