Ecosystem recovery after the end-Permian event, Sydney Basin, Australia: Diversity and ecological interactions of the Early Triassic Dicroidium floras

The end-Permian event (EPE; c. 252 Ma) resulted in the loss of keystone plant species from humid tropical and high-latitude ecosystems and the extinction of several major insect groups. The subsequent Early to Middle Triassic evinced diminished terrestrial productivity, punctuated by a series of second-order biotic crises that hindered recovery. End-Permian ecosystem collapse resulted in the extirpation of productive wetland ecosystems, the primary carbon sinks on land, represented by the cessation of significant coal formation until the Middle Triassic. The gymnosperm seed fern Dicroidium (Order: Umkomasiales) emerged as the dominant floral component of most known terrestrial ecosystems of the Early Triassic across southern Gondwana and, by the Middle Triassic, was the principal coal-forming plant. Understanding when and how this ecologically important taxon rose to dominance will provide a gauge of ecosystem recovery and carbon sink stabilisation in Gondwana following the worst mass extinction event in Earth’s history.

While there have been many large-scale investigations into Middle Triassic plants and biodiversity, the Early Triassic interval of ecological recovery immediately following the EPE is poorly studied. In addition to examination of the fossil plants themselves, trace fossils of plant–arthropod interactions (PAIs) provide an independent window into assessing terrestrial ecosystem states through geological time. In this context, PAI records can be used for evaluating changes in herbivorous arthropod feeding guilds in the wake of global biotic crises. Here, we investigated three well-preserved early records of Dicroidium from the well-age constrained Lower Triassic strata of the Sydney Basin, Australia (the Skillion, Turimetta Head and Mona Vale). In this study, we: 1, systematically described the Dicroidium species from these localities; 2, interpreted their palaeoenvironmental contexts; 3, compared their diversity and morphological trends over time; and 4, recorded evidence of PAIs.

The floras exhibited a generally low species richness of Dicroidium overall, but an increase in richness and leaf size with increasing time from the EPE. Similarly, Dicroidium leaf fragments from each locality revealed evidence of PAIs (including margin feeding, hole feeding, galling, and oviposition), with the highest proportion of PAIs from the youngest locality. Increasing numbers of PAIs on the dominant plant genus in Gondwanan ecosystems indicate that foundational trophic interactions between plants and arthropods were slowly re-establishing in the early Mesozoic. Given the broadly similar depositional conditions, these changes cannot readily be attributed to differences in local environments. Collectively, our findings evidence the recovery of terrestrial ecosystems and carbon sinks over several millions of years following the worst warming-driven mass extinction in Earth’s history.