Defences against herbivory in Gondwanan Triassic plants

Following the end-Permian extinction event (c. 252 Ma), which saw the demise of the temperate glossopterid forest biome in Gondwana, a new flora emerged that came to be dominated by umkomasialean (corystosperm) gymnosperms through the course of the Triassic. The re-diversification of gymnosperms in the Gondwanan Triassic was matched by diversification of insects, particularly holometabolous groups (including Coleoptera, Diptera and Hymenoptera) with a broad spectrum of mouthparts and feeding strategies. The range of herbivory damage features that these animals left on plants also diversified and increased in abundance through this period. This diversification and increased intensity of arthropod herbivory in the terrestrial vegetation ought to have been matched by specialization of plant defences. We established a scale (of 1–5) for qualitatively ranking the degree of physical defences within fossil plants. Our survey of Gondwanan Triassic plant fossil assemblages revealed relatively modest levels of obvious defensive structures compared to extant plants, and no examples of individual species or genera with very high levels of multiple physical and chemical defences in the form of dense glandular trichomes, spines, extensive biomineralization, nut-like protection of seeds, or abundant resin canals. However, we observed a weak trend of increased representation of cuticular elaborations (prominent papillae and trichomes) and leaf margin cusps and spines through the course of the Triassic. Given the relatively modest physical defensive features developed by these plants, we infer that many groups produced secondary metabolites in common with extant relatives hosting an array of anti-herbivore chemical defences, but that these allelochemicals do not preserve well in the fossil record.