Evidence of plant-insect interactions in the upper triassic Molteno formation of South Africa

The Molteno Formation of Late Triassic (Carnian) age in South Africa has yielded more than 200 plant species from 100 plant assemblages at 69 localities (30000 catalogued slabs) as well as more than 300 species of insect. Damage to leaves caused by insects is widespread on many plant species at numerous localities. The damage includes feeding traces, predominantly continuous marginal feeding traces, leaf mines including linear and possible blotch varieties and probable leaf galls. Damaged taxa include a wide variety of gymnosperms including the conifer Heidiphyllum, the ginkgoaleans Ginkgo and Sphenobaiera, the peltasperms Dicroidium and Dejerseya, the pentoxyalean Taeniopteris and the gnetopsid Yabeiella. Quantitative data (>3000 specimens examined) on the insect damage were obtained for four sites: Aasvoelberg, Birds River, Kapokkraal and Waldeck. Quantitative data indicate that leaf damage between sites varies from 3 to 25% and within species from 1 to 50%. The variation in damage to the same taxon between sites, and even in the overall folivory seen at the four sites, makes the interpretation of the general levels of herbivory in the Molteno difficult to assess. Herbivory levels in Northern Hemisphere Triassic plant assemblages appear to be significantly lower than those in the Molteno, and succeeding Jurassic floras worldwide also show a low level of plant-insect interactions. It is not yet clear to what extent taphonomic bias may influence the calculation of overall herbivory levels for a given time period.