DOES PALEOCLILMATE ACCOUNT FOR THE DIVERSIFICATION OF PLANTS AND INSECT HERBIVORES DURING THE LATE TRIASSIC IN SOUTH AFRICA?

An impressive adaptive radiation of herbivorous insects occurred on a gymnosperm-dominated flora during the Late Triassic of South Africa. This event is penecontemporaneous with similar events in Australia, U.S. Southwest and Western Europe, all which exhibit parallel expansions of generalized and mostly specialized insect herbivory on plants, expressed as insect damage on a various plant organs and tissues. The Molteno Formation of the Karoo Basin in South Africa is the best documented and most diverse Late Triassic biota, consisting of 106 localities and supplying extensive evidence for insect herbivore colonization of seven distinct habitats. Within each habitat is preferential targeting of particular plant hosts, assessed both as whole plant taxa and as unaffiliated organs, resulting in specific and complex herbivory patterns involving the frequency and diversity of 79 distinctive damage types (DTs) on about 220 whole-plant species. These DT patterns show that external foliage feeders, piercer-and-suckers, leaf miners, gallers, seed predators, borers and oviposition culprits were intricately using almost all tissue types from the dominant host plants of liverworts, lycopods, horsetails, ferns, cycads, peltasperms, corystosperms, hamshawvialeans, ginkgoaleans, voltzialean conifers, bennettitaleans and gnetophytes.

What explains the timing and intensity of this spectacular ecological expansion of herbivory around 228 Ma, probably lasting a few m.y., about 90 m.y. earlier than the earliest angiosperms? One possibility involves consideration of atmospheric O₂ and CO₂ curves during this interval. Early and Middle Triassic times experienced a dramatic decrease of O₂ to 15 % of the present atmospheric level (PAL) of 21 %, increasing to 18 % PAL by earliest Late Triassic. Simultaneously, earliest Late Triassic CO₂ levels were rising from a low of 2 % of PAL, elevated several-fold compared to the current level of 0.4 %. This combination of a rapidly rising O₂ level promoting arthropod respiration and growth was contrasted to rising CO₂ levels allowing increased herbivory of plants as nitrogen became nutritionally limiting. Both rising O₂ and CO₂ levels, beyond minimal thresholds, provided a “perfect storm” for an herbivore diversification event paralleling that of the angiosperms.