HERBIVORE AND POLLINATOR EVENTS DURING THE PREANGIOSPERMOUS MESOZOIC: A CONNECTION TO GLOBAL CLIMATE?

Two major events characterize plant–insect associations during the preangiospermous Mesozoic. First was Gondwanan diversification of plant lineages and expansion of herbivory during the Middle to Late Triassic interval, documented in the Molteno Formation of South Africa. This event hosts a diverse spectrum of 79 damage types inflicted by insect herbivores, including external feeders, piercer-and-suckers, leaf miners, gallers, seed predators and ovipositing insects on a broad spectrum of horsetails, ferns, cycads, conifers, peltasperms, corystosperms, ginkgophytes and gnetaleans. Many of the herbivores were host specialists that targeted particular host-plant taxa and their tissues. Molteno herbivore associations often are local, suggesting a complex mosaic of spatiotemporally fleeting associations.

The second event was expansion of associations during the Middle Jurassic of western Eurasia, involving long-proboscid, fluid- and pollen feeding lineages of Mecoptera (aneuretopsychine scorpionflies), Neuroptera (kalligrammatid lacewings), and Diptera (brachyceran flies). Suspect plant taxa accommodating these long siphonate mouthparts were cheirolepidiaceous conifers, caytonialeans, ginkgophytes, bennettitaleans and gnetaleans providing secretory fluid and pollen rewards. This distinctive feeding type formed a gymnosperm pollination mode lasting to the mid Cretaceous, at which time angiosperm pollination originated and replaced older gymnosperm associations.

Importantly, Middle Triassic to Early Cretaceous atmospheric concentrations range from 12–19 pO₂, well below the current norm. Such low partial pressures exert distinctive responses from insects with a respiratory system of ramifying tubules originating from valvular spiracles. Also, atmospheric pCO₂ was considerably more elevated than current levels, suggesting low levels of accessible nitrogen from foliage, tissue secretions and pollen. Insect herbivores and pollinators require both O₂ levels adequate for respiratory activity and access to nitrogen sufficient for body maintenance, reproduction and activities such as flight. Both requirements may explain the expansion of herbivory and pollination as a consequence of greater access to nitrogen while maintaining safe levels of respiratory metabolism.