The origin of leaf galling in the permian, and the macroevolutionary implications thereof

Though modern leaf galls overwhelmingly occur on angiosperm leaves, their origin during the Permian Period (299 - 252 mya), long before the rise of angiosperms, is supported by three lines of evidence: insect damage on fossil leaves, insect body fossils, and global paleoclimate trends. The Permian was a time of increased aridity, particularly at latitudes immediately peripheral to the paleoequator. Recently, numerous types of fossil galls have been found on leaves in Euramerican and Gondwanan deposits. Two floras from the Early Permian of Texas, Colwell Creek Pond and Mitchell Creek Flats, contain new types of galls and demonstrate that gall induction on leaves became widespread during the Kungurian Stage in a variety of habitats. Insects from several lineages became smaller during the Permian; small size is the principal prerequisite for gall induction in an arthropod. Interestingly, all potential gall-inducers of this time interval were members of the Modern insect fauna, which incrementally replaced the Paleozoic insect fauna throughout the Permian. Because gall-inducers invest less energy in feeding and protecting themselves, and therefore devote more energy to reproduction, the evolution of gall induction may have been a key development in the transition between the only two taxonomically distinctive global assemblages of insects in deep time.