LEAF GALLING: A KEY BEHAVIORAL ADVANCE AMONG INSECTS OF THE LATE PALEOZOIC

Galls are complex, specialized plant‒animal interactions that often involve a microbial symbiont. Currently, insects belonging to many lineages induce galls. Though modern leaf galls overwhelmingly occur on angiosperm leaves, their origin during the late Paleozoic, long before the rise of flowering plants, is supported by three lines of evidence: insect damage on fossil leaves, insect body fossils, and global paleoclimate trends. These lines of evidence involve the Permian Period, which was a time of increased aridity, particularly at latitudes geographically peripheral to the paleoequator. Recently, numerous types of fossil galls have been found on leaves in several 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 in a variety of habitats during the Kungurian Stage. Insects from several lineages became smaller during the Permian, and small size is the principal prerequisite for gall induction of a plant host by 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. Though these lineages are long-extinct, their relatives and descendants have galled broadleaf foliage to the present day. Crucial toward understanding this replacement is that gall-inducers invest less energy in feeding and protecting themselves, and therefore devote more energy to reproduction. Consequently, the evolution of gall induction may have been a key development in the transition between the only interval of time wherein the two taxonomically distinctive global assemblages of insects overlapped. The transition from the Paleozoic to the Modern Insect Fauna was ultimately caused by the Permian-Triassic Extinction, but it may have been set in motion by the evolution of novel galling behavior during the earlier Permian.