INSECT DIVERSITY BEFORE THE ORIGIN OF FLOWERING PLANTS

Wings and holometaboly (complete metamorphosis, such as a maggot's transformation into a fly) are the two key innovations that facilitated the tremendous diversification of insects, and these both appear in rapid succession in the fossil record of the Pennsylvanian Period. By the Permian, true bugs and holometabolous insects had diversified tremendously, such that the higher-level taxonomic distribution of insect diversity already began to resemble what we see today. In contrast, flowering plants—which account for over 90% of plant diversity in modern ecosystems—were extremely rare or entirely absent on terrestrial landscapes until the mid-Cretaceous. Therefore, for over half of their evolutionary history, insects were fundamentally modern in a way that plants were not.

Despite the lack of flowering plants from the Permian through mid-Cretaceous, insects' taxonomic and behavioral diversity increased during this interval. This presentation will cover two aspects of this diversification: the prevalence of specialized insect feeding behaviors that are currently associated with flowering plants, and the number of taxonomic families present on the landscape.

Galls are tumor-like plant growths that protect insects from desiccation, fire, predators, and parasites. Although leaf galls are almost exclusively known from flowering plants in modern ecosystems, three lines of evidence (climate, insect body size, and direct evidence from fossilized leaves) indicate that insects induced galls on the leaves of seed plants during the Paleozoic. The trajectory of insect evolution, rather than plant evolution, appears to be the primary determinant of the origin of this behavior.

The quality of the insect fossil record can vary tremendously from one interval to the next, complicating the task of reconstructing insects' taxonomic diversity in deep time. The capture–mark–recapture technique for reconstructing diversity curves is particularly well-suited to the insect fossil record, and is in agreement with the shareholder quorum subsampling technique in reconstructing a peak in family-level insect diversity millions of years before the diversification of flowering plants. This diversification is better attributed to the selective pressures exerted by parasitoid insects than to any phenomena in plant evolution.