Fossil Perspectives on the Evolution of Insect Diversity

A key contribution of palaeontology has been the elucidation of macroevolutionary patterns and processes through deep time, with fossils providing the only direct temporal evidence of how life has responded to a variety of forces. Thus, palaeontology may provide important information on the extinction crisis facing the biosphere today, and its likely consequences.

Hexapods (insects and close relatives) comprise over 50% of described species. Explaining why this group dominates terrestrial biodiversity is a major challenge. In this thesis, I present a new dataset of hexapod fossil family ranges compiled from published literature up to the end of 2009. Between four and five hundred families have been added to the hexapod fossil record since previous compilations were published in the early 1990s. Despite this, the broad pattern of described richness through time depicted remains similar, with described richness increasing steadily through geological history and a shift in dominant taxa after the Palaeozoic. However, after detrending, described richness is not well correlated with the earlier datasets, indicating significant changes in shorter term patterns. Corrections for rock record and sampling effort change some of the patterns seen. The time series produced identify several features of the fossil record of insects as likely artefacts, such as high Carboniferous richness, a Cretaceous plateau, and a late Eocene jump in richness. Other features seem more robust, such as a Permian rise and peak, high turnover at the end of the Permian, and a late-Jurassic rise.

The growth rate of hexapod family richness appears to have significantly slowed through time, and short term increases in hexapod richness, after adjustment for sampling bias, tend to reduce origination in the following interval, consistent with density-dependent processes. Increases in plant family richness are associated with higher hexapod extinction and lower family richness. Several potential abiotic drivers are identified, though the important drivers are different before and after adjusting for sampling bias in the hexapod record. In unadjusted data, higher richness is associated with periods of low temperature, high atmospheric oxygen concentrations, and seas rich in organic nutrients, whilst after adjusting for sampling bias, high richness is associated with high sea levels, and high marine productivity.

Tests on the origination and extinction rates of subgroups of hexapods suggest that the origin of wings represented a major macroevolutionary event, which led to greater faunal turnover. The Holometabola have achieved their present high family richness not by great changes in the average rates of origination or extinction but by a subtle widening of the difference between origination and extinction relative to some other groups, and by peaks in origination at key moments in evolutionary history.