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The K/P (Cretaceous/Paleogene, or end-of-Mesozoic), or K/T (Cretaceous/Tertiary, as commonly denoted) extinction event may have terminated the sphecomyrmine ants, or these insects could have vanished before the event, but anatomically primitive ponerines soldiered on. During the reassembly and continued expansion of the flowering plants, which replaced much of the old gymnosperm flora worldwide, forest litter became more complex. (In all respects, especially structural but also chemical and microclimatic, the litter of angiosperm forests is much better for ant colonies than that of gymnosperms.) Insects inhabiting the soil, ground litter, and vegetation of the forests and savannahs grew correspondingly diverse and abundant. In Paleocene and Early Eocene times, the ponerines experienced an adaptive radiation, and some of the genera appearing back then have survived to the present time.

During, or perhaps more precisely, toward the end of the ponerine expansion, and probably no later than the Early Eocene, the myrmicines began their own radiation. They became formidable competitors of the ponerines for both prey and nest sites. In time, they equalled and then surpassed the ponerines in biomass and diversity. Many also added seeds and elaiosomes to their diets and, at least partly as a result of these important new sources of oils and carbohydrates, were able to expand more effectively into deserts and dry grasslands.

Most importantly, some of the myrmicines added symbioses with homopterans to their repertory: largely, scale insects and treehoppers in tropical and warm temperate vegetation, aphids in cool temperate vegetation, and mealybugs underground everywhere. Similar symbioses were contracted with the caterpillars of honeydew-secreting butterflies.

Dolichoderines and formicines also diversified, perhaps with the myrmicines but more likely later, in Early to Middle Eocene times. They were less successful than the ponerines and myrmicines in the forest litter environment, having been pre-empted there by these two groups, but more successful at creating homopteran symbioses. However, they were able to penetrate environments less available to predators, including cool-temperate climates and tropical forest canopies. Their success is reflected in their high levels of abundance in amber (especially worker specimens) and rock fossils (winged specimens), as would be expected from a preponderance in arboreal habitats.

In a phrase, the breakout of the dolichoderines and formicines, and to some extent that of the myrmicines, was due to a change in diet. This shift, in turn, was aided by the rising dominance of angiosperms over much of the land environment, an expansion that began in the Cretaceous and culminated in the Paleocene and Eocene. It was furthered by the expansion of the honeydew-producing homopterans and lepidopterans, groups also favoured by the angiosperm dominance.

Taken from Edward O. Wilson's The rise of the ants: A phylogenetic and ecological explanation, 18th March 2005

Tags: Evolution

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