THE CO-RADIATIONS OFPOLLINATING INSECTS ANDANGIOSPERMS IN THECRETACEOUS'David Grimaldi2ABSTRACT The origins of many groups of flower-visiting insects are generally believed to have been in the Cretaceous. However,a recent hypothesis has concluded that many modern familles of insects originated in the Jurassic, and that theCretaceous radiation of angiosperms had little positive effect on the diversity of insect families. It is shown here, basedon critical and phylogenetic interpretation of Mesozoic fossils, that radiations of major anthophilic groups of insectstook place in the late part of the Lower Cretaceous to Upper Cretaceous: the bees (Apoidea/Apidae sensu lato), pollenwasps (Vespidae: Masarinae), various familles of brachyceran flies (Acroceridae, Apioceridae, Bombyliidae, Empididae,Nemestrinidae, Stratiomyidae, and Syrphidae), and the Lepidoptera. The pattern of diversification of these insects,centered in the mid-Cretaceous, is consistent with the chronology of appearance of entomophilous syndromes in Cre-taceous flowers, and not with a model of late Jurassic or earliest Cretaceous diversification of pollinating insects. Despitea more refined understanding of the timing of Cretaceous insect-angiosperm co-radiations, cause and effect relationshipsremain obscure. Working on a group of organisms with perhaps 5million species, entomologists are not easily im-pressed by groups other than insects. Nonetheless,even they admit to the central role that the angio-sperms have in terrestrial communities. Based sim-ply on the dazzling array of colors, patterns, andmorphologies of flowers specialized for attractinginsects, it is reasonable to estimate that at leasttwo-thirds of the 250,000-300,000 living angio-sperm species are insect pollinated. On this basisalone insects would be the most ecologically im-portant group of terrestrial animais, without eventaking into consideration their other ecologicalroles. The intimate and obligate associations thathave evolved between thousands of species of an-giosperms and insects are among the most signifi-cant mutualistic relationships to occur among ailorganisms. Understanding the origins of this rela-tionship is, thus, hardly a trivial consideration. Insects feeding on, or from, the reproductivestructures of plants is an ancient habit, probablybeginning in the Carboniferous with the Paleodic-tyopteroidea (Taylor & Scott, 1983; Labandeira,1998). This assemblage of extinct insect orders hadsucking mouthparts, presumably used for obtainingplant fluids or reaching into small spaces, such asthe sporangia of Carboniferous medullosan pteri-dosperms. Indeed, the very large pollen (to 600 p.mdiam.) of some of these plants is thought to haveprecluded wind pollination, and they may havebeen pollinated by paleodictyopterans. It was theColeoptera, though, that clearly set the stage forpollination of the early seed plants, probably be-ginning as early as the debut of beetles in thePermian (reviewed in Carpenter, 1992). Evidencethat Mesozoic beetles were significant pollinators islargely circumstantial, and is based on the fact thatvarious kinds of beetles today are facultative, andsome even obligate, visitors to flowers of general-ized morphology and exposed floral rewards (Arm-strong & Irvine, 1990; Dafni et al., 1990; Gazit etal., 1982; Proctor et al., 1996). For example, En-' Thanks are due to colleagues at the Paleontological Institute of the Russian Academy of Sciences who providedinformation on published and unpublished records: Vladimir Zherikhin, Miskhail Mostovsky, and Alexandr Rasnitsyn.John Wenzel (Ohio State Univ.) provided advice on fossil aculeate nests: Michael Engel (Cornell Univ.) providedinformation and advice on fossil becs; and Tarn Nguyen (AMNH Entomology) did the graphics. The research by DavidYeates (Univ. Queensland)-much of it donc as a postdoc at the AMNII fromn 1990 to 1992-allowed much of thediscussion here on bombyliid flies. The original manuscript was vastly improved by comments and advice from WilliamCrepet, Else Marie Friis, Niels Kristensen, and Conrad Labandeira, with especially detailed commentary by PeterCrane. I am very appreciative to the Paleontological Institute, Moscow, for permission to reproduce figures from theirpublications: and to Agriculture and AgriFood Canada for publication of figures from vols. 1 and 2 of the "Manual ofNearctic Diptera," reproduced with the permission of the Minister of Public Works and Government Services Canada,1998. 2 Department of Entomology, American Museum of Natural History, New York, New York 10024-5192, [email protected] ANN. MISSOURI BOT. GARD. 86: 373-406. 1999.