Cretaceous 14565 mya

Until about 30 years ago, the Cretaceous (145-65 mya) was one of the poorest known geological periods for insects; now it is one of the best known. At least 25 major deposits of Cretaceous insects occur around the world (and at least as many less significant ones), but the Northern Hemisphere (with 97 Cretaceous deposits) has nearly four times as many known deposits as the Southern Hemisphere (23 deposits: Eskov, 2002). The Cretaceous is of exceptional biological significance for five main reasons, which we will discuss in greater detail later in this book.

1. This is the period when the origin and radiation of the angiosperms took place (approximately Hauterivian/ Barremian to Turonian: 135-90 mya). Angiosperms are the predominant life form on land, the diversity of which defines biomes from Arctic tundra to tropical forests. Because insects are intimately associated with plants, especially as pollinators and phytophages of angiosperms, the radiation of angiosperms appears to be both a cause for and an effect of contemporaneous radiations of insects.

2. The Cretaceous is when most of the Recent families of insects first appeared, many of which are probably associated with the angiosperm radiations, though not all. There are large groups, though, that largely radiated in the Cenozoic, like the families of ditrysian Lepidoptera and schizophoran Diptera. It is during the Cretaceous that there first appeared ants, termites, and vespid wasps - the three main groups of insects with advanced sociality.

3. The fragmentation of most of the land masses into present-day continents (though not their modern configurations) occurred in the Cretaceous. By the Early Cretaceous Pangaea had already separated into the northern (Laurasia) and southern continents (Gondwana), and further fragmentation of Gondwana, took place in the later part of the Early Cretaceous, approximately 115-110 mya. This fragmentation dramatically affected global climate; during the Early Cretaceous it was very hot (particularly in the dry interiors of equatorial regions) but became more temperate and even seasonal in the Late Cretaceous.

4. The very end of the Cretaceous is marked by the most famous mass extinction event in earth history (though hardly the largest), since this is when the nonavian dinosaurs, ammonites, and rudist bivalves and some other marine life became extinct. It has been thoroughly established that a large meteorite crashed near the Yucatan Peninsula at or just before 65 mya, and that it had global effects. What is not perfectly established is the effect of this catastrophe on the extinctions of dinosaurs and other organisms because many of these groups were in decline before 65 mya. The end-Cretaceous, or K/T, extinctions appear to have had minor or only regional effects on insects, though there is some controversy about this.

5. During the Cretaceous, amber appeared in abundance as a remarkable preservation medium. Nodules of fossilized resin, or amber, are known since the Triassic, but they are small and scattered until the Cretaceous. For reasons that are not entirely clear, large quantities and globules of amber appear in the Early Cretaceous, ca. 130 mya, formations of which then continue through the Tertiary. There are nine major Cretaceous deposits of amber that yield diverse insect inclusions. These deposits may be attributed to the origin and spread of certain resin-producing conifers in the Cretaceous. The family Araucariaceae (kauri, Agathis, etc.) is routinely implicated as the source of all or most Cretaceous ambers by a few workers, but for only a few deposits (e.g., Alava amber: Alonso et al., 2000) is the evidence compelling. Taxodiaceae (redwoods and cedars) (Grimaldi et al., 2000a,b), the extinct family Cheirolepidiaceae (Azar, 2000), and other families probably produced many Cretaceous ambers. Alternatively, and perhaps additionally, large

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