Fossil Record

David Grimaldi

American Museum of Natural History, New York

Insects are the most evolutionary successful group of organisms in the 4-billion-year history of life on earth, with perhaps 5 million species alive today and untold millions of extinct species. Although fossils of insects are not as abundant as has been found for some other types of organisms, the insect fossil record extends back for 400 million years, making them among the oldest terrestrial animals known, and the fossils contribute unique insight into the evolutionary history of insects. Particularly significant periods in the evolution of

FIGURE 1 Various kinds of fossil insects, modes of fossilization, and degree of preservation. Different scales. (a—c) Iron hydroxide concretion of a heteropteran in Cretaceous limestone from Brazil, showing preservation of thoracic muscles [b, light micrograph; c, scanning electron micrograph (SEM)]. (d) Silvery carbon film of a belostomatid on fine-grained, Triassic shale (Virginia). (e) Nymph of tHerdina (in this article, t signifies an extinct group) in ironstone concretion from the Carboniferous of Mazon Creek, Illinois. (f) Trichopteran case of sand pebbles in volcanic shale from Florissant, Colorado (late Eocene/early Oligocene). (g) Part of the head of tabanid fly from Florissant, showing the eye facets. (h) Portion of the wing of tTypus ("Protodonata"), from the Permian of Elmo, Kansas, with the wing fluting preserved.

FIGURE 1 Various kinds of fossil insects, modes of fossilization, and degree of preservation. Different scales. (a—c) Iron hydroxide concretion of a heteropteran in Cretaceous limestone from Brazil, showing preservation of thoracic muscles [b, light micrograph; c, scanning electron micrograph (SEM)]. (d) Silvery carbon film of a belostomatid on fine-grained, Triassic shale (Virginia). (e) Nymph of tHerdina (in this article, t signifies an extinct group) in ironstone concretion from the Carboniferous of Mazon Creek, Illinois. (f) Trichopteran case of sand pebbles in volcanic shale from Florissant, Colorado (late Eocene/early Oligocene). (g) Part of the head of tabanid fly from Florissant, showing the eye facets. (h) Portion of the wing of tTypus ("Protodonata"), from the Permian of Elmo, Kansas, with the wing fluting preserved.

insects are the Paleozoic, Triassic, and Cretaceous. Key features that gave rise to their spectacular success, notably flight and complete metamorphosis, originated at least 300 and 250 million years ago (mya), respectively.

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