Fossils

Fossils are generally, but not necessarily, extinct species whose remains have been preserved for thousands to millions of years. The remains are most commonly mineralized replacements of original tissues (Figs. 1 and 2); in rare situations portions of the original organism are preserved (Figs. 2b—2e, 2g, and 5d—5f). Remains of apparently existing (or extant) species that are thousands to several million years old are sometimes called subfossils. Earth's fossils are dominated by organisms from marine continental shelves, where deep sediments efficiently preserved durable calcified parts such as shells and skeletons. Terrestrial life is less well known in the fossil record and largely is preserved as bones, leaves, and pollen in freshwater sediments. Well-preserved insects, by comparison, are rare; their occurrence depends on conditions under which

FIGURE 2 Exceptional preservation of fossil terrestrial arthropods. (a) Centipede from Upper Devonian (New York). (Courtesy of William Shear, Hampton—Sydney College.) (b) Mycetobia woodgnat in Miocene amber (Dominican Republic), with parasitic nematodes bursting from abdomen. (c-e) Flight muscles of meliponine bee in Dominican amber, showing ultrastructural preservation of myofibrils (d) and even the fingerprint-like mitochondria (e). (f) Silicified replicas of early instar dytiscoid beetle, from Miocene of California (inset, photomicrograph; SEM is larger). (g) Cuticular remains of extant beetle species from the Wisconsin stage (ca. 10,000-80,000 years ago) of Alaska (left, weevil head; right, carabid elytron). (Courtesy of Scott Elias, University of Colorado.)

FIGURE 2 Exceptional preservation of fossil terrestrial arthropods. (a) Centipede from Upper Devonian (New York). (Courtesy of William Shear, Hampton—Sydney College.) (b) Mycetobia woodgnat in Miocene amber (Dominican Republic), with parasitic nematodes bursting from abdomen. (c-e) Flight muscles of meliponine bee in Dominican amber, showing ultrastructural preservation of myofibrils (d) and even the fingerprint-like mitochondria (e). (f) Silicified replicas of early instar dytiscoid beetle, from Miocene of California (inset, photomicrograph; SEM is larger). (g) Cuticular remains of extant beetle species from the Wisconsin stage (ca. 10,000-80,000 years ago) of Alaska (left, weevil head; right, carabid elytron). (Courtesy of Scott Elias, University of Colorado.)

the sediments were fine grained, anoxic (i.e., lacking oxygen), and deposited rapidly but without significant disturbance. Even amber is usually deposited in lacustrine or swampy sediments; it would otherwise completely decompose from oxidation and other processes. Insects preserved in such sediments were aquatic or semiaquatic and died in situ (autochthonous), or their bodies were transported via winds or water from surrounding habitats (allochthonous).

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