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2.37. The human louse, Pediculus humanus, found on a 1,000-year-old human mummy from Peru (cf. Figure 8.7). AMNH.

2.38. A beautifully preserved mayfly in Pleistocene clays from North America. AMNH; wing spread 25 mm.

described the stratification of rock layers and their marine animal fossils, their intentions were to define a succession of extinct faunas, and work started small. The Devonian Epoch, for example, is named for a sequence of rocks described in 1839 from Devonshire, England; the Cambrian and Silurian, from sequences in Wales, named after ancient tribes from there. Correlating sequences of fossils in various geological columns (stratigraphic correlation) on a global scale transformed a provincial system into one that is as universal to geology as the Periodical Table of the Elements is to physics and chemistry, and the Linnean Hierarchy is to biology.

Correlation requires fossil taxa that are widespread, common, and diverse and that have fairly long histories (usually a million years or more). Most fossilization has involved invertebrates from marine continental shelves that had hard parts, and certain of these groups have become particularly important in defining epochs and ages. Graptolites (an extinct group of marine, colonial hemichordates), for example, are very diagnostic of ages within the Paleozoic, ammonites (extinct nautilus-like animals) within the Mesozoic, and planktonic Foraminifera (minute, shelled protists) and pollen for the Cenozoic. Typical index fossils for terrestrial sediments are pollen and spores. By describing stretches of similarity, change, and gaps, the early stratigraphers were essentially defining periods of biotic stasis and extinction events. Indeed, the boundaries between periods, epochs, and ages largely reflect abrupt events of extinction and biotic turnover. Geological strata continue to be studied using fossil correlation, but assigning absolute ages to strata ultimately requires physical methods.

The most commonly used physical dating method uses isotopes, wherein the proportion of isotopes of an element is measured. Because isotopes of an element are formed at a steady rate (the decay constant or half-life), the amount of isotope reflects the age of the substance. Elements have vastly different decay constants, which is why 235U (ura-nium)-207Pb (lead) is used for Paleozoic ages; 40K (potas-sium)-40Ar (argon) is used for Mesozoic and Cenozoic strata; and 14C (carbon) is used for ages of 40,000 years or less. Another constraint to isotope dating involves the composition of the strata or fossil. The minerals used in isotope dating are most common in igneous and metamorphic rocks, but most terrestrial fossils occur in sedimentary rocks. Dating fossiliferous rocks thus requires overlaid or intrusive igneous or volcanic ashes that contain datable crystals like zircons.

2.39 (Left). Hardened forewing, or elytron, of a carabid beetle from Wisconsin-aged (Holocene) bog deposits in Alaska. Photo: Scott Elias.

2.40 (Right). Head of a Holocene weevil from bog deposits in Alaska. Photo: Scott Elias.

2.41. The geological time scale, with the system of periods used in this book. Based on Palmer and Geissman (1999) and others.

Dating with carbon-14 (14C) obviously requires an organic material.

Geomagnetic Polarity Timescale (GPTS), another physical dating technique, measures the orientation of iron oxide crystals aligned to the earth's magnetic field when the rock in which the crystals occur was formed. Earth's magnetic polarity is constantly reversing over periods of less than a million years. Regardless of the location on earth, crystals from contemporaneous paleomagnetic reversals all have the same orientation, and these reversals must be calibrated using radiometric techniques. GPTS is useful for most of the history of winged insects, to approximately 300 mya (Late Carboniferous), but the error range in dating is far smaller for strata younger than half this age (Jurassic-Cretaceous boundary), which can be as little as several thousand years.

Depending on the time frame and types of fossils, fossil correlation has proved to be a very reliable dating method for estimating ages, particularly when the fossils are correlated with a column dated with physical methods. Physical dating methods, however, have revolutionized estimates of absolute ages (Figure 2.41 - the system used in this book). Over the past century, physical dating, for example, indicates an age of the earth that is more than 100 times Lord Kelvin's estimate of 40 million years.

The geological or fossil record of insects is often dismissed by paleontologists and even by entomologists as too incomplete. In the words of one Oxford evolutionary biologist, the fossil record is "corrupted." Such adjectives gloss over the fact that hundreds of deposits of fossil insects occur on all seven continents, from the Devonian to the Holocene. The deposits vary greatly in preservation and diversity but collectively form a geological record that is, while not the envy of paleo-botanists and vertebrate paleontologists, actually more impressive than for most groups of terrestrial animals. In some aspects the insect fossil record is unique or virtually so, such as the myriad life forms embalmed in amber and preserved as three-dimensional mineralized replicas. The mostly tiny insects preserved these ways together with larger insects preserved in sediments provide a complementary and vivid fossil record.

Our review here is not complete; it does not discuss all fossil deposits known to have yielded insects. Instead, we have focused discussion on the largest and most diverse deposits, ones that have yielded particularly significant finds, or those that are from poorly represented corners of the globe. An overview of fossil insect deposits was by Hennig (1981), which is rather out of date and general and which has now been replaced by the review in Rasnitsyn and Quicke (2002), itself incomplete but the most comprehensive to date. Schl├╝ter (1990, 2003b) reviewed insect deposits from Gond-wana, or the southern continents plus India. Evenhuis (1994) provided an extensive list of Mesozoic and Tertiary deposits in which flies are preserved (thus, no Paleozoic localities), but there was very little discussion, and the ages of some deposits require updating, which we have done here. The encyclopedic references on fossil insects - Rohdendorf (1962, 1991) and Carpenter (1992) - provided no overview at all on the various deposits.

Most fossil insect deposits occur in the Northern Hemisphere, which may be attributable simply to centuries more exploration and study in regions where paleontology developed, namely Europe and then North America. Arid regions of uplift, like the American west, Patagonia, Mongolia, and parts of Australia, harbor many fossil formations where sparse vegetation and erosion exposes fossil beds. Formations overgrown with thick rain forest are only occasionally exposed by mudslides or eroded river banks, which partly explains the paucity of deposits from tropical countries.

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