Nearctic and Palearctic

Paleocene (65-55 mya). The Paleocene is a very poorly known period in the geological record of insects. Though major lineages of insects, like families, were largely unaffected by the mass extinctions at the end of the Cretaceous, we have very few details about how insect faunas responded to this cataclysm (see, for example, Labandeira et al., 2002). North America was in the direct wake of the ejecta from the giant meteorite that fell at Chixculub, Mexico 65 mya, so Paleocene insect faunas from North America would be particularly interesting.

The most significant Paleocene insect deposit in North America is from the Paskapoo Formation of Red Deer River, near Blackfalds, Alberta (Mitchell and Wighton, 1979; Wighton, 1982), a region that was probably protected within the shadows of the Rocky Mountains when the meteorite hit. Insects from the Paskapoo Formation are preserved as detailed, carbonized compressions in finegrained, calcitic limestone. The insects were largely autochthonous, with a preponderance of aquatic insect larvae (including many beetles); 8-9 orders and about 20 families are reported.

In Eurasia, the Paleocene is slightly better represented than in North America, albeit it is still underrepresented. The two most significant deposits with insects are those of the lacustrine shales of Menat in France (Gaudant, 1979; Olliveier, 1985), and the marine diatomites of the "Mo Clay" in the Fur Formation (Thanetian) of Denmark (Willmann, 1990c; Thomsen and Schack-Pedersen, 1997; Rust, 1999) (e.g., Figure 2.63). Perhaps one of the most significant discoveries from the Paleocene of Europe has been the discovery of silky lacewings (Andersen, 2001), a lineage today known only from southern Africa, southeast Asia, and Australia. Among the most interesting fossils of the Eurasian Paleocene are those preserved in amber from Sakhalin Island in the Russian Far East (Zherikhin, 1978). The amber has been found in the Due Formation and is of approximately Late Paleocene (Thanetian) age. The major collection of Sakhalin amber is located in the Paleontological Institute, Russian Academy of Sciences, Moscow.

Eocene (55-38 mya). Climatically, the Eocene is the most dramatic period in the Tertiary. Changes during this time had profound impact on the global distributions of insects. During the Early Eocene no ice occurred on earth, even at the poles, and tropical organisms ranged to the highest latitudes. Lemurs and crocodiles roamed among forests where arctic tundra is today. By the end of the Eocene and the early part of the Oligocene, the glaciation of Antarctica had begun. The Eocene is particularly well represented in North America, along with the Miocene. The farthest east Eocene insects occur in North America is Kentucky; all other sites are restricted to western North America.

green river formation. This is one of the largest fossil lake systems in the world; it is some 65,000 km2 in area and 600 m thick, and is also among the most prolific sources of compression fossils in the world, including insects. An informative review of the Green River Formation was provided by Grande (1984), which focused mostly on the fishes. The formation was formed by three paleolakes in what are now eastern Utah, southwestern Wyoming, and western Colorado: "Fossil Lake" (the smallest, Early Eocene), "Lake Gosiute" (Early to mid-Eocene), and "Lake Uinta" (the largest, Late Paleocene to Late Eocene). "Green River" is perhaps best known for the diverse and beautifully articulated fish, some birds, reptiles (including a boid snake), and the oldest known bat, Icaronycteris index. Most of the fossil insects derived from the U-2 (or "Ray-domed") and the U-4 (or "Bonanza") localities, both in the Parachute Creek Member of Lake Uinta. Vertebrates that are so abundant and diverse elsewhere in the Green River Formation are scarce in these localities, but plants are diverse and well studied (MacGinitie, 1969). Thus, the paleofloral context of the insect fauna is well known. The flora indicates that the paleoenvironment was warm temperate to subtropical, represented by species that are a curious

2.63. The Mo-Clay of Denmark is one of the few diverse deposits of Paleocene insects in the world, and these giant ants are distinctive to the deposit. Assessing the impact of the Cretaceous-Tertiary extinctions on insects will require discovery of more Paleocene deposits. Photo: Zoological Museum, University of Copenhagen.

2.63. The Mo-Clay of Denmark is one of the few diverse deposits of Paleocene insects in the world, and these giant ants are distinctive to the deposit. Assessing the impact of the Cretaceous-Tertiary extinctions on insects will require discovery of more Paleocene deposits. Photo: Zoological Museum, University of Copenhagen.

2.64. A bibionid (March fly) from the Eocene of Washington state. Bibionids are extremely common and diverse in Cenozoic compression deposits of the Northern Hemisphere. University of Washington Burke Museum (UWBM); length 5.1 mm.

mixture of tropical and temperate, Asian and North American. Approximately 14 orders, and nearly 100 families and 300 species of aquatic and terrestrial insects are recorded from the Green River, among the most famous of which are several very rare butterflies. T. D. A. Cockerell described many of the Green River insects. Large historical collections of Green River insects are in Harvard's Museum of Comparative Zoology; another large collection is at the Smithsonian Institution.

okanongan highlands. Eocene outcrops in western British Columbia and northern Washington state have produced diverse plants and insects (e.g., Figure 2.64), preserved in mudstone, in shales, and in volcanic tuffs formed during periods of extensive volcanism. Most of the deposits belong to the Early to mid-Eocene Allenby Formation (48-50 myo); the one from Republic, Washington, is in the contemporaneous Klondike Mountain Formation. Insect fossils from this region were known since the time of Scudder (1890a), but the deposits are still being explored for insects (Wilson, 1977, 1978; Lewis, 1992; Douglas and Stockey, 1998; Archibald and Mathewes, 2000) and plants (Wolfe and Wehr, 1987; Wehr, 1996). The paleoflora from the Klondike Mountain Formation, in fact, is the richest one known for the Eocene of western North America (approximately 250 species and 23 families of angiosperms alone), even more diverse than the Green River Formation. Paleoclimate of the Okanongan Eocene was more temperate (12-13°C mean annual temperature) than the Green River (15-21° C), and is a particularly early record of montane conifer forests. The insects have not been studied as long as those from the Green River Formation, but systematic study of insects from Quilchena, British Columbia, for example has revealed at least 13 orders and 40 families thus far (Archibald and Mathewes, 2000).

claiborne group. Two thick clay formations in this mid-Eocene unit of the Gulf Coastal Plain have yielded significant fossil insects: the Claiborne Formation of Arkansas and the Tallahatta Formation in Mississippi (the Holly Springs Formation may also belong here). The Tallahatta Formation contains diverse compressions of leaves, flowers, seeds, fruits, and various terrestrial and freshwater aquatic arthropods, including six orders of insects. Among them are abundant caddisfly (Trichoptera) cases (Johnston, 1993, 1999). In very large clay pits in Malvern County, Arkansas, are deep exposures of the Claiborne Formation, within which is abundant fossiliferous amber (Saunders et al., 1974). The amber occurs as large (1-5 cm), rounded lumps with a thick weathered rind. The translucent (rarely transparent) interiors of the amber contain an array of arthropod families typical for Tertiary amber; even several flowers have been found. Quality of preservation is not very good, no doubt as a result of the chemistry of the original resin. The amber was not formed by a conifer, but probably from a dipterocarp-like tree. This is the most productive of only several known Tertiary deposits of fossiliferous amber in North America. Despite its significance it has not been fully exploited.

florissant formation. The first major fossil insect deposit to be studied in North America is the now famous Florissant fossil beds (Figure 2.65), or Florissant Formation, a deposit from the latest Eocene to lowermost Oligocene (Evanoff et al., 2001), approximately 100 km south of Denver, Colorado, 9,000 ft (3,000 m) in elevation. Prolific study of Florissant began as early as 1873 by S. H. Scudder (1878a, 1889, 1890a) and then by T. D. A. Cockerell, A. L. Melander, and many others. Nearly 200 families and 1,100 species of insects, and 140 species of plants, are known from the Florissant beds, though the taxonomy of some groups (particularly the beetles) require restudy (Meyer, 2002). Ancient Lake Florissant was formed around 38 mya by volcanic mudflows that dammed a river valley. One to two million years later, repeated eruptions produced thick ash that blanketed the area, preserving organisms on fine-grained shales in exqui-

2.65. Historical photo, c. 1890s, of the outcrops excavated by Samuel Scudder, at Florissant, Colorado. These outcrops are now known to have been formed in the latest Eocene to earliest Oligocene, 38 mya. The Florissant Formation has been among the most prolific of all fossil insect deposits, perhaps second only to the deposits of Baltic amber. Photo: H. Meyer, U.S. National Park Service.

2.65. Historical photo, c. 1890s, of the outcrops excavated by Samuel Scudder, at Florissant, Colorado. These outcrops are now known to have been formed in the latest Eocene to earliest Oligocene, 38 mya. The Florissant Formation has been among the most prolific of all fossil insect deposits, perhaps second only to the deposits of Baltic amber. Photo: H. Meyer, U.S. National Park Service.

site two-dimensional detail, including minute setae, eye facets, and scales on the wings of Lepidoptera. As one early entomologist put it, ancient Florissant was an "insectan Pompeii." The area was verdant and forested; some of the plants have relatives found only in southeast Asia today, like Koelreuteria (rain tree) and Ailanthus (tree of heaven). Just like the evolutionary history of horses, fossils throughout western North America reveal that Ailanthus was once a native of North America, but then it became extinct on that continent. It is now back, but as an introduced weed that grows amidst concrete in northeastern U.S. cities. North American "castaways" among Florissant insects are many, the most famous including spoon-winged lacewings (Mar-quettia americana [Nemopteridae: Figure 9.18], the genus also occurs in the Ruby River beds), and tsetse flies (Glossinidae: Figure 12.106). Nemopteridae are now found only in Australia, South America, southern Eurasia, and Africa, but the most diverse are found in Africa. Tsetse flies from Florissant were twice the size of extant species, which occur exclusively in Africa. Butterflies are exceptionally rare in all the fossil deposits in the world, but Florissant has yielded 12 species - far more than any other place (e.g., Figures 4.12, 13.65, 13.66). The unique specimen of Prodryas persephone (Figure 13.66) was shown to Frank Carpenter as a boy, who was so impressed with the exquisitely preserved wing veins and color patterns that it sparked an interest that developed into a 70-year career in paleoentomology. Much of the Florissant Formation outcrops became officially protected in 1969, as the Florissant Fossil Beds National Monument ( Major collections of Florissant insects are at the Museum of Comparative Zoology, Harvard University; University of Colorado; and the American Museum of Natural History, New York.

Assorted other Eocene deposits have yielded sparse remains of insects, the most significant being the Wilcox Group. The Wilcox Group is comprised of the Wilcox Formation, exposures of which have yielded Coleoptera and leaf-mined and galled leaves in Arkansas and Kentucky. Some place the Holly Springs Formation in this Group (alternatively in the Claiborne Group). Exposures of the Holly Springs Formation have yielded Coleoptera in Arkansas, and five orders of insects from several localities in Tennessee. Among the more significant Holly Springs Formation insects are a giant ant, Eoponera berryi (wingspan of 57 mm [2.4 in.]), and the termite Blattotermes, which belongs to the relict family Mastotermitidae, extant in Australia.

Across Eurasia, deposits with insects abound; however, most of these localities preserve relatively few fossils, with low diversity and unimpressive preservation. Several of the world's most prolific deposits of Eocene insects, though, are from northern Europe. Additional sites of significance include Monte Bolca, Italy (Lutetian); Pesciara di Bolca (Lutetian); and Menat, France (Ypresian).

Messel and eckfeld, Germany. The central European fossil Lagerstätten Eckfeld and Messel have been of special interest to paleontologists because their diversity includes almost everything from lithified bacteria to articulated mammals with gut contents and soft tissue preservation (e.g., Schaal and Ziegler, 1992; Lutz and Neuffer, 2001). Specimens are preserved in a fine oilshale with little or no turbation. The diversity of insects at both localities is high, with most specimens preserving coloration and minute structural details (Lutz, 1990, 1992; Tröster, 1992, 1993). Eckfeld represents the Middle Lutetian, with an established age of ca. 44.3 myo (Franzen, 1993; Mertz et al., 2000). Messel is slightly older, marking the lowermost Lutetian (ca. 49 myo). Not surprisingly, several taxa occurring in the Messel and Eckfeld deposits are similar to those found in the somewhat contemporaneous Baltic amber (e.g., Wappler and Engel, 2003).

The crater of the Eckfeld Maar near Manderscheid, Eifel, Germany, originally had a diameter of 900 m and a depth of about 170 m. The depth of the maar lake initially exceeded 110 m and might have reached 150 m (Pirrung, 1992, 1998; Fischer, 1999; Pirrung et al., 2001). Anoxic, alkaline conditions, and a raised content of electrolytes help to explain the perfect preservation of both the lamination of the oilshale and of the fossils (Wilde et al., 1993; Mingram, 1998). To date more than 30,000 fossils have been excavated, all of which document a highly diverse terrestrial flora and fauna (e.g., Schaal and Ziegler, 1992; Neuffer et al., 1996; Lutz et al., 1998; Wilde and Frankenhäuser, 1998; Lutz and Neuffer, 2001; Wappler and Engel, 2003).

Messel is located on the eastern shoulder of the northern Rhine rift valley. The Messel Pit today has a diameter of 700-1000 m. Despite the fact that Messel's perfectly preserved fossils have been studied for nearly a century (e.g., Ludwig, 1877; Lutz, 1990; Tröster, 1991, 1992, 1993, 1994; Hörnschemeyer, 1994), its formation was a matter of considerable debate until recently (Schaal and Ziegler, 1992; Pirrung, 1998; Liebig and Gruber, 2000). New geophysical data (Harms, 2002) demonstrate that the Messel locality was a maar-like Eckfeld (Lutz et al., 2000; Pirrung et al., 2001), as had previously been proposed by Pirrung (1998). However, with an initial diameter of about 1,500 m, the Messel maar was certainly considerably larger than Eckfeld.

Baltic amber. The most productive and extensive amber deposits in the world stretches across northern Europe in what is known as the blau Erde ("Blue Earth"). Pockets of amber can be found from Denmark and Sweden into Lithuania and south into Germany and Poland, although the greatest concentration can be found on the Samland Peninsula. The formation containing the amber runs approximately 45 m below the surface and approximately 5 m below sea level. Amber frequently erodes from the shores of the Baltic and can be carried as far as the East Anglian coast. Stratigraphic studies of the blaue Erde indicate it to be middle

Eocene (ca. 44 myo) in age (Kosmowska-Ceranowicz and Müller, 1985; Kosmowska-Ceranowicz, 1987), a dating congruent with K-Ar radiometric measures (Ritzkowski, 1997). The Albertus Universität in Königsberg (present day Kaliningrad), situated near the base of the Samland Peninsula, once held the most significant and largest collections of Baltic amber, with inclusions numbering near 100,000 at one time. Most of this material was, unfortunately, lost or destroyed during World War II, although a surviving portion is preserved in the Institut und Museum für Geologie und Paläontologie in Göttingen. The Saxonian amber (also known as Bitterfeld amber) is actually contemporaneous and chemically identical to Baltic amber. Although now located in Miocene deposits, Saxonian amber has been shown to be Baltic amber that had eroded and been redeposited in Miocene sediments (Weitschat, 1997). Ukrainian amber, of slightly younger age, was perhaps also part of the Baltic amber forest, although at an extreme of the distribution, having a slightly different faunal composition. The same can likely be said for the Paris Basin amber (Oise, France) of slightly older age (Ypresian) (Plöeg et al., 1998).

Baltic amber has historically been considered the resin of an extinct species in the pine family (Pinaceae). Even though some authors have argued against this hypothesis based on chemical analyses and favor an araucarian origin (e.g., Langenheim, 1969), a pinaceous origin for Baltic amber has gained additional support. Among the thousands of inclusions in Baltic amber are numerous cones and needles of pines (e.g., Weitschat and Wichard, 1998; Wichard et al., 2002) as well as wood fragments with microstructural details indicative of Pinaceae (Pielinska, 1997; Turkin, 1997). No araucariaceous inclusions have ever been found in Baltic amber. Moreover, it has been discovered that both extant and extinct species of the pine genus Pseudolarix produce succinic acid; indeed, Pseudolarix resin is chemically similar to Baltic amber (Anderson and LePage, 1995). Today Pseudolarix occurs in Asia but included at least the Arctic during the Eocene. Numerous Baltic amber fossils have relationships to taxa today occurring in southeast Asia or sub-Saharan Africa (Larsson, 1978; Engel, 2001a). Indeed, Baltic amber was perhaps produced by taxa related to Pseudolarix or even by an extinct species of this genus. Preservation of the insects in Baltic amber generally does not have the fidelity seen in some other ambers, such as Dominican amber. Insects in Baltic amber commonly have a milky coating (e.g., Figure 10.88), which is a microscopic froth that exuded from the bodies during decomposition.

Many tons of Baltic amber have been gathered by humans for nearly 13 centuries, but systematic study of the fossils did not begin until the 1700s (Sendel, 1742). Several thousand species of insects alone have been described from Baltic amber, making this the most prolific source of fossil insects (Figures 11.84 to 11.86, 11.93, 12.40, 12.62, 12.73, 12.103,

14.37). Major summaries of the fauna include Larsson (1978), Weitschat and Wichard (1998), and Wichard et al. (2002).

Oligocene (38-23.6 mya)

creede and antero formations. Approximately contemporaneous with Florissant are two other Colorado formations also formed by lake deposits and volcanic ash. Outcrops of the Creede Formation are approximately 100 mi (160 km) northeast of Florissant, near Pike's Peak. The paleoflora was dominated by conifers, not by angiosperms as at Florissant. It is quite productive, with some five orders and eight families represented among 2,000 specimens housed in the Museum of Comparative Zoology, Harvard. Outcrops of the Antero Formation in South Park, Colorado, have been less productive, though it has preserved an in situ aquatic fauna dominated by Heteroptera and Diptera. Some of the insect species in the Creede and Antero Formations also occur in the Florissant Formation (Carpenter et al., 1938; Durden, 1966).

kishenehn basin. Outcrops of the Kishenehn Basin of northwestern Montana and southeastern British Columbia contain abundant plants, terrestrial and aquatic gastropods, fishes, mammals, and 6 orders and more than 14 families of terrestrial insects (Constenius et al., 1989). The Kishenehn deposits were dated by K/Ar and fission track methods as approximately 30 myo (mid-Oligocene).

renova formation, Montana. Outcrops of the Late Oligocene-Early Miocene Renova Formation near Alder, Ruby River Basin (Passamari Member), and Canyon Ferry Reservoir, both in southwestern Montana, have yielded 13 orders and over 30 families of insects. The Ruby River Basin insects were discovered by Zuidema (1950), although Lewis (e.g., 1971, 1973, 1978) published many papers on assorted insects from here. Among them are rare bee flies (Bombyli-idae), eumastacid grasshoppers, and sialid "alder flies" (Megaloptera). The Canyon Ferry outcrops were only recently discovered (CoBabe et al., 2002). The Ruby River Basin deposits are typical lacustrine sediments, though they are called paper shales because they are more finely laminated than most such sediments. The Canyon Ferry deposits are lacustrine and volcanic tuffs, dominated by abundant corixids ("water boatmen"), but most of the diversity comprises families that feed on terrestrial plants. A satyrid butterfly wing is a notable record from Canyon Ferry. Both deposits have preserved an impressive record of insects now extinct in this region, or even for North America and the Western Hemisphere. For example, in the Ruby River Basin there is a nemopterid lace wing, and a stalk-eyed fly (Diopsidae). Diopsidae today occur in Africa and southeast Asia, with two species of the basal genus Sphyracephala living in eastern North America, but the Ruby River basin diopsid is more closely related to Old World species. Hodotermitid termites occur in both deposits, though today they are found in western North America and

Eurasia. As at Florissant, Canyon Ferry has preserved an osmylid lacewing in the subfamily Kempyniinae, a group that today is austral. Also at Canyon Ferry are giant hornets in the genus Vespa, whose natural distribution today is Eurasian.

bembridge marls. These are highly fossiliferous limestones from the northern portion of the Isle of Wight, United Kingdom, formed near the Eocene-Oligocene boundary. The limestones are very finely grained and preserved insects in microscopic detail but also with complete relief (Figure 2.11). The body cavities of many insects are preserved (Figure 2.12), in some cases with muscles still preserved (Figure 13.65). Thousands of insects have been collected from this deposit; most of them are stored in the Natural History Museum in London. Among the more significant finds are several beautifully preserved butterflies (Figure 13.65), and termites belonging to the presently Australian genus Mastotermes (Figure 7.80). An important reference on the deposit is by Jarzembowski (1980), but much work remains on understanding this paleofauna.

aix-en-provence. The Late Oligocene gypsum marls that occur in Aix-en-Provence, France, contain abundant insects and plants. Though productive and also known and studied much longer than the Bembridge Marls, these deposits are not quite as diverse for insects as this other deposit. Most of the insects were described by Theobald (1937) but are in need of revision before meaningful comparisons can be made to other Oligocene sites. Among the more significant fossil insects from Aix are six species of butterflies, which are exceedingly rare as fossils (Table 13.3).

rott. The most famous Oligocene deposit in Europe occurs in Rott, Germany near Bonn, which are also historically among the longest studied insect compression fossils in Europe. Insects from this deposit are preserved with very fine detail (e.g., Figure 11.87), partly because the matrix consists of extremely fine-grained paper shales. Age of the deposit is slightly ambiguous, being either latest Oligocene (Chattian) or earliest Miocene (Aquitanian). A very large collection of Rott fossil insects is at the Natural History Museum of Los Angeles in California, which is the collection of Georg Statz, who published several large papers on this fauna between 1936 and 1950.

oeningen and radoboj. Perhaps the richest Miocene insect deposits in Europe are from Oeningen in Switzerland and Radoboj in Croatia. Fossils from these localities were extensively monographed by Oswald Heer (Heer, 1849), and relatively little work has been done on this deposit since. As a result, much of the fauna is in need of modern revision. Insects from Oeningen are of Late Miocene (Messinian) age and are preserved as compressions in freshwater limestone. Insects from Radoboj are Early Miocene (Burdigalian) in age and are preserved similar to those at Oeningen. Among the more significant insects from Radoboj are three species of very rare fossil butterflies (Table 13.3). Classic reconstructions of the biota and climate of Switzerland during the Miocene were made by Heer (1865), based principally on the Oeningen fossils. The Early Miocene Randecker Maar in Germany has received recent attention (e.g., Schweigert and Bechly, 2001), although the insect fauna is much smaller than that of Oeningen or Radoboj.

rubielos de mora, spain. This deposit is particularly interesting and has been monographed recently (Penalver Molla, 1998). The deposit is located in Teruel, in the Rubielos de Mora Basin, which formed during the Early Miocene (ca. 20 mya) from the deposits of a meromictic lake. Diversity of the insects is not exceptional; some seven orders of insects are preserved. But, the preservation is remarkable (Figure 2.66). The matrix is a fine-grained, light oil shale, against which the dark, shiny cuticle of the insects stands out. Resolution of preservation is remarkable, including the fringe of setae around the margin of thrips' wings, and even the microtrichia on the wing. Adults and soft-bodied larvae are preserved.

shanwang formation. This formation of diatomites occurs near Linqu, in Shandong Province, China, from which approximately 400 species of insects in 84 families have been recorded (e.g., Zhang, 1989; Zhang et al., 1994).

latah formation. Numerous and prolific outcrops of this mid-Miocene (ca. 18 mya) formation occur in western Idaho and eastern Washington state, including compressions of entire insects and occasional insect mines and galls among diverse fossil plants (e.g., Carpenter et al., 1931; Lewis, 1969, 1985). Preservation varies greatly, that of outcrops near Spokane, Washington and Juliaetta, Idaho, being mostly carbonized wings. Among the more significant fossils are Bombus proavus and rhinotermitid termites that are closely

2.66. A beautifully preserved aphid from the Miocene of Rubielos de Mora, Spain, showing the long, slender cornicles and venation. MPZ-96/18. Photo: Enrique de Penalver.

related to species from southeast Asia (Emerson, 1971). Perhaps the most famous of the outcrops is an unpretentious roadcut at Clarkia, Idaho, which yields beautifully preserved plants and completely articulated insects. Cellular-level preservation of these fossils led to early efforts in the extraction of ancient DNA. At Clarkia alone some 9 orders and 30 families of insects have been identified.

savage canyon formation (stewart valley). This is probably the most complete paleocommunity known for the Cenozoic of North America. It occurs in southwestern Nevada, and K/Ar dating indicates an age of 16-10.5 myo (mid- to Late Miocene). The hot, arid environment in the region today contrasts with the paleoenvironment, which was cool, wet, and covered in mixed conifer-deciduous forest. Fossils from Stewart Valley include 50 families of mammals, 30 families of plants, and terrestrial and freshwater molluscs; fine-grained shales contain articulated fish skeletons, bird feathers, and complete leaves and insects. Even small, delicate insects like midges are preserved, intact with fine setae and often with color patterns. Ten orders and 50 families of insects are known, 50% of the individuals being Diptera, with Hymenoptera second in abundance due mostly to ants (Schorn et al., 1989).

barstowformation (calico mountains). This is one of the most distinctive fossil insect deposits in the world. The Barstow Formation occurs approximately 100 miles northeast of Los Angeles, California, in the Mojave Desert near the town of Yermo. While fossiliferous outcrops of the formation occur at Mt. Pinos and in the Frazier Mountains, there are three beds in the upper part of the Barstow Formation unique to the nearby Calico Mountains that yield insects some 13-14 myo. Insects occur within 5- to 60-cm-sized nodules, one or two of which are extracted from each nodule by digesting it with acids. The insect remains resemble microscopic glass sculptures (Figures 2.5 to 2.7). Original work on the deposit was by Palmer (1957), and there has been a recent paleoeco-logical study of it (Park and Downing, 2001). The fauna was mostly preserved in situ in a drying "alkali" or "soda"/"salt" lake, similar to lakes found today in the western United States (Mono Lake, California, or Big Soda Lake, Nevada), and throughout the Middle East and eastern Africa. This paleoen-vironment accounts for the remarkable preservation and impoverished autochthonous fauna. The fauna is dominated by fairy shrimp (Anostraca), dytiscid beetles, and immature stages of Dasyhelea midges (Ceratopogonidae), much as one would find in alkali lakes today. Rare arthropods include mites, spiders, thrips, psyllids, leafhoppers, heteropterans, and even a caterpillar, which apparently wafted into the lake. W. D. Pierce, whose taxonomy is notorious, described diverse species, genera, and even some new families from this deposit, the last of which is extremely odd for such relatively young fossils (e.g., Pierce, 1963, 1966; Pierce and Gibron, 1962).

alaska. Two fairly diverse deposits of insects occur in the Late Miocene of Alaska, formed during a period of considerable biogeographic importance: formation of the Bering Land Bridge. One deposit is uppermost Miocene (5.7 myo), from the northern part of the Seward Peninsula (Hopkins et al., 1971). The other is slightly older (6.7 myo), from Sun-trana, central Alaska (Grimaldi and Triplehorn, unpubl.).

Pliocene (5.2-1.7 mya) and Pleistocene (1.7 mya-10,000 ya)

Evolutionary significance of the Pliocene and Pleistocene for insects largely concerns the origins of modern species and sweeping changes in their distributions in the more recent past.

North American Pliocene insects are scarce, known from only four modest deposits. These occur in Alaska (Matthews, 1970), California (Squires, 1979), Nevada (Sleeper, 1968), and Texas (Carpenter, 1957). Remains of beetle elytra, chirono-mid head capsules, and other durable parts of insects preserved in Pleistocene lake beds and bogs are often identifiable to species. As a result, changes in the distribution of extant species can be tracked. Just as elephants (i.e., mammoths), lions, cheetahs, camels, and other "African" megafauna became extinct from North America, there have been dramatic changes among Pleistocene insects. Pleistocene and Holocene insects are well represented throughout the world, which is presented in detail by Elias (1994). One fascinating Pleistocene deposit is the La Brea tarpits outside of Los Angeles, which is famous not only for the impressive preservation and diversity of vertebrates but also for its diverse insects (Miller, 1983).

As in North America, Pliocene deposits of insects are uncommon and known from only scattered deposits in Greenland (Bennike and Bocher, 1990; Heie, 1995; Bocher, 1995, 1997), Italy (Pedroni, 1999, 2002), Sicily (Kohring and Schlüter, 1989; Schlüter and Kohring, 1990), France (Balazuc, 1989; Nel, 1987, 1988a,b, 1991b), Germany (Weidner, 1979; Harz, 1980; Schlüter, 1982; Rietschel, 1983; Grabenhorst, 1985, 1991; Kohring and Schlüter, 1993; Briggs et al., 1998b; Brauckmann et al., 2001), Turkey (Nel, 1988a,b), Georgia (Kabakov, 1988), Japan (Hayashi, 1999, 2000, 2001a,b; Hayashi and Shiyake, 2002; Mori, 2001), Chad (Duringer et al., 2000a,b; Schuster et al., 2000), Malawi (Crossley, 1984), Tanzania (Ritchie, 1987; Sands, 1987; Kaiser, 2000), and Antarctica (Ashworth et al., 1997; Ashworth and Kuschel, 2003).

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