Devonian 414358 mya

The earliest evidence of hexapods, the group of arthropods to which insects belong, comes from a few fragmentary remains of apterygote lineages from the Devonian. Although plants had long since colonized the land, this invasion was slow, and in the Early Devonian most plants were confined to moist, lowland environments or still consisted of mats growing on the surface or edges of pools. These primitive vascular plants were not complex and consisted of relatively simple shoots that generally reached no higher than a meter. The entire biotic world literally resided at knee-height or below, but Devonian plants rapidly diversified and took over an essentially empty landscape. During the Late Pragian through Givetian (412-370 mya), arborescence evolved, and by the end of this time period the structural support provided by woodi-ness allowed plants to dramatically increase their physical size. The first forests developed at this time, and plants began to venture further from the ecologically restricted moist, riparian environment. By the end of the Devonian, medium-sized to giant tree ferns would appear (e.g., cladoxylopsids, lycopsids) as well as archaeopterid and aneurophyte pro-gymnosperm trees. Some Late Devonian archaeopterids actually exceeded 30 m in height. Near the end of the Devonian (during the Famennian, ca. 364 mya), the first seed plants appeared. As can be imagined, such a considerable explosion of plant life on land had a dramatic impact on the world's environment. In fact, the impact on CO2 levels was significant, resulting in a precipitous decline between the Late Devonian and Early Carboniferous (Berner, 1997; Alego et al., 2001), and this even caused a brief episode of continental glaciation at the end of the Devonian (Caputo, 1985). The origin and spread of forests also transformed the soil, particularly acidification of the soil and slow weathering processes that retarded the transport of sediments.

During this dramatically changing world insects appeared and spread. The earliest hexapods are known from the Early Devonian, a "miniature" terrestrial world, and their rise would follow that of the plants that formed their microcosm.

Rhyme Chert. The first and perhaps the most famous Devonian hexapod is Rhyniella praecursor Hirst and Maulik (1926), a springtail from the Rhynie chert of Scotland (Figure 3.31). The age of the hot spring chert from Rhynie, Scotland, has been assigned to the Lockhovian-Pragian (as a maximal age) from spore evidence, while radiometric dating has indicated a slightly younger age of Emsian (Trewin, 1994; Rice et al., 1995: ca. 396 mya). The Rhynie paleoenvironment was likely one of a marsh or swamp (Tasch, 1957). Rhynie chert is exceptional among Paleozoic deposits for the detail of the remains. Most fossils are preserved as inclusions, three-dimensionally embedded in the microcrystalline, translucent chert. Somewhat mimicking the quality of preservation known from considerably younger amber inclusions, specimens entombed in Rhynie chert preserve fine microscopic details such the structure of setae and cuticle. The fauna mostly includes arachnids, such as trigonotarbids and mites, but also contains remains of crustaceans, eurypterids, and centipedes (Shear et al., 1987, 1998; Anderson and Trewin, 2003) and a pair of mandibles - described by Tillyard as Rhyniognatha hirsti in 1928b, but recently reported as the earliest definitive insect (Engel and Grimaldi, 2004a) (Figure 5.8). Although the identity of the hexapod remains from Rhynie was challenged by Crowson (1985), who believed the remarkably modern Rhyniella to be a later contaminant, the recovery of additional specimens have established that springtails were definitively present in the Early Devonian environment of Scotland (Scourfield, 1940a,b; Whalley and Jarzembowski, 1981). Even with the excellent preservation of Rhynie chert, the higher-level assignment of Rhyniella has been difficult but appears to be within the modern family Neanuridae (Massoud, 1967) or more likely Isotomidae (Greenslade and Whalley, 1986).

Gaspé Bay, Canada. In eastern Québec on the northern and southern shores of Gaspé Bay are outcrops representing a terrestrial environment in the Battery Point Formation. Spore and brachiopod assemblages date the formation from the beginning of the Middle Devonian (near the Emsian-Eifelian boundary, ca. 390 mya) (Boucot et al., 1967; Richardson and MacGregor, 1986). The flora of the Gaspé fossil beds is perhaps one of the most thoroughly studied Devonian botanical assemblages. The deposits are understood to represent a relatively tropical fluvial and delta-plain environment, with freshwater marshes and lacustrine deltas (Lawrence and Williams, 1987; Hotton et al., 2001). From macerated material taken from the Gaspé fossil beds, two fragments of a bristle-tail head and thorax were recovered (Labandeira et al., 1988). These remains represent the oldest record of insects in North America, although Jeram et al. (1990) suspected them to be recent contaminants. The Gaspé fragments preserve only primitive features of the Archaeognatha and thereby of the Insecta as a whole and accordingly may represent an extinct lineage basal to Recent apterygotes. However, not enough evidence presently exists to make a more conclusive assignment.

Gilboa, New York. In upstate New York near the town of Gilboa rests a layer of mudstone famous for its fossils from the Middle Devonian (Givetian). The fossils are extracted from the mudstone matrix by macerating the material in an acid wash and then sorting the minute pieces of cuticle from the dissolved rock. From this site many fragmentary remains of early arthropods have been recovered. They are mostly chelicerates and myriapods (Figure 3.24) but also include tantalizing pieces of cuticle with scales, indicative of Archaeognatha or Zygentoma (Shear et al., 1984, 1987).

Together these three deposits represent the earliest evi-

2.42. An protodonatan nymph preserved in an ironstone concretion from the famous Upper Carboniferous deposits at Mazon Creek in northcentral Illinois. FM PE30272; length of concretion 51 mm.

dence of terrestrial hexapods. Interestingly, all these sites occur within 10° of the Devonian equator. Potential sites that deserve attention in the future, but that to date have revealed no insects, are from the Falkland Islands and southern Greenland.

throughout the world: the central United States as far east as Pennsylvania, southern England, western Europe, Moravia Brazil, and Argentina (e.g., Sellards, 1904; Handlirsch, 1906b Bolton, 1916; Pruvost, 1927; Carpenter, 1933, 1940, 1963c 1970; Laurentiaux, 1952; Laurentiaux and Laurentiaux-Vieira 1980; Durden, 1984, 1988; Pinto, 1986, 1990; Nelson and Tidwell, 1987; Shear et al., 1992), but three deposits, in particular, have revealed the most significant material in terms of diversity.

Commentry and Montceau-les-Mines, France. Some of the earliest discovered and most famous Carboniferous deposits are the fossil beds of Commentry in Allier, France, and Montceau-les-Mines of Central Massif, France. Both deposits are of the same geological formation (dating from the lower Bashkirian) but represent rather different faunas (Burnham, 1981). It is from these deposits that the earliest glimpses of the giant insects (e.g., griffenflies = Protodonata) were obtained (Brongniart, 1884, 1893). The Coal Measures of the Commentry Basin include a remarkable diversity of early insects (Brongniart, 1878, 1884, 1885a,b, 1893; Bolton, 1917; Carpenter, 1943b, 1951, 1961, 1963a,b, 1964a; Kukalova, 1969a,b, 1970), particularly of the extinct lineage Palaeodictyopterida as well as "protorthopterans" (ancestral polyneopterous insects). Specimens are preserved in fine-grain sandstone at Commentry and in ironstone nodules at Montceau-les-Mines; both preserve remarkable detail of both wing and body characters.

Carboniferous (358-289 mya)

The Carboniferous consists of two major periods, the Mississippian (358-324 mya) and the Pennsylvanian (323289 mya). By the Early Carboniferous, vast coal swamps had developed, and large arborescent plants dominated the landscape. Land plants had finally taken hold, and forests covered many regions of the world. Insects also radiated and had become diverse in the Carboniferous. Unfortunately, from the Early Carboniferous (i.e., the Mississippian) no outcrops have as of yet yielded insects. Thus, there is a considerable gap in our knowledge from this critical time period. Essentially, the fossil record terminates with the few fragmentary fossils in the Early and Late-Middle Devonian and resumes with a diverse fauna in the earliest Pennsylvanian. The recovery and characterization of the Mississippian insect fauna is one of the greatest challenges and discoveries awaiting insect paleontology. While in the Devonian, insects were flightless, terrestrial creatures, by the time their record resumes in the Late Carboniferous, there are numerous winged forms representing most of the major superordinal lineages, albeit as distinctly plesiomorphic forms. Thus, the Early Carboniferous, and perhaps the Devonian (Engel and Grimaldi, 2004a), witnessed the incredible radiation of winged insects.

By the Late Carboniferous several deposits are found

Mazon Creek, Illinois. Among the most famous invertebrate fossils in North America are those preserved in ironstone concretions of the Carbondale Formation near Mazon Creek, Illinois (Shabica and Hay, 1997) (Figures 2.4, 2.42, 2.43, 3.7). The material is scattered in old coal strip mines and predominantly includes rather large and robust specimens (Nitecki, 1979). The deposits are of Upper Carboniferous age (ca. 300 mya) and represent a coastal region during that time period (Baird, 1997). Numerous accounts have been written on the insect fauna preserved at Mazon Creek (e.g., Handlirsch, 1911; Richardson, 1956; Carpenter and Richardson, 1968, 1971; Carpenter, 1997; Kukalova-Peck, 1997).

Hagen-Vorhalle, Germany. The rich deposits in the former brickyard quarry of Hagen-Vorhalle in the Ruhr area of Germany are of lowermost Bashkirian age and are therefore tantalizingly on the borderline with the Early Carboniferous (Brauckmann et al., 1994: ca. 315 mya). Many specimens beautifully preserve body structures in addition to wing venation, making them critical for a broader understanding of early insect evolution. To date nearly two dozen species are recognized from five orders (Brauckmann and Koch, 1982, 1994; Brauckmann, 1984, 1986, 1988, 1989, 1991; Kukalova-Peck and Brauckmann, 1990; Brauckmann et al., 2003) (e.g., Figures 6.28, 6.29, 7.1).

2.43. Prospecting for Carboniferous nodules at Mazon Creek, late 19th century. This formation yields the rede-posited remains of coal swamps, including insects. Mazon Creek fossils include a remarkable array of terrestrial arthropods, particularly arachnids, myriapods, and insects. Photo: The Field Museum, Chicago; negative GEO 85145.

2.43. Prospecting for Carboniferous nodules at Mazon Creek, late 19th century. This formation yields the rede-posited remains of coal swamps, including insects. Mazon Creek fossils include a remarkable array of terrestrial arthropods, particularly arachnids, myriapods, and insects. Photo: The Field Museum, Chicago; negative GEO 85145.

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