Types Of Preservation

Compressions and Impressions

Compressions and Impressions are the most extensive types of insect fossils, occurring in rocks from the Carboniferous to Recent. Impressions are like a cast or mold of a fossil insect, showing its form and even some relief (like pleating in the wings) but usually little or no color from the cuticle (Figure 2.2). Compressions preserve remains of the cuticle, so color also distinguishes structures (Figure 2.3). In exceptional situations microscopic features such as microtrichia on sclerites and wing membranes are even visible, but preservation of this scale also requires a matrix of exceptionally fine grain, such as in micritic muds and volcanic tuffs. Because arthropod sclerites are held together by membranes, which readily decompose, many fossil arthropods are known only by isolated sclerites. Far more desirable are complete fossils.

Just as bones and teeth are to the vertebrate fossil record, sclerites and wings are the enduring record in arthropod evolution. Sclerites in some fossil arthropods may appear remarkably preserved, but rarely is the molecular composition intact; that is, rarely are the sclerites actually cuticle. Insect cuticle is composed of a readily denatured protein component, and chitin, which is a pleated polymeric sheet of mucopolysaccharides. Chitin is crosslinked by other molecules to the protein, which makes it particularly durable. By chemically analyzing a spectrum of insect remains from the Holocene to the Carboniferous, it has been possible to determine how

2.2. A roach forewing from the Cretaceous of India, with the intricate pleating still preserved. Most insect fossils occur only as isolated wings because these structures are so durable. Fortunately, wing venation is very informative about the identity and relationships of insects. Museum of Comparative Zoology, Harvard University (MCZ) 20084012; length 24 mm.

2.2. A roach forewing from the Cretaceous of India, with the intricate pleating still preserved. Most insect fossils occur only as isolated wings because these structures are so durable. Fortunately, wing venation is very informative about the identity and relationships of insects. Museum of Comparative Zoology, Harvard University (MCZ) 20084012; length 24 mm.

2.3. A dragonfly wing from the mid-Miocene of Oregon, with the dark coloration still preserved. MCZ 4895a; length 38 mm.

and where insect cuticle preserves (Stankiewicz et al., 1997a,b, 1998a,b; Briggs et al., 1998a,b; Briggs, 1999).

The chitinous portion of fossil insect cuticle preserves much longer than does protein, as expected. Even in relatively recent fossils, like those in bog peats or from tar pits, 2,000 to 40,000 years old, protein is highly degraded or virtually absent, but much of the chitin is still present. Age, however, has probably less effect on preservation of chitin than does the conditions under which the cuticle was preserved and the nature (i.e., the thickness) of the cuticle itself. Insects that settled into anoxic levels of a lake decompose more slowly than those that settled into oxygenated levels, and so preserved better. Chitin also preserves better in freshwater sediments than in marine sediments. Cuticle that is thick and heavily sclerotized preserves far better than thin, poorly scle-rotized cuticle, which explains why roach tegmina and beetle elytra are so widespread in the insect fossil record. But chitin has definite limits to its lifespan. The most ancient chitin known thus far is from the elytra of 25 myo weevils preserved in ancient lake sediments of Westerwald, Germany. Older insect fossils (and younger ones that are less well preserved) have sclerites composed largely of just aliphatic and aromatic hydrocarbons, probably a product of the polymerization of lipids that coat arthropod cuticles (epicuticular waxes) and lipids that are contained within the body. As the chitin in the insect cuticle polymerized, it was chemically transformed, leaving a layer visually indistinguishable in some cases to arthropod cuticle. Fragmentary arthropods and isolated sclerites from the Devonian, for example, appear strikingly like modern cuticle (e.g., Shear et al., 1984; Subias and Arillo, 2002) (Figure 3.24), and have even been interpreted by some as original cuticle. Though the composition of these Devonian remains has not yet been determined, it is almost certainly completely modified.

Detailed looks at compression fossils sometimes reveal astonishing preservation. Perhaps the most interesting is the preservation of spores and pollen from carbonized gut remains (Krassilov and Rasnitsyn, 1982; Rasnitsyn and Kras-silov, 1996a,b). Spores and pollen are among the most persistent, and pervasive, biological structures in terrestrial sediments, which makes them so useful for dating. Pollen grains are even durable enough to be voided in the feces of bees, flower flies, and other pollen-grazing insects, so it is not surprising that they have endured within long-extinct insects. Spores and pollen have been recovered from the guts of generalized Permian Hypoperlida and Grylloblattida (sensu lato), and from xyelid sawflies from the Early Cretaceous. Amounts and content of the meals indicated the insects actually foraged on the spores and pollen, and thus probably had a significant impact on pollination. Detailed examination of fossil insect guts is rarely done, so systematic study of them using scanning electron microscopy may reveal many more examples like these.

There is no relationship between detail of preservation, mode of preservation, and age. Insects from the Triassic of Virginia (ca. 220 myo), for example, are entirely two-dimensional silvery films on a very fine-grained, black shale. In this situation, the body of the insect was compressed into metamorphosed carbon, and this acted as a template for the precipitation of aluminosilicates in clays (D. Briggs, pers. comm.), which is what gives them the silvery appearance (Figures 8.69, 10.16, 10.26). But even microtrichia less than 1 ^m thick are routinely preserved in these fossils, which is detail few Cenozoic deposits show. Similar preservation occurs in a deposit of Early Cretaceous insects from South Korea (Engel et al., 2002, unpublished data). One of the most celebrated Lagerstätte is Grube Messel, a mid-Eocene deposit of oil shales southeast of Frankfurt, Germany (Schaal and Ziegler, 1992). It was originally a 30-40 m deep, anoxic lake periodically choked by algal blooms. It preserved a great variety of completely intact organisms, including insects, and some beetles even retained iridescence.

Concretions

These are stones with a fossil at the core whose chemical composition differs from that of the surrounding matrix, usually formed as a result of mineral precipitation from decaying organisms (Selles-Martinez, 1996). The most significant deposit consists of various localities of the Late Carboniferous Francis Creek Shale of the Carbondale Formation at Mazon Creek, Illinois, which are composed of shales and coal seams yielding oblong concretions. Within most concretions is a mold of an animal (Figures 2.4, 3.7) and sometimes a plant that is usually marine in origin. Rare insects were preserved when their bodies were transported to flooded coastal swamp-forests and then rapidly buried in anoxic sediments. This is the most diverse deposit of early winged insects, and many of the specimens are preserved complete. Carbonates in the sediments were replaced by siderite (FeCO3), assisted by decay from anaerobic bacteria. This process caused the formation of a hard rind of mud around the insect that metamorphosed into the stones. Insects within the concretions have relief, though they are not completely three-dimensional nor do they have significant detail. The diversity

2.4. An arachnid preserved in an ironstone concretion from the Upper Carboniferous at Mazon Creek, Illinois. Mazon Creek arthropods are often articulated and have relief, but the detail of preservation varies greatly. Yale University Peabody Museum of Natural History (YPM) 66-576; body length 27 mm.

and taphonomy of Mazon Creek fossils were reviewed by Baird et al. (1986) and Shabica and Hay (1997). Another deposit of insect-bearing concretions is from the Early Miocene of Izarra, northern Spain (Barron et al., 2002). Fragmentary plants and diverse terrestrial arthropods in the Izarra concretions are replicated in calcite, many of which are allochthonous.

Crustacea, Tardigrada, and even Pentastomida. Pentastomida are parasitic, probably highly modified crustaceans, presently known only from the respiratory tracts of terrestrial vertebrates. The phylum Tardigrada (Figure 3.4), discussed later, comprises minute, membranous animals that are probably a living sister group to the arthropods. Though the Paleozoic Orsten deposits were formed 150 my before the origin of insects, these fossils provide unique information on the early evolution of arthopods and closely related phyla. Insects were preserved in similar fashion, some even also by phosphates, but much later.

Perhaps the most famous and extensive deposit of replicated insects comes from the Barstow Formation (Miocene, 18 mya), in the Calico Mountains of southern California in the Mojave Desert (Palmer, 1957). The deposit was originally a shallow, highly alkaline lake, which preserved insects living in it as well as ones that wafted into the water from surrounding areas. By dissolving calcareous nodules with acids, minute arthropods can be extracted, preserved like minute glass sculptures (Figures 2.5 to 2.7). Hairs and fine appendages are preserved, and even internal organs like the brain and digestive tract are known for some specimens. Most of the specimens are composed virtually entirely of celestite (SrSO4), quartz (SiO4), or both, with trace quantities of other minerals.

Mineral Replication

When an insect is partly or wholly replaced by minerals, usually completely articulated and with three-dimensional fidelity, it is replicated. This is also called petrifaction, as in "petrified" wood. Insects preserved this way are often, but not always, preserved as concretions, or within nodules of minerals that formed around the insect as its nucleus. Such deposits generally form where the sediments and water are laden with minerals, and where there is also quick mineralization of the carcass by coats of bacteria (Seilacher et al., 1985; Allison, 1988b). Permineralization is a form of replication that, strictly speaking, is a result of microbial decay (Briggs, 2003).

The most significant replicated arthropods are the so-called Orsten (nodule)-preserved animals from the Paleozoic, some 550-500 mya, which are composed of phosphate (Figure 3.4). They come from the Early Ordovician of Sweden and Newfoundland, the Late Cambrian of Poland and Sweden, the Middle Cambrian of Australia, and the Lower Cambrian of Shropshire, England (Müller and Walossek, 1986, 1991; Hinz, 1987; Andres, 1989; Roy and Fähraeus, 1989; Walossek and Szaniawski, 1991; Müller and Hinz, 1992; Müller and Hinz-Schallreuter, 1993; Walossek et al., 1993, 1994). These arthropods were tiny (<2 mm), benthic marine species, many of them the larval stages of stem-group arthropods and other phyla, including Pantopoda (Chelicerata),

2.5. Pupae of ceratopogonid midges replicated in silica, from the Miocene-aged Barstow Formation of California. AMNH; body length 2.8 mm.
2.6. An early instar beetle larva of the aquatic family Dytiscidae, from the Barstow Formation. Like the midge pupae (Figure 2.5), it too is replicated in silica. Scanning electron micrograph. AMNH; body length 4.7 mm.

Replicas in gypsum (CaSO3 ■ 2H2O) also occur, but are rare (Palmer, 1957; Doberenz et al., 1966; Park, 1995).

Miocene (20 myo) arthropods and soft-bodied invertebrates from Mfwangano Island in Lake Victoria, Kenya (Leakey, 1952) are also three-dimensional; they are, however, composed of calcite and do not occur in concretions. Among the specimens are an earthworm, a caterpillar, arachnids, various insects, roach nymphs, and even a colony of weaver ants, Oecophylla (Wilson and Taylor, 1964) (Figure 2.8). This is one of only several known fossil ant colonies. Over 350 ants were found in the colony, including larvae, pupae, and adults. Excellent preservation allowed measurement of the size distribution of major and minor workers, which is remarkably

2.7. Head of the beetle larva in Figure 2.6, showing the detail of preservation under scanning electron micrography.

similar to living species of the genus. Another deposit of three-dimensional insects not preserved within concretions occurs in Oligo-Miocene (25 myo) limestone from Riversleigh, Queensland, northern Australia (Duncan and Briggs, 1996; Duncan et al., 1998). Rare insects replicated in calcium phosphate [Ca5(PO4)3] are found here among abundant and diverse vertebrates. Internal tissues decayed, but microscopic structure of cuticle and ommatidia are preserved in fine detail. Similar phosphatized, three-dimensional insects occur from the Eocene of Quercy, France (Handschin, 1944) and the Oligocene of Ronheim, Germany (Hellmund and Hellmund, 1996).

The huge Cretaceous Lagerstätte from Ceara, northeastern Brazil, has preserved insects, plants, and various other organisms with consistent detail and relief (Martill, 1988; Grimaldi, 1990a) (Figure 2.9). Insects occur in fine-grained limestone of the Crato Member, Santana Formation, which was an evaporating, shallow, alkali lake approximately 120 mya. Small fish (Dastilbe) died en masse in the lake, probably as it evaporated and the minerals concentrated, as did aquatic insects and diverse terrestrial ones that wafted into the water from amongst vegetation that surrounded the lake. The organisms were preserved quickly and with very little disturbance; almost all are completely articulated and show little sign of decay (e.g., Figure 8.48). Unlike many three-dimensional fossil insects preserved by phosphates, these are preserved as goethite (FeO(OH)) - a form of rust -though portions of many also have calcite in the body cavity. Internal tissues replicated in goethite have preserved even myofibrils in the muscles (Grimaldi, 2003a) (Figure 2.10), similar to the scale of preservation known for diverse fish from the Romauldo Member of the Santana Formation (Martill, 1988, 1990). The gut contents of some insect specimens are preserved, some even with pollen. Significant difference between the density of the insect replica and the softer limestone matrix has allowed high-resolution CT scanning (HRCT), providing images of the complete insect from any view, even of structures concealed deep in matrix (Figure 2.9).

Similar to the Santana Formation insects, though less extraordinary, are insects from the Late Eocene (ca. 40 mya) Bembridge Marls of the Isle of Wight, southern England (McCobb et al., 1998). A diverse insect fauna (220 species in 12 orders) was preserved in dense mudstone produced by the sediments of a brackish lagoon or estuary. Preservation is a spectrum from isolated wings to three-dimensional, fully articulated specimens (Figure 2.11). Some three-dimensional specimens are mere voids inside (Figure 2.12); others have remnants of soft tissues, even myofibrils of the muscles, replicated in calcite.

2.9. A predatory, aquatic water bug, family Belostomatidae, from the Early Cretaceous Santana (Crato) Formation of Brazil. The bug is replicated in iron hydroxide and phosphates and lies in a matrix of soft limestone. This allows high-resolution CT scan images (right), which reveal hidden details on the ventral surface of the bug. AMNH; body length 15 mm.

2.8. Phosphatized replicas of a roach nymph (left) and a weaver ant (Oecophylla) pupa, from the Miocene of Kenya. Natural History Museum, London (NHM); body length of pupa 5.5 mm.

2.9. A predatory, aquatic water bug, family Belostomatidae, from the Early Cretaceous Santana (Crato) Formation of Brazil. The bug is replicated in iron hydroxide and phosphates and lies in a matrix of soft limestone. This allows high-resolution CT scan images (right), which reveal hidden details on the ventral surface of the bug. AMNH; body length 15 mm.

2.10. A plant bug (Heteroptera) in limestone from the Early Cretaceous Santana Formation, showing the preservation of muscle tissue. Scanning electron micrographs reveal bundles of muscle fibers and even their striations. AMNH; body length 6.5 mm. a: whole insect. b, c: thorax with exposed muscles (b, light Photomicrograph; c, scanning electronmicrograph). d, e: details of muscle fibers.

2.11. A beetle preserved with complete relief in Late Eocene-Early Oligocene clays from the Bembridge Marls of Isle of Wight, UK. Because of the detailed, three-dimensional preservation of insects from this deposit, one entomologist has called the Bembridge clay "opaque amber." NHM In. 26121; body length 6 mm.

2.13. "Gold bugs," pyritized beetle remains preserved in the Eocene London Clay. NHM; length of largest 5.1 mm.

Exceptional situations are insects replicated by the metallic mineral pyrite ("fool's gold") and its less stable form, mar-casite (both FeS2). These generally form in highly reducing environments where sulphur is abundant, such as in clays and other fine sediments from brackish or marine waters that have stagnated with decaying organisms. Fresh waters are usually sulphate-poor, so pyritization is rare in lacustrine sediments. Pyritization is common in marine sites, such as Beecher's Trilobite beds of New York state, United States (Late Ordovician), and the Early Devonian Hunsrückschiefer (Hunsrück slate) of western Germany (reviewed by Martin, 1999). The former is a depauperate fauna of trilobites; the latter has preserved diverse echinoderms, and molluscs, even worms and other soft-bodied invertebrates, many in virtually complete relief. External and internal soft tissues of various marine invertebrates, even an octopus, are remarkably replicated in calcite, apatite, and pyrite from the Jurassic (ca. 160 mya) of La Voulte-sur-Rhone, southern France (Wilby et al., 1996). The uppermost Paleocene and lowermost Eocene London Clay is one of the rare Tertiary marine deposits in which insects are preserved, in this case also by pyritization. In it are scattered small (<7 mm), woodland insects, particularly beetles and even larvae (Rundle and Cooper, 1971; Allison, 1988a; Jarzembowski, 1992) (Figures 2.13, 2.14). Pyritized insects occur sporadically in amber (e.g., Baroni-Urbani and Graeser, 1987; Grimaldi et al., 2000a). Water laden with sulphurous minerals can seep into fine cracks in the amber and replace the insects' tissues when the pyrite crystallizes. Even though insects preserved this way may be in turbid amber, they can be readily imaged using high-resolution radiographs (Schlüter and Stürmer, 1984).

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