Insect Fossilization

Too often insect fossils are thought to be merely impressions of wings scattered in slabs of rock. Wings are indeed a common insect fossil. They do not readily decay or digest, which is why birds and spiders typically leave the wings after devouring the rest of an insect. Fortunately, wing veins are also a veritable road map to the identification and phylogeny of insects, much the way crowns and pits are on the teeth of mammals. Most isolated insect wings, in fact, are readily identifiable to the family level. Also, the generally small size of terrestrial arthropods, along with their durable cuticle, have allowed them to be preserved in many more ways than have fossil vertebrates, and certainly in many more ways than just as compressed wings.

Terrestrial vertebrates are almost always preserved just as bony remains (or inorganic casts thereof), the original bone usually having been replaced by the mineral apatite [Ca5(PO4)3(F,Cl,OH)]. Occasionally, mummified or frozen vertebrates are found, but their age is usually no more than several thousand years. Fossils of insects, in contrast, are preserved as organic compressions and inorganic impressions; as three-dimensional, permineralized, and charcoalified replicas; and as inclusions in amber and even within some minerals (Schopf, 1975). There is also abundant fossil evidence for the behavior of extinct insects, including feeding damage on fossil vegetation and in wood, fecal pellets, and nests in fossil soils. Dinosaur behavior, by contrast, is recorded mostly as footprints and coprolites.

The common denominator among most deposits of fossil insects and terrestrial plants is the lake environment. Those insects that became preserved were either living in the fossil lake (autochthonous) or carried into it from surrounding habitats by winds, stream currents, or their own flight (allochthonous). Drowning and dying insects not eaten by fish and other predators settle to the bottom, where they may be preserved in the lake's sediments (lacustrine) under appropriate conditions. Even amber, or fossil resin from trees, requires a watery environment that is lacustrine or brackish in order to be preserved. Without protection in anoxic sediments, amber would gradually disintegrate; it is never found buried in fossil soils. Various factors contribute greatly to what kinds of insects become preserved and how well, if indeed at all, including lake depth, temperature, and alkalinity; type of sediments; whether the lake was surrounded by forest or vast and featureless salt pans; and if it was choked in anoxia or highly oxygenated.

There are some major exceptions to the lacustrine theme of fossil insects, the most famous being the Late Jurassic limestones from Solnhofen and Eichstätt, Germany, which are marine (Barthel et al., 1990). These deposits are famous for pterosaurs and the earliest bird, Archaeopteryx. The limestones were formed by a very fine mud of calcite that settled within stagnant, hypersaline bays isolated from inland seas. Most organisms in these limestones, including rare insects, were preserved intact, sometimes with feathers and outlines of soft wing membranes, indicating that there was very little decay. The insects, however, are like casts or molds, having relief but little detail (Figures 2.1, 6.43). In some cases iron oxides precipitated around wing veins, revealing better detail.

Lagerstätten are exceptional fossil deposits. Konservat Lagerstätten are deposits in which organisms, including soft-bodied ones, are exceptionally well preserved in any one of a variety of ways. Konzentrat Lagerstätten are ones that preserve an exceptional assemblage of individuals, from one to many species. Use of these terms often depends on the scale of diversity. For vertebrates and a great variety of marine invertebrates, Solnhofen limestones can be considered a Lagerstätte. For plants and insects, however, Solnhofen is not particularly remarkable. The finest terrestrial Lagerstätten are lacustrine, and these have provided most information on the evolutionary history of insects.

Taphonomy, or the study of how organisms fossilize, is essential for understanding differential preservation of organisms in the fossil record, and can help explain extinctions and the ecology of extinct organisms and communities (Allison and Briggs, 1991; Donovan, 1991; Briggs, 1995;

2.1. A dragonfly from the famous Jurassic limestone quarries of Solnhofen, Germany. The wing venation is clearly visible, which is exceptional preservation for insects from this deposit. Private Collection; wingspan 93 mm.

Martin, 1999). One aspect of taphonomy is biostratinomy, or those environmental factors that affect which species become trapped and fossilized. The vegetation surrounding a water body, for example, dictates which kinds of insects waft into the water. Deep, cold, still, and anoxic waters will usually preserve specimens better than shallow, warm, oxygenated waters where dead organisms will decay faster and also become fragmented by currents and scavengers. Diagenesis refers to changes that take place after burial. Very deep sediments and faulting, for example, can dramatically transform a fossil with extreme pressure, and even obliterate it.

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