Significance Of Fossils

There are at least five reasons why fossils are uniquely significant for understanding the evolutionary history of organisms:

1. Fossils provide the only direct record of extinct lineages, such as giant dragonfly-like forms from the Carboniferous and Permian (e.g., tMeganeuridae) (in this article, t signifies an extinct group).

2. Fossils reveal patterns and timing of extinctions and radiations. The mass extinction at the end of the Permian, for example, was the most cataclysmic event in the history of life and may have caused the extirpation of the tPaleodictyop-teroidea (known almost exclusively from the Carboniferous and Permian). The extinctions at the Cretaceous/Tertiary boundary that extinguished the remaining dinosaurs, ammonites, and other groups, appear to have had little impact on families of insects. Although insects have been affected by some mass extinction events, major lineages of insects appear particularly resistant to extinction.

3. Fossils provide the only direct information on the ages of lineages. Because there is never assurance that a fossil is the earliest, original occurrence of a taxon, the age of the earliest fossil is the minimum age of a taxon. This information, in conjunction with the phylogenetic positions of all fossils in a group, can be used to estimate actual ages and significant gaps in the fossil record. Such information is further useful for calibrating and then estimating rates of change, such as rates of genetic change among living species and dates of divergence.

4. Fossils may assist in reconstructing the phylogeny of a group. While fossils are widely acknowledged to possess combinations of characters unique from those of living species, their significance in reconstructing phylogenetic relationships is controversial. Nonetheless, fossils also provide the only direct evidence for the evolutionary sequence of character change. For example, in the fossil record of the Blattaria (cockroaches and their primitive roach-like relatives), tegminous forewings and a large, discoid pronotum appear well before the loss of an external ovipositor—a sequence not revealed by the study of living species alone.

5. Fossils can provide evidence that a taxon is old enough for its distribution to have been affected, for example, by Cretaceous continental drift. Indeed, many families of insects extend to the Cretaceous or even earlier, whereas some large groups are apparently too young to have been affected by continental drift, such as the ditrysoan Lepidoptera and the schizophoran Diptera. Often, too, a fossil is found outside the present-day range of its group, indicating formerly widespead distribution. A famous example is the occurrence of Glossinidae (tsetse flies) in the Cenozoic of North America and Europe.

preservation of fossil insects

The small size and external cuticle of insects are largely responsible for the many modes of fossilization, which are much more varied than for vertebrates and plants. Insect fossils are most commonly encountered as impressions or compressions in sediments (Figs. 1d, 1g, 1h, 5a and 5b), generally as disarticulated cuticle and particularly as wings because these are especially resistant to decay. Because wing venation has many systematically significant features, isolated wings often can be identified at least to family level. Generally, remains in sediments are highly compressed, but can still reveal microscopic structures such as flagellomeres, tarsomeres, microtrichia, wing scales, and even color patterns. Some are preserved as concretions, which are three-dimensional permineralized replicas of the original animal (Figs. 1a-c, 1e and 2f).

The finest preservation of insects is in amber (Figs. 2b—2e and 5d—5f). These formed when the resin was originally viscous and sticky, and small organisms became mired and then engulfed by the flows; they were embalmed so thoroughly as to preserve parasites, soft internal organs and tissues, and even organelles of cells (Figs. 2b—2e). The putatively most ancient DNA in the geological record is reported from insects preserved in amber, but authenticity of the DNA is disputed by those who unsuccessfully attempted to replicate these results.

Exceptional preservation is also seen in some insects preserved free in sediments. Terrestrial arthropods in several Devonian deposits of eastern North America are preserved as original cuticle, with even microscopic sensilla and setae preserved (Fig. 2a). In several Miocene deposits from California, insects are preserved in nodules as perfect three-dimensional silicified replicas (Fig. 2f). Similar relief and microscopic fidelity are found in carbonized remains in Cretaceous clays, rendered by ancient forest fires that charcoalified small organisms buried in leaf litter. Traces of insects have also been preserved as tracks, burrows, nests, galleries, feeding damage, and larval cases (e.g., Fig. 1f). Lepidoptera, for example, are very rarely preserved in rocks, probably because they are so soft-bodied, but larval mines characteristic of various microlepidopterans occur in some fossil leaves.

The various modes of fossilization each have their biases. Entrapment in amber is biased against larger insects that could extract themselves from the resin and against insects that live in open, nonforested habitats. Also, the earliest insects preserved in amber are from only the Lower Cretaceous, some 275 million years after the earliest known hexapods appeared.

FIGURE 3 Living and extinct orders of insects, their possible relationships, and chronology. Width of lineages is a rough approximation of diversity. Some groups with a meager or nonexistent fossil record (i.e., Phthiraptera, related to Psocoptera) are not included.

FIGURE 4 Representative Paleozoic hexapods. (a) Devonian. (c and d) Carboniferous. All others Permian. (a) tRhyniella (Collembola). (b) tDasyleptis (Archaeognatha). (c, d) tMischoptera adult (c) and nymph (d) (tMegasecoptera). (e) tMeganeuropsis ("Protodonata"). (f) tProtereisma (near Ephemeroptera). (g) tLiomopterum (Paraplecoptera). (h) tLemmatophora (near Plecoptera). (i) tDichentomum (near Psocoptera). (j) tProtelytron (tProtelytroptera). (k) tPermopanorpa (Mecoptera). (l) tSojanoraphidia (Raphidioptera). (m) tSylvacoleus (Coleoptera). Not to the same scale. (Reproduced, with permission of the publisher, the Geological Society of America, Boulder, Colorado, from Carpenter, 1992.)

FIGURE 4 Representative Paleozoic hexapods. (a) Devonian. (c and d) Carboniferous. All others Permian. (a) tRhyniella (Collembola). (b) tDasyleptis (Archaeognatha). (c, d) tMischoptera adult (c) and nymph (d) (tMegasecoptera). (e) tMeganeuropsis ("Protodonata"). (f) tProtereisma (near Ephemeroptera). (g) tLiomopterum (Paraplecoptera). (h) tLemmatophora (near Plecoptera). (i) tDichentomum (near Psocoptera). (j) tProtelytron (tProtelytroptera). (k) tPermopanorpa (Mecoptera). (l) tSojanoraphidia (Raphidioptera). (m) tSylvacoleus (Coleoptera). Not to the same scale. (Reproduced, with permission of the publisher, the Geological Society of America, Boulder, Colorado, from Carpenter, 1992.)

Preservation in rock is biased against smaller insects, and microscopic features are usually not visible against the grain of the matrix. Collectively, though, the fossil record of insects is actually much better than most paleontologists realize.

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