Understanding Insects through Fossils

Although it has been estimated that insects account for roughly one-third of all animal species alive today, insects are, on the whole, poorly represented in the available fossil record, where many species are known from just a single specimen, and a high proportion of fossil insects come from exceptional fossil deposits that are sporadically distributed in time and space. Nonetheless, about 40,000 species of insects have been described as fossils, with many more awaiting description. Foremost among insect-rich deposits are ambers in which complete external preservation of insects is routine. Amber is the fossilized resin of a few particular kinds of trees. Oozing out of the bark, this resin had the ability to trap and surround insects, as well as other small animals, protecting them from the normal processes of organic decay as it hardened into transparent, yellow or orange amber. The chemical process of "amberization" could take up to 10million years. During this time, it was common for amber initially buried in the soil to be washed out by rivers and redeposited in the sea.

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Although the oldest amber comes from the Carboniferous period (360-290 million years ago), the great majority of amber deposits were formed between the start of the Cretaceous period（146 million years ago）and the present. They provide priceless windows on the insects and other small animals living at the time in the forests where amber-producing trees grew. Elsewhere in the fossil record, insects can be found in fine-grained sedimentary rocks, such as clays and silts deposited in freshwater lakes and sluggish rivers. Unlike the insects in amber, these fossils generally comprise only fragments, particularly of wings or wing cases, although more complete examples can be found, such as the dragonflies of the famous Jurassic Solnhofen Limestone of Bavaria.

The fossil record describes a multiplicity of insects that scientists have grouped according to their features. The most basic categorization of insects is into a primitive group without wings, called the Apterygota, and the winged Pterygota. Surviving apterygotes include the springtails and silverfish. They are now relatively rare, comprising less than 1 percent of all insect species. Pterygotes are divided into those with wings that cannot be folded, which are called the Palaeoptera, and a larger, more advanced group, the Neoptera, capable of folding their wings close to the body. Mayflies, dragonflies and damselflies are all palaeopteran insects, while the neopterans include locusts, butterflies, and wasps.

In spite of its imperfection, the fossil record holds a lot of useful information about the times of origination of insect groups that are alive today. Primitive wingless insects-the apterygotes-appear to have undergone an initial diversification during the Devonian period (416-359 million years ago), possibly even the Silurian period (444-416 million years ago). Unfortunately, however, relatively few fossil insects of this age are known and there is a great need for further discoveries. The oldest known fossil insect is currently Rhyniognatha hirsti from the early Devonian fossils in Scotland. However, this species, preserved in sinter (mineral sediment) from an ancient hot water spring that was active between 400 and 412 million years ago, exhibits some advanced characteristic, implying that there are more primitive, older insects still to be discovered.

Fossil insects with preserved wings （Pterygota）first occur in the mid Carboniferous. The evolution of wings was accompanied by an increase in maximum body size. A remarkable dragonfly called Meganeura with a wingspan approaching 70 cm has been described from the late Carboniferous. This inhabitant of the forests is one of the largest insects ever to have lived. The huge size of Meganeura has led to speculations about the composition of the atmosphere at the time, the powered flight of such a large insect perhaps demanding an atmosphere containing higher levels of oxygen than that of the present day. Unfortunately, the fragmentary insect fossil record sheds little light on the origin of flight, as the oldest winged insects already had fully formed wings.