Fossil Insects

Frank M. Carpenter.

Stenomema canadense, a common mayfly.

Written in the rocks of Colorado, Kansas, Oklahoma, and many other places is the story of insects in the ages before man appeared on earth.

The insects were trapped, caught in mud or sticky resin, and thereby left a permanent record as did dinosaur, mollusk, and plants that broadens our knowledge of their evolution. About 12,000 species of fossil insects have been described. Countless thousands of specimens have been collected.

Fossil insects are not found in as many deposits or localities as most other invertebrates. Like other organisms, insects are preserved as fossils by a sequence of events that results in their burial in a suitable medium. Immediate burial is necessary to preserve the whole insect; otherwise the body parts soften and fall apart, and only the wings remain. The wings decompose more slowly and therefore can be preserved under less favorable conditions. That is the reason why many specimens of fossil insects consist of wings alone. When conditions were good for preserving insects, large numbers of fossils usually occur.

An example of such abundance is provided by the Tertiary shales at Florissant, Colo., which have yielded upwards of 60,000 specimens. The shale originated about 40 million years ago in a shallow lake, extending into several narrow valleys and rimmed by granitic hills. Several neighboring volcanoes frequently erupted and scattered ashes and debris over a wide area. Whatever insects were flying or were being blown over the lake at those times were forced into the water by the falling ashes and were promptly buried.

Fossil insects have been found at nearly 150 localities in various parts of the world. About nine-tenths of the specimens have been collected at 12 of these deposits. The remainder has come from less productive rocks. Some of the latter are important because of their geological position, however. One of them is the Commentry shales of central France. These were deposited by a deep fresh-water lake, which existed during the Upper Carboniferous period some 250 million years ago. About 1,500 specimens have been found in the shales. They are well preserved and are almost the oldest insects known.

Another deposit, notable for the abundance of fossils as well as their ages, is the Elmo limestone in eastern Kansas. The rock, fine-grained and nearly white, was deposited by a shallow fresh-water lake inhabited by aquatic insects, crustaceans, and small king crabs. A collector who carefully breaks the limestone, after it has been dug up and dried, may get as many as 50 good insects a day. Most of the fossils are strikingly well preserved. Some show even the coloration and minute hairs on the wings. About 10,000 specimens so far have been collected there.

A similar but more extensive limestone formation was discovered in 1940 in northeastern Oklahoma. It originated in a shallow, saline lake, barren of life except for algae and bivalve crustaceans (Conchostraca). Most of the insects preserved there were presumably carried to the lake by floods.

The lithographic limestone of Bavaria, famous for such fossil vertebrates as the flying reptiles and the earliest birds, is not nearly so important for its insects. Several thousand specimens have been found there, but fewer than one-tenth of them are well preserved.

The richest of all deposits is the Baltic amber from Germany. The material is itself the fossil resin from an extinct pine tree (Pinites succinifera) . The Amber Pine Forest existed for Several million years during the early Tertiary period, and extended from about the site of Bornholm and Rugen in the south to that of the White Sea and Ural River in the east. The northern and western borders are uncertain because those regions are covered by the ocean. At any rate, the local accumulation of the amber along the coast of East Prussia is the result of the washing out of the flooded forest. Insects and other small invertebrates, which were caught in the resin on the tree trunks, are preserved in great detail and perfection. At least 150,000 insects have been found in the amber.

The earliest geological record of the insects is still uncertain. Fragments of small arthropods, which have been recovered in a Devonian chert in Scotland, have been determined by some entomologists as Collembola (springtails), but the identity will remain doubtful until more is known about them. The oldest unquestionable insects have been found in rocks of early Upper Carboniferous age, about 250 million years ago. Only three of these fossils are known one each from Czechoslovakia, Germany, and Pennsylvania and each consists of a single wing. Whatever else may be inferred from the specimens, it is certain that insects with fully developed wings existed then.

Insects are much more abundantly represented in the later Upper Carboniferous rocks than in the earlier ones, so that we have at least a working knowledge of the insect fauna of the time. Six orders of insects have been recognized, all but one of them extinct. The most interesting was the Palaeodictyoptera, which were of medium size and resembled mayflies. Since some of the Palaeodictyoptera were more generalized than any of the other winged insects known, the group as a whole is usually considered to be the ancestral stock from which all other winged insects have been evolved. As far as we know, all species of the order had a pair of membranous lobes on the first thoracic segment. The lobes appear to be homologous with the functional wings of the other two thoracic segments and are regarded as indicating the steps by which functional wings arose. Unfortunately nothing is known about the immature stages of the Palaeodictyoptera. The order reached its maximum development in the Carboniferous period but persisted through the Permian period.

The most spectacular insects of the Carboniferous and Permian were the Protodonata. They resembled dragonflies. Their chewing mouth parts were powerful, and their legs, like those of true dragonflies, were covered with strong spines. They were undoubtedly predaceous, catching their victims in flight and devouring them while resting on tree ferns or other ancient plants. All of the Protodonata were large and some were veritable giants, having a wing expanse of 3o inches and a body length of 15 inches. Specimens of such large species have been found in rocks in France, Kansas, and Oklahoma. Since birds and other flying vertebrates did not exist at that time, these huge insects presumably ruled the air. Their nymphs have not been found, but they were probably aquatic and like those of true dragonflies or damselflies.

The only living order or group of families of insects known to have existed in Carboniferous time is the Blattidae, or cockroaches. Their remains make up a high percentage of insects of that period, but that is probably due partly to the favorable conditions prevailing in the Carboniferous swamps that produced the deposits. Some formations of that period, such as the coal beds of Pennsylvania, have yielded no insects except roaches. The average size of the Carboniferous roaches was somewhat greater than that of living species, but none of the fossil forms exceeds in size certain living species of the Tropics. The difference between the ancient roaches, existing some 250 million years ago, and those of today is exceedingly slight, involving chiefly position of wing veins.

By the beginning of the Permian period, about 5o million years after the appearance of the first insects, a marked change had taken place in insects. Although the several extinct orders which arose in the Carboniferous still existed, several living orders besides the roaches were represented. Along with the giant dragonflies were minute barklice, only one-eighth of an inch across the wings.

Altogether, the lower Permian insect fauna was very diverse more so, in fact, than any other insect fauna known. There was about equal representation of the extinct orders of the Carboniferous and relatively specialized existing orders. Also adding to this diversity were several other extinct orders, known. only from Permian strata. One of them, the Protelytroptera, included beetle-like insects, having well-developed elytra, but they were closely related to the roaches and had no affinities with the Coleoptera. The living orders that appeared in early Permian time include such types as the Odonata (dragonflies), Ephemeroptera (mayflies), Corrodentia (barklice), Hemiptera (bugs), Neuroptera (lacewings), and Mecoptera (scorpionflies). The lacewings and scorpionflies are especially noteworthy because the living species have complete metamorphosis. Coleoptera and Plecoptera are first found in late Permian strata, but they probably existed earlier in the period.