Since insects lack bones, they didn't leave behind skeletons for paleontologists to unearth millions of years later. How do scientists learn about ancient insects without fossilized bones to study? They examine the abundant evidence found in the different types of insect fossils described below. For the purpose of this article, I've defined a fossil as any preserved physical evidence of insect life from a time period prior to recorded human history.
Much of what we know about prehistoric insects is derived from evidence trapped in amber, or ancient tree resin. Because tree resin is a sticky substance – think of a time when you've touched pine bark and come away with sap on your hands – insects, mites, or other tiny invertebrates would quickly become trapped upon landing on the weeping resin. As the resin continued to ooze, it would soon encase the insect, preserving its body.
Amber inclusions date as far back as the Carboniferous period. Scientists can also find preserved insects in resin dated just a few hundred years old; these resins are called copal, not amber. Because amber inclusions form only where trees or other resinous plants grew, the insect evidence recorded in amber documents the relationship between ancient insects and forests. Put simply, insects trapped in amber lived in or near wooded areas.
If you've ever pressed your hand into a freshly poured bed of cement, you've created the modern equivalent of an impression fossil. An impression fossil is a mold of an ancient insect, or more often, a part of an ancient insect. The most durable parts of the insect, the hard sclerites and wings, comprise the majority of impression fossils. Because impressions are just a mold of an object that was once pressed in the mud, and not the object itself, these fossils assume the color of the minerals in which they are formed.
Typically, insect impressions include only a mold of the wing, frequently with sufficiently detailed wing venation to identify the organism to order or even family. Birds and other predators that might have eaten the insect would find the wings unpalatable, or perhaps even indigestible, and leave them behind. Long after the wing or cuticle has decayed, a copy of it remains etched in stone. Impression fossils date back to the Carboniferous period, providing scientists with snapshots of insect life from up to 299 million years ago.
Some fossil evidence formed when the insect (or part of the insect) was physically compressed in sedimentary rock. In a compression, the fossil contains organic matter from the insect. These organic residues in the rock retain their color, so the fossilized organism is conspicuous. Depending on how coarse or fine the mineral comprising the fossil is, an insect preserved by compression may appear in extraordinary detail.
Chitin, which makes up part of the insect's cuticle, is a very durable substance. When the rest of the insect body decays, the chitinous components often remain. These structures, such as the hard wing covers of beetles, comprise most of the fossil record of insects found as compressions. Like impressions, compression fossils date back as far as the Carboniferous period.
Paleontologists describe dinosaur behavior based on their study of fossilized footprints, tail tracks, and coprolites – trace evidence of dinosaur life. Similarly, scientists studying prehistoric insects can learn a great deal about insect behavior through the study of trace fossils.
Trace fossils capture clues to how insects lived in different geologic time periods. Just as hardened minerals can preserve a wing or cuticle, such fossilization can preserve burrows, frass, larval cases, and galls. Trace fossils provide some of the richest information about the co-evolution of plants and insects. Leaves and stems with obvious insect feeding damage comprise some of the most abundance fossil evidence. The trails of leaf miners, too, are captured in stone.
Younger fossils – if one can call 1.7 million year old fossils young – are recovered from sediment traps representing the Quaternary period. Insects and other arthropods immobilized in peat, paraffin, or even asphalt were entombed as layers of sediment accumulated over their bodies. Excavations of such fossiliferous sites often yield tens of thousands of beetles, flies, and other invertebrates. The La Brea tar pits, located in Los Angeles, is a famous sediment trap. Scientists there have excavated well over 100,000 arthropods, many of them carrion feeders that were preserved along with the large vertebrate carcasses on which they fed.
Sediment traps provide scientists with more than a catalogue of species from a certain geological time frame. Quite often, such sites also offer evidence of climate change. Many, if not most, of the invertebrate species found in sediment traps are extant. Paleontologists can compare their fossil finds with the current known distributions of living species, and extrapolate information about the climate at the time those insects were entombed. Fossils recovered from the La Brea tar pits, for example, represent terrestrial species that inhabit higher elevations today. This evidence suggests the area was once cooler and moister than it is now.
In some fossil beds, paleontologists find perfect mineralized copies of insects. As the insect's body decayed, dissolved minerals precipitated out of solution, filling the void left as the body disintegrated. A mineral replication is an accurate and often detailed 3-dimensional replica of the organism, in part or whole. Such fossils typically form in places where water is rich with minerals, so animals represented by mineral replications are often marine species.
Mineral replications give paleontologists an advantage when excavating fossils. Because the fossil is usually formed of a different mineral than the surrounding rock, they can often dissolve the outer rock bed to remove the embedded fossil. For example, silicate replications can be extracted from limestone using an acid. The acid will dissolve the calcareous limestone, leaving the silicate fossil unscathed.