Transitions: The Evolution of Life

Studying the evolution of insects can be difficult because they don’t fossilize well. But there are ways to study insect evolution. All life affects it’s environment in one form or another. In some cases the affect can be large, in others small. Occassionally, these affects remain behind long after the organism that caused them has died. Animal footprints, such as those of two dinosaurs below (from Glen Rose trackway), are good examples.

But organism leave other types of traces besides footprints. Below is a picture of a leaf. If you look closely you can see a black line zig-zagging around the leaf. This represents a track of an insect burrowing through the leaf and eating as it goes. The black bits are - well, I’ll leave that to your imagination.

The next picture shows what happens when an insect feeds on a leaf. The blackened edges, in this case, are caused by the plant trying to heal itself. The white parts (center middle) are plant cells that have become swollen and discolored due to the damage.

The above two picture are of fossil leaves found in Colorado and date to about 35 million years ago. Scientists interested in the study of insect evolution realized that different insects leave different kinds of damage and that the type of leaf damage could be used to identify the type of insect that caused the damage. You can go here for a quick overview on how this is done.
With this information you can learn a lot about how insects lived in the past. As one scientist puts it:

“Insect damage on leaves, the remains of insect meals, is uniquely valuable data,” … “While actual insect fossils can give us taxonomic information, leaf damage provides unique ecological data about which and how many kinds of insects were eating and interacting with ancient plant species in the deep past. Also, insect damage on fossil plants, which can be very abundant, can give us a great deal of information about insects at times and places with very few insect fossils.”

Recently, fossils in Patagonia were analyzed using the above ideas. Almost 3,600 plant fossils were collected and compared to fossils collected in North America. The fossils were examined for insect damage:

The researchers classified damage by feeding group and damage type. The four feeding groups are those insects that feed on the external leaf, chewing holes, edges and other leaf parts; those insects that mine tissues inside the leaf; those that produce bulbous galls and those that pierce and suck the leaves. Because different insects chew, mine, gall and pierce in different ways, the researchers recognized 52 discrete damage types from the four feeding groups. They applied these categories to both bulk samples from single quarries and to individual leaf species.

By comparing the Patagonian fossils with the North American fossils, researchers were able to learn about past environments and how past environments affected the number of different types of plants and insects in each area.

The current evidence from South America suggests that there were a large number of different insect lineages feeding on a large number of plant species.

Above is a 52 million year old fossil of a laurel leaf. The white circles with black centers represent the feedin activities of the fairy moth (pictured below)

“There was tremendous diversity and abundance of insects and plants in the Eocene,” … “Insects depend on plants to survive. If you have diverse plants, you get diverse animals. We know that plant and insect diversity are linked today and our study shows that plant and insect diversity were linked in the past as in today’s South America.”

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