Older Than Dinosaurs

Arthropods were swimming in lakes, crawling on land, and flying through the air long before dinosaurs. In fact, millipedes are one of the oldest land animals on Earth and have been around for about four hundred million years. Insects are more than 380 million years old. Scientists know this by studying their fossils (FAH-suhls), or remains of animals that lived long ago, usually found set into rock or earth. Scientists who study fossils are called paleontologists (PAY-li-un-TA-luh-jists). Paleontologists study fossils to understand how life has developed and changed over time. The location and chemical makeup of fossils helps paleontologists to determine their age. By studying fossils scientists know that some groups of organisms, such as horseshoe crabs, millipedes, silverfish, and cockroaches, have changed very little over millions of years. The process of organisms changing over time is called evolution (EH-vuh-LU-shun). Organisms must adapt in form and behavior to survive in an environment that is also changing. Studying fossils not only reveals clues about the evolution of these ancient animals but also gives a glimpse of the environments in which they lived.

How to become a fossil

For arthropods, the process of fossilization (FAH-suhl-ih-ZAY-shun) begins when footprints, bodies, or body parts are quickly covered in mud or sand made up of fine particles. The finer the particles, the greater the detail preserved in the fossil. Under just the right circumstances, the mud and sand will be compressed, or squeezed, as more mud and sand settle on the remains, until it becomes rock. This process takes millions of years. Fossils are only impressions of the ancient animals. Their tissues are replaced, molecule by molecule, with surrounding minerals. In time, the remains of the arthropod are transformed physically and chemically and resemble the surrounding rock.

Some of the most detailed remains of ancient arthropods are preserved in hardened tree sap called amber (AM-bur). Amber comes from the sticky sap, or resin (REH-zin), of trees. Trees produce resin to heal wounds and to defend against insect borers. Insects, spiders, and other organisms became trapped and

Some of the most detailed remains of ancient arthropods are preserved in hardened tree sap called amber. (JLM Visuals. Reproduced by permission.)

Some of the most detailed remains of ancient arthropods are preserved in hardened tree sap called amber. (JLM Visuals. Reproduced by permission.)

completely encased in the sticky stuff. The resin quickly hardened and eventually fell to the ground, where it was buried by decomposing plants and soil. Storms washed the hardened resin into low-lying areas that were eventually covered by the ocean. These ancient sea bottoms eventually changed into layers of limestone and sandstone that are now filled with scattered bits and chunks of fossilized resin, or amber. Over millions of years, as new mountains and islands formed, the sea bottoms were lifted above sea level, exposing pieces of amber with the ancient remains of arthropods. The oldest known insect fossils in amber are about one hundred twenty million years old.

Only a tiny fraction of all the arthropods that ever lived during ancient times were preserved as fossils. The remains of species living on land were less likely to be preserved than those living in freshwater and marine habitats. Fossilization in stone or amber depends on animals dying in the right place, at the right time, and under the right circumstances. The odds of this all happening are extremely low. Even today arthropods are quickly eaten by other animals or decompose and break up within hours or days of their deaths.

Flights of fancy

Insects are one of only four groups of animals (with pterosaurs, birds, and bats) to have achieved true flight and were the first to take to the air. The power of flight gives many insects the opportunity to find food and mates over wide areas. Flying insects also have the ability to avoid being eaten by other animals and to colonize new and suitable habitats. Birds, bats, and pterosaurs all evolved wings from their forelimbs, or front legs, but insects did not have to give up a pair of legs to fly. So where did insect wings come from?

Based on the study of fossils, the first winged insects appeared between three hundred fifty and three hundred million years ago. Their wings may have evolved from flexible structures that were first used as gills for breathing underwater. Or they may have developed from stiff projections growing out of their midsection, or thorax, and eventually evolved into more flexible, winglike structures. But why did the first insect wings evolve? Did the oldest winged insects use them for gliding through their habitat, or did they use them as solar panels to collect heat to warm their bodies? The discovery of even older fossils of winged insects may help to unravel the mystery.

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