Building the Fossil Case
1The entomology club at Larchmont Middle School studies insects. Members raise mealworms, keep a beehive with help from a local beekeeper and pin their best finds in glass cases. This spring, the club's adviser, Ms. Celestine Obuya, borrowed a set of insect fossil casts from the state geological survey. Club members Mirela Kovač and Tobin Achterberg are building a hallway case called "Six Legs Through Time" that puts the casts in order from oldest to youngest.
2Insects are hard to fossilize, because most are small and their bodies break down quickly. The best insect fossils formed where a body was buried fast in fine mud at the bottom of a lake, or where an insect was trapped in sticky tree resin that later hardened into amber. The ages of the rocks and amber come from the layers they lie in and from measurements of certain elements in nearby volcanic ash. Scientists give these ages in millions of years ago (mya). Often a single wing is all that is left, but that can be enough, because the pattern of veins in a wing differs from one insect group to another. The oldest insect fossils known are about 400 million years old.
3Table 1 lists the oldest and youngest known fossils of seven insect groups. Griffinflies were relatives of today's dragonflies. Some had wings that spread about 70 cm from tip to tip, more than three times the wingspan of any living dragonfly. The oldest known fossils of flowering plants are about 130 million years old.
4About 252 million years ago, Earth went through the largest mass extinction in its history, often called the Great Dying. Most kinds of sea animals and many kinds of land animals and plants disappear from the fossil record at that time.
5Scientists sort insects into large groups called orders. Beetles are one order, true flies are another, and so are moths and butterflies. Tobin found a chart of how many insect orders are known from fossils at different times, and the club decided to hang it above the case as a graph. Mirela added a note: the counts depend partly on how many rocks of each age have been searched for insect fossils.