Evolution and extinction
You may notice that fossils of certain organisms have different morphologies, or forms, through time. This is because some organisms develop new features, like shells that are thicker or shaped differently. This process of change is called evolution. How and why these changes come about, or why they don’t, are the focus of much research.
Fossils also show us that many groups of creatures eventually become extinct. Trilobites, once plentiful in the Cambrian seas of Wisconsin, are extinct today, along with countless other creatures. Why are trilobites extinct, but direct descendants of other creatures that lived during the Paleozoic era, such as snails, abundant today? Some creatures survive during periods during which many other organisms become extinct. Does this happen because they are better adapted to their environment than those that became extinct, or are they just lucky? Paleontologists are working to answer these questions.
What was the area we now call Wisconsin like 400 million years ago? It is possible to determine the environmental conditions of the past on the basis of the organisms that lived in an area at a certain time. Using information from living relatives of extinct creatures and assuming that similar, related creatures lived in similar habitats, we can make inferences about the past. For example, one indicator of marine environments is coral. Today, corals and coral reefs require specific conditions to flourish—warm temperatures (25°C to 29°C [77°F to 84°F]), shallow depths (less than 56 meters [165 feet]), and normal salinity (the total quantity of dissolved salts in water; normal salinity is approximately 35 parts per thousand). The abundance of coral fossils in Wisconsin’s Silurian-age rock suggests that a warm, shallow sea of normal salinity covered Wisconsin during that time.
Many other organisms can be used to reconstruct aspects of Earth’s ancient environment as well. Knowing how life has responded to environmental changes in the past may help us understand how future environmental changes, such as global warming, will influence life on Earth.
Age of rocks
Fossils can generally be considered the same age as the rocks in which they are found. The age of rocks can be determined relatively or absolutely. Different rock layers can be dated relatively on the basis of where they fall in a “stack” of layers. For example, if you were to drop some sheets of paper on the ground, one on top of the other, it’s obvious that the ones on the top got there more recently than the ones on the bottom. In the same way, rock layers (and the fossils they contain) on top of others are relatively younger than the rock layers below, if the rock has not been overturned by geologic processes.
One way you may be able to determine the geological period (age range) from which you are collecting is the presence of index fossils in the rock layers. Index fossils are fossil groups that lived for very short, specific periods of time. That makes them more useful for relative dating of rocks than long-lived fossil groups because they allow you narrow down a rock’s age range. Geologists can use several index fossils together to make determinations about relative rock ages.
Absolute age of rocks can be determined through a process using radioactivity. By knowing the decay rate (half life) of an element in a rock sample, geologists and paleontologists can determine how much decay has occurred over time and therefore how much time has passed. Normally, scientists use a combination of relative and absolute dating to determine the age of rocks and fossils.