Many people view the earth and its lifeforms as what we see today. However, both have changed dramatically through time; the evidence for this comes from something most of us see every day, but think little about: rock. Much of the rock in Wisconsin contains fossils, the remains of ancient organisms. After these organisms died, parts of their bodies were preserved in rock. Paleontologists—people who study ancient life on the basis of fossilized plants and animals—use them as clues about the Earth’s past. The fossils found in rock throughout Wisconsin were formed from creatures that lived in the warm, shallow seas that once covered the state.
When did the creatures that were to become fossils live? To appreciate fully just how long ago these organisms existed, it is helpful to develop an understanding of the concept of geologic time. Scientific methods for determining ages indicate that the Earth is about 4.6 billion years old. It is difficult for most of us to imagine such a vast amount of time, so geologists have developed a timetable that breaks geologic time into major units. The most expansive units, covering the longest amounts of time, are called eons. The next unit of time is the era, which is further subdivided into periods.
Geologic time is divided into three eons. From oldest to most recent, these eons are the Archean, Proterozoic, and Phanerozoic. The Archean and Proterozoic (sometimes collectively referred to as the Precambrian) encompass geologic history prior to 570 million years ago. Fossils representing these two eons, when the earliest forms of primitive life developed, are uncommon. The oldest known single-celled organisms—bacteria that were beautifully preserved in rock that is 3.5 billion years old—were discovered in Africa. Fossils of soft-bodied multicellular organisms about 700 million years old have been found in Australia’s Ediacara Hills.
The Phanerozoic has been divided into three principal eras. From oldest to youngest, they are the Paleozoic, Mesozoic, and Cenozoic.
Diversity of lifeforms and the complexity within them developed during the early part of the Phanerozoic, in the Paleozoic Era, 570 to 245 million years ago. This era has been divided into (from oldest to most recent) the Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, and Permian Periods. Many major groups of shell-bearing invertebrates (animals with hard shells and no backbones) appeared throughout the oceans during an immense proliferation of lifeforms at the beginning of the Paleozoic. This era ended with the largest extinction in Earth’s history: 80 percent of all types of marine invertebrates became extinct at the end of the Paleozoic, during the Permian Period.
The Mesozoic Era (245 to 66 million years ago) is also called the Age of Reptiles because of the extensive proliferation of land and sea reptiles. Dinosaurs were one of the dominant animal groups during the Mesozoic Era. Although Wisconsin contains only a sparse geologic record of the Mesozoic, we know from the fossil record in other areas that many marine and terrestrial animals, including dinosaurs, became extinct near the end of this era. Mammals and flowering plants became common during about the last two-thirds of the Mesozoic.
The Cenozoic Era encompasses the past 66 million years of Earth’s history. The most recent Ice Age began about 2.5 million years ago, during the Cenozoic.
Because fossils form under specific conditions, only a small percentage of organisms is preserved. Hard skeletal parts such as shells, bones, and teeth are the most commonly fossilized remains because they are more durable than the soft tissues, which decay rapidly.
Fossils are preserved in sedimentary rock, which is formed from what is essentially scrap material: weathering processes break up and erode rocks at the Earth’s surface, and wind and water carry away the scraps—pebbles, sand, silt, and clay. Many of these particles, called sediment, then settle on sea bottoms. As sea creatures die and also settle to the seafloor, they eventually are buried by sediment (see the figure on fossil formation).
Eventually, compaction of the sediment consolidates (solidifies) the sediment into layered rock, and, if conditions are right, the remains of organisms preserved in this rock become fossils.
Fossils can be preserved in such sedimentary rock types as sandstone, shale, limestone, and dolomite. To learn more about the specific way in which each rock type forms fossils, please see the publication Common Paleozoic Fossils of Wisconsin.
Conditions at a burial site determine how an organism is preserved. Fossils of marine invertebrates in Wisconsin are most commonly preserved as replacements or molds. Replacements are produced when minerals settle out of water and replace the original skeletal parts.
Molds are three-dimensional impressions left in rock after the skeletal parts of an organism dissolve. A mold can be either internal or external. The impression of the outside of a preserved shell is called an external mold; if minerals or sediment fill the space between two shells, but the shells dissolve, an internal mold is formed.
Under certain conditions, the remains of some ancient plants and animals become fossil fuels. Coal is formed from the accumulation and burial of plant material under conditions lacking oxygen, such as in swamps. Over millions of years, high pressures and temperatures force oxygen and hydrogen out of the remains, leaving carbon, which we burn as fuel. Natural gas and petroleum form similarly, but they may include animal remains as well and form under higher pressures and temperatures. About 85 percent of the energy used in the United States comes from these fossil fuels.