The history of life on our planet is a long and winding path. Much of what we know about how life has evolved comes from fossils. The physical remains of organisms and the evidence of behaviors, such as footprints, tell us where, how, and who ancient life was. Uncovering the fossil record takes a lot of detective work through the constant development and testing of hypotheses. This requires scientists with many different areas of expertise: biochemistry, statistics, biomechanics, ecology, and of course, geology. Understanding geology is critical to studying paleontology because fossils are preserved in rock. The characteristics of the rocks that hold fossils can be as informative as the fossils themselves.
Almost all fossils are preserved in sedimentary rock. This is primarily because sedimentary rock is the type most commonly formed at or near the surface of the earth, where all organisms live. There are some exceptions, such as lava, which can occasionally surround tree trunks and preserve them as casts, but these are pretty rare. The rock cycle is a concept geologists use to describe the interplay between the three main types of rock: metamorphic rock, igneous rock, and sedimentary rock. Metamorphic rock is formed under high heat and pressure, which generally destroys any fossils that were in the rock before it was metamorphosed. Marble is an example of metamorphic rock. Marble is created when limestone is pressure-cooked within the earth, causing the minerals that compose the stone to re-form into new crystals. Igneous rock is formed by the cooling of molten rock, either at the surface (lava), or underground (as granite or other similar rocks). Its extreme heat and molten nature generally makes igneous rock unsuitable for preserving organic material. Sedimentary rock is formed by the accumulation of minerals and other material. Think of a riverbed: silt, or thin soil, is carried through the water and deposited elsewhere, forming a layer. Over time, these layers build up and solidify under pressure, becoming sedimentary rock.
As rocks at the earth's surface break down by physical or chemical means, a process called weathering, the particles are moved by forces such as water and wind into low places where they collect. Organisms that live in topographically low places (such as lakes or ocean basins) have the best chance of being preserved. This is because they are already in locations where sediment is likely to bury them and shelter them from scavengers and decay. Mudstone, shale, and limestone are examples of sedimentary rock likely to contain fossils. The smaller the grains of sediment that make up the rock, the better they seal out oxygen, which means that more detail is likely to be preserved. As the layers of sediment build up on top of one another, they create a physical timeline. The oldest layers, along with the organisms that were fossilized as they formed, are deepest. The youngest layers are found at the top. Reading the layers is complicated by the fact that as continents move and mountains rise, the layers are often tipped sideways and altered in other ways.
Throughout the earth's history, each of the three rock types has been continuously formed and recycled into other types of rock. This makes the rock record fragmentary and difficult to read, but enough rock has been produced over time to tell us a lot about the history of our planet.
Geologic Time is a crucial concept to understanding the history of the earth – including the evolution of life. Many different representations of the timeline have been created, and many approaches developed by informed, trusted sources. We will provide a list of web sites with relevant information.
We are including several pages with Geologic Time Scale data for use in the classroom. We suggest using a threefold approach to connect with the investigations in the Fossil Kit.
Each group working on an investigation should have a time scale so that they can locate the time that is relevant to the set of fossils they are working with. As they work through the investigation, the group should come to a consensus about where on the time line they are.
Each student should have a copy of the time scale that they can use when they are working on the investigations. As they come to consensus with the group, they should locate the relevant time on their timeline and draw a picture of some of the plants and animals living at the time.
There should be a large shared time scale that is constructed by the class as a whole. This will be a starting point for discussion of the investigations the students have completed. Start with a blank sheet of kraft paper the height of the blackboard. The most important divisions for students to focus on are the Eras.
The Paleozoic, Mesozoic and Cenozoic are the eras during which there is life on earth. We have included charts for each of these, with images of some of the life forms that were abundant during these times.
Students should search for other images online, or draw pictures of plants and animals to place on the time scale. Thought should be given to both when an organism appears on the time scale and when it is gone because it has become extinct.
In addition to organisms, the images of the continents at different geologic time points can also be placed on the time scale. The complete set of these images can be found at: http://www.scotese.com/.