How Do Fossils Form

12.1 The Fossil Record
1. Form in Several Ways (The following are some of the processes that make fossils.) a. Permineralization minerals carried by water are deposited around a hard structure (may replace the hard structure) The most common fossils result from this process.
A1. Natural Casts form by water removing the bone/tissue, leaving an impression in sediment. Minerals fill in the mold, recreating the original shape of the organism
A2. Trace fossils record activity of an organism. Ex. nests, burrows, Imprints, footprints
A3. Amber-preserved fossils are organisms that become trapped in tree resin that hardens into amber after the tree gets buried underground
A4. Preserved remains form when an entire organism becomes encased in
material such as ice or volcanic ash or immersed in bogs.
b. Most fossils form in sedimentary rock (layers of sediment). The best environments for fossilization include wetlands, bogs, and where sediment is continuously deposited Ex. river mouths, lakebeds, and floodplains
c.
For Permineralization, the organism must be buried or encased in some type of material (sand, sediment, mud, tar) soon after death while the organism’s features are still intact. Then groundwater trickles into tiny pores and spaces. The excess minerals in the water are deposited on the remaining tissues. The mineral deposits are left behind making a fossilized record to replace the organic tissues with hard minerals. It has the same shape as the original structure and may keep tissue.
2. Radiometric Dating Provides an Accurate Estimate of a Fossil’s Age. a. Relative Dating estimates the time during which an organism lived. It’s done by comparing the placement of fossils of that organism with the placement of fossils in other layers of rock. b. Radiometric Dating is a technique that uses the natural decay rate of unstable isotopes found in materials in order to calculate the age of that material. (Done to calculate actual age of Fossil) c. Isotopes are atoms of an element that have the same number of protons, but a different number of neutrons. Isotopes are named by their number of protons plus their number of
neutrons.
C1. Isotopes with unstable nuclei have their nuclei undergo radioactive decay (break down) over time. This releases radiation in the form of particles and energy. A decaying isotope may turn into a new element.. A half-life is the amount of time it takes for half of the isotope in a sample to decay into a different element, or its product isotope
3. Radioactive Dating
a.
A fossil’s age can be estimated by comparing the ratio of a stable isotope, to a radioactive isotope. Carbon-14 dating can be used to date objects only up to about 45,000 years old. Older objects can be dated using isotopes with longer half-lives.
b. Scientists have used radiometric dating to determine the age of Earth by using meteorites
12.2 The Geologic Time Scale
1. Index Fossils Are Another Tool to Determine the Age of Rock Layers a. Index Fossils are fossils of organisms that existed only during specific spans of time over large geographic areas. b. The best Index fossils are common, easy to identify, found widely around the world, and only existed for a relatively brief time.
2. The Geologic Time Scale Organizes Earth’s History a. The Geologic Time Scale is a representation of the history of Earth. It organizes Earth’s history by major changes or events that have occurred, using evidence from the fossil and geologic records. b. The time scale is divided into a series of units based on the order in which different groups of rocks and fossils were formed. It consists of 3 basic units of time: (Periods are most common)
B1. Eras last tens to hundreds of millions of years and consist of two or more
periods
B2. Periods last tens of millions of years. Each period is associated with a particular type of rock system.
B3. Epochs are the smallest units of geologic time and last several million years.