Earth’s History Chapters 29 & 30

Paleontology is the study of the life that existed in Prehistoric times. Traces of ancient life (fossils) are the basis for the geologic time scale and an important part of the rock record.

A fossil is the remains, impression, or any other evidence of a plant or animal preserved in rock. Fossils include shells, bones, petrified trees, footprints, impressions made by leaves, or even burrows made by worms.

Fossilization usually occurs when a dead plant, animal, or impression (ex. footprint) is buried by sediments that gradually turn to rock. Fossils may form in several ways, from original remains or from replaced remains, by being preserved in either molds or casts, as trace fossils, or in carbonaceous film.

Types of fossilization A. Original remains: Fossils are rarely the original unchanged material of plants or animals, but in rare cases, an organism is preserved in its entirety. Example: amber

B. Replaced remains: Some fossils form as remains are slowly replaced, molecule by molecule, by rock forming minerals. Examples: Bones, teeth, shell, petrified wood

C. Molds and Casts: 1. A brachiopod has died and fallen into soft sediments. The hard shell remains after the soft body parts decay. 2. The sediments become rock. The shell decays, leaving a mold of the shell. 3. Minerals fill the mold, creating a cast of the original shell.

D. Trace fossils: When indirect evidence of life is preserved. Trace fossils include any impressions left in rock by an animal such as trails, footprints, tracks, burrows, and even bite marks on other fossils.

E. Carbonaceous films: Sometimes the only fossil trace is a thin carbon film resembling a silhouette.

Relative dating is the process of placing events in the sequence in which they occurred. Relative dating does not identify the actual dates on which the events occurred; it places them in order. 4th 3rd 2nd 1st 5th unconformity

Back scattered electron (BSE) imaging reveals multiple cross-cutting relationships which provide evidence that the meteorite experienced at least three distinct impacts prior to its arrival on Earth. This image shows that vein 1 is cut by vein 2, and both 1 and 2 are cut by vein

Index fossils are the remains of organisms that lived and died within a particular time segment of Earth’s history and can be used to correlate rock layers.

Absolute Time Radiometric dating: A technique that measures the ratio of radioactive decay a sample has gone through to determine an age. Examples: Radiocarbon Dating Uranium-Lead Dating Rubidium-strontium Dating Potassium-Argon Dating

Absolute Time # of half-lives elapsed Fraction remaining Percentage remaining 01/ /250 21/425 31/ / / / / Half-life: The time it takes for half the atoms in a sample of radioactive element to decay to a stable product. Question: If you have 20g of a radioactive isotope that has gone through two half lives, how many grams were in the original sample? Answer: 80g

4.6 BYA

The geologic time scale is a summary of major events in Earth’s past that are preserved in the rock record.

The geologic time scale is divided into eons, eras, periods, and epochs. The eon represents the longest segment of geological time. Eons are divided into eras. Eras are divided into periods. Periods are divided into epochs. (Shortest amount of time.)