Fossils are the remains, imprints, or traces of prehistoric organisms. Formation of Fossils Most fossils are the hard parts of organisms, such as fossil teeth, or bones.

Most hard parts of organisms such as bones, teeth, and shells have tiny spaces within them. If the hard part is buried, groundwater can seep in and deposit minerals in the spaces. Types of Preservation—Mineral Replacement Permineralized remains are fossils in which the spaces inside are filled with minerals from groundwater.

Sometimes fossils contain only carbon. As sediment piles up, the organism’s remains are subjected to pressure and heat. Carbon Films These conditions force gases and liquids from the body. A thin film of carbon residue is left, forming a silhouette of the original organism called a carbon film.

The hard part might decay or dissolve, leaving behind a cavity in the rock called a mold. If mineral-rich water or other sediment enter the cavity, form new rock, and produce a copy or cast of the original object. Molds and Casts Impressions form when seashells or other hard parts of organisms fall into a soft sediment such as mud.

Trace fossils are fossilized tracks and other evidence of the activity of organisms. In some cases, tracks can tell you more about how an organism lived than any other type of fossil. Trace Fossils

Index fossils are remains of species that existed on Earth for relatively short periods of time, were abundant, and were widespread geographically. Because the organisms that became index fossils lived only during specific intervals of geologic times, geologists can estimate the ages of the rock layers based on the particular index fossils they contain.

Superposition—Oldest Rocks on the Bottom According to the principle of superposition, in undisturbed layers of rock, the oldest rocks are on the bottom and the rocks become progressively younger toward the top.

Relative age determination doesn’t tell you anything about the age of the rock layers in actual years. Relative Ages The relative age of something is its age in comparison to the ages of other things. Geologists determine the relative ages of rocks and other structures by examining their places in a sequence.

Unconformities Most rock sequences are incomplete—layers are missing. These gaps in rock sequences are called unconformities. Unconformities develop when agents of erosion such as running water or glaciers remove rock layers by washing or scraping them away.

Absolute Ages Absolute age is the age, in years, of a rock or other object. Geologists determine absolute ages by using properties of the atoms that make up materials.

Radioactive Decay Some isotopes are unstable and break down into other isotopes and particles. Sometimes a lot of energy is given off during this process. The process of breaking down is called radioactive decay.

Half-Life The half-life of an isotope is the time it takes for half of the atoms in the isotope to decay. After two half-lives, one fourth of the original isotope still remain. After three half-lives, one eighth of the original isotope still remain. As time passes, the amount of parent isotope in a rock decreases as the amount of daughter product increases.

The longest subdivisions— eons—are based upon the abundance of certain fossils. Major Subdivisions of Geologic Time The division of Earth’s history into time units makes up the geologic time scale. Next to eons, the longest subdivisions are the eras, which are marked by major, striking, and worldwide changes in the types of fossils present.

Periods are units of geologic time characterized by the types of life existing worldwide at the time. Periods can be divided into smaller units of time called epochs. Eras are subdivided into periods. Major Subdivisions of Geologic Time Epochs are also characterized by differences in life-forms, but some of these differences can vary from continent to continent.

Natural Selection Charles Darwin was a naturalist who sailed around the world from 1831 to 1836 to study biology and geology. He proposed that natural selection is a process by which organisms with characteristics that are suited to a certain environment have a better chance of surviving and reproducing than other organisms.

The exoskeleton of a trilobite consists of three lobes that run the length of the body. Trilobites The trilobite’s body also has a head (cephalon), a segmented middle section (thorax), and a tail (pygidium).

Precambrian time is the longest part of Earth’s history and includes the Hadean, Archean, and Proterozoic Eons. Precambrian time lasted from about 4.5 billion years ago to about 544 million years ago.

Early Life Cyanobacteria are blue-green algae thought to be one of the earliest forms of life on Earth. They contained chlorophyll and used photosynthesis. During photosynthesis they produced oxygen, which helped oxygen become a major atmospheric gas.

The Paleozoic Era, or era of ancient life, began about 544 million years ago and ended about 248 million years ago. The Paleozoic Era An abundance of organisms with hard parts, such as shells, marks the beginning of the Paleozoic Era. Many of the life-forms scientists know about were marine, meaning they lived in the ocean. Vertebrates, or animals with backbones, also evolved during this era.

Reptile’s skin helps it retain bodily fluids. This characteristic, along with their shelled eggs, enabled reptiles to adapt readily to the drier climate of the Mesozoic Era. The Mesozoic Era: Reptiles Rule Reptiles became the most conspicuous animals on land by the Triassic period.

Dinosaurs ranged in height from less that 1 m to enormous creatures like Apatosaurus and Tyrannosaurus. Throughout the Mesozoic Era, new species of dinosaur evolved and other species became extinct. Dinosaurs and Gymnosperms During most of the Mesozoic Era, gymnosperms dominated the land. Gymnosperms are plants that produce seeds but not flowers.

The Mesozoic Era ended about 65 million years ago with a major extinction of land and marine species. End of an Era Many paleontologists hypothesize that a comet or asteroid collided with Earth, causing a huge cloud of dust and smoke to rise in the atmosphere, blocking out the Sun.

The Himalayas formed as India moved northward and collided with Asia. Cenozoic Era: Times of Mountain Building Mammals and angiosperms became the new dominant life-forms The Cenozoic Era, or era of recent life, began about 65 million years ago and continues today.