| A | B |
|---|
| Fossil | Preserved remains or traces of an organism that lived in the past |
| Mold | A type of fossil that is a hollow area in sediment in the shape of an organism or part of an organism |
| Cast | A fossil that is a solid copy of an organism’s shape, formed when minerals seep into a mold |
| Petrified Fossil | A fossil in which minerals replace all or part of an organism |
| Carbon Film | A type of fossil consisting of an extremely thin coating of carbon on rock |
| Trace Fossil | A type of fossil that provides evidence of the activities of ancient organisms |
| Paleontologist | A scientist who studies fossils to learn about organisms that lived long ago |
| Evolution | Change over time; the process by which modern organisms have descended from ancient organisms |
| Extinct | Term used to refer to a group of related organisms that has died out and has no living members |
| Relative Age | The age of a rock compared to the ages of other rocks |
| Absolute Age | The age of a rock given as the number of years since the rock formed |
| Law of Superposition | The geologic principle that states that in horizontal layers of sedimentary rock, each layer is older than the layer above it and younger than the layer below it |
| Extrusion | An igneous rock layer formed when lava flows onto Earth’s surface and hardens |
| Intrusion | An igneous rock layer formed when magma hardens beneath Earth’s surface |
| Fault | A break in Earth’s crust along which rocks move |
| Index Fossil | Fossils of widely distributed organisms that loved during a geologically short period |
| Unconformity | A gap in the geologic record that shows where rock layers have been lost due to erosion |
| Radioactive Decay | The process in which the nuclei of radioactive elements break down, releasing fast-moving particles and energy |
| Half-life | The time it takes for half of the atoms of a radioactive element to decay |
| Geologic Time Scale | A record of the geologic events and life forms in Earth’s history |
| Era | One of the three long units of geologic time between the Precambrian and the present |
| Period | One of the units of geologic time into which geologists divide eras |
| Comet | A loose collection of ice and dust that orbits the sun, typically in a long, narrow orbit |
| Invertebrate | An animal without a backbone |
| Vertebrate | An animal with a backbone |
| Amphibian | A vertebrate whose body temperature is determined by the temperature of its environment, and that lives its early life in water and its adult life on land |
| Reptile | A vertebrate whose temperature is determined by the temperature of its environment, that has lungs and scaly skin, and that lays eggs on land |
| Mass Extinction | When many types of living things become extinct at the same time |
| Mammal | A vertebrate whose body temperature is regulated by its internal heat, and that has skin covered with hair or fur and glands that produce mild to feed its young. |
