| A | B |
|---|
| fossil | The preserved remains or traces of living things. |
| paleontologist | A scientist who studies fossils to learn about organisms that lived long ago |
| sedimentary rock | The type of rock that is made up hardened sediment, most fossils are found in this type of rock. |
| petrified fossil | A fossil in which minerals replace all or part of an organism. |
| mold | A fossil formed when an organism buried in sediment dissolves, leaving a hollow area. |
| cast | A fossil that is a copy of an organism’s shape, formed when sediment and minerals seep into a mold and harden. |
| 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. |
| evolution | The theory that states that all different kinds of living things have gradually changed over time. |
| extinct | Describes a type of organism that no longer exists anywhere on Earth. |
| scientific theory | A well-tested concept that explains a wide range of observations. |
| relative age | is a rock’s age compared to the ages of other rocks |
| absolute age | is the number of years since the rock formed |
| law of superposition | according to this law, in horizontal sedimentary rock layers the oldest is at the bottom |
| extrusion | lava that hardens on the surface |
| intrusion | magma that hardens into a mass of igneous rock beneath the surface |
| unconformity | a gap in the geological record |
| index fossils | tell the relative ages of the rock layers in which they occur, must have been widely distributed and represent an organism that lived only briefly in time |
| radioactive decay | atoms of one element break down to form atoms of another |
| half-life | the time it takes for half of the radioactive atoms to decay in an element |
| carbon-14 dating | used to date more recent fossils, tells its age, breaks down into stable nitrogen-14 |
| eras | time between Precambrian Time and the present, divided into three long units of time |
| periods | are eras subdivided into units of geological time |
| epochs | periods of the Cenozoic Era subdivided further by geologists |
| Precambrian time | the longest period of time in the Geologic Time Scale, with very few fossils due to the simple soft-bodied organisms from this time |
