| 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 |
| paleontology | the study of ancient life |
| sedimentary rock | The type of rock that is made of hardened sediment |
| 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 minerals seep into a mold |
| carbon film | A type of fossil consisting of an extremely thin coating of carbon on rock |
| trace fossils | A type of fossil that provides evidence of the activities of ancient organisms |
| scientific theory | A well-tested concept that explains a wide range of observations |
| evolution | The process by which all the different kinds of living things have changed over time |
| extinct | Describes a type of organism that no longer exists anywhere on Earth |
| relative age | The age of a rock compared to the ages of rock layers |
| 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 |
| uniformitarianism | the present is the key to the past |
| 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 or crack in Earth’s lithosphere along which the rocks move. |
| unconformity | A place where an old, eroded rock surface is in contact with a newer rock layer |
| index fossils | Fossils of widely distributed organisms that lived during only one short period |
| atom | The smallest particle of an element |
| element | A type of matter in which all the atoms are the same |
| radioactive decay | The breakdown of a radioactive element, releasing particles and energy |
| half-life | The time it takes for half of the atoms of a radioactive element to decay |
| geologic time scale | 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 |
| invertebrate | An animal without a backbone |
| period | One of the units of geologic time into which geologists divide eras |
| epochs | Subdivisions of the periods of the geologic time scale |
| vertebrate | An animal with a backbone |
| amphibian | A vertebrate that lives part of its life on land and part of its life in water |
| reptile | A vertebrate with scaly skin that lays eggs with tough, leathery shells |
| mass extinction | When many types of living things become extinct at the same time |
| mammal | A warm-blooded vertebrate that feeds its young milk |
