Most commonly, a generic name for all fossil resins, although it has been restricted by some to refer only to succinite, the mineralogical species of fossil resin that includes most of the Baltic Coast deposits. The time necessary to transform resin to amber (Fig. 1) is in the millions of years.

The establishment of the temporal sequence of prehistoric events. An event can be defined as a duration of time during which certain features came together to form a set, for example, when certain materials were formed into an object (manufacturing event) or when a set of objects were deposited at a single location (occupation event). An event can be dated only if the relevant features have properties that are dependent on time. A distinction is drawn often between dating events, which are those events that can be dated, and target events, which are the events of interest. Where these do not coincide, “bridging arguments” are required to link them.

The ongoing lower level of species extinction intensity occurring between episodes of mass extinctions. Extinction is a normal part of the history of life. Out of every 100 species that have ever lived on Earth, 99 are no longer in existence. However, occurrences of species extinction can be split into two categories: mass extinctions, in which substantial percentages of the global diversity of life disappear within a geologically brief interval; and background extinctions, which represent a much lower rate of species loss that occurs routinely over geologically long intervals between mass extinctions (see illustration). Background extinctions are ongoing consequences of normal environmental changes, local catastrophes, or interspecies competition. On the basis of the fossil record, evolutionary paleontologists have generally estimated that the baseline level of background extinction is approximately 1 extinction per million species per year (however, other analyses indicate that the rate might be as low as a tenth of that figure, so further investigations are necessary). Background extinction is mostly a local phenomenon. It befalls only one or a few species at any time, usually within a particular area. In contrast, hundreds or thousands of species can be affected worldwide during mass extinctions. Five mass extinctions have occurred during the past 550 million years on Earth, and investigators believe that the Earth is currently amid a sixth mass extinction, termed the Anthropocene extinction, caused predominantly by environmentally destructive human activities. See also: Anthropocene extinction; Biodiversity; Extinction; Extinction and the fossil record; Fossil; Geologic time scale; Macroevolution; Mass extinctions; Speciation; Species concept

Part of a clay and silt sequence that accumulated at the foot of a colossal “reef” during the Cambrian explosion, a dramatic evolutionary radiation of animals beginning about 545 million years ago. Although this explosion is most obvious from the geologically abrupt appearance of skeletons, the bulk of the radiation consisted of soft-bodied animals (Fig. 1). The Burgess Shale fauna, located near Field in southern British Columbia, is Middle Cambrian, approximately 520 million years old.

The now-buried remains of a large asteroid impact that occurred approximately 66 million years ago off the Yucatán Peninsula in Mexico, causing mass extinction on a global scale. Measuring approximately 200 km (124 mi) in diameter, the Chicxulub (pronounced CHIK-shoo-loob) impact crater (Fig. 1) is unique. It is the only known terrestrial impact structure that has been linked directly to a mass extinction event, and the only terrestrial impact with a global layer of ejecta (sediment material that is forced out of the Earth as a result of an asteroid impact, volcano discharges, or other explosions). Of the three largest impact structures on Earth, Chicxulub is the best preserved. Chicxulub is also the only known terrestrial impact structure with an intact, unequivocal topographic peak ring. Chicxulub's role in the Cretaceous–Paleogene (K–Pg) mass extinction [formerly called the Cretaceous–Tertiary (K–T) mass extinction] and its exceptional state of preservation make it an important natural laboratory for the study of (1) large impact crater formation on Earth and other planets, and (2) the effects of large impacts on the Earth's environment and ecology. See also: Asteroid; Impact; Mass extinctions; Paleoecology; Sedimentology

A group of extinct marine animals that are often abundant in strata of Late Cambrian to Late Triassic age, a time span of about 300 million years. Only the mineralized elements, which are usually 0.2 to 2 mm (0.008 to 0.08 in.) in dimension (the largest known reach 14 mm or 0.6 in.), are normally preserved. They are routinely extracted as isolated discrete specimens by chemical degradation of the rock in which they occur. The apatite (calcium phosphate) of which conodont elements are composed is laid down as lamellae. In the earliest euconodonts (“true” conodonts, as opposed to the more primitive and possibly unrelated, protoconodonts and paraconodonts), the elements comprise an upper crown and a basal body. The basal body occupies a cavity in the base of the crown, but is not present in the majority of post-Devonian species. In advanced conodonts the crown incorporates regular patches of opaque, finely crystalline, white matter.

The term (meaning terrible lizards) assigned to a group of certain large, ancient bipedal and quadrupedal reptiles that lived during the Mesozoic Era. The term Dinosauria [from the Greek deinos (fearful or terrible) and sauros (reptile or lizard)] was coined by the British comparative anatomist and paleontologist Sir Richard Owen in 1842 to represent three partly known, impressively large fossil reptiles from the English countryside: the great carnivore Megalosaurus, the plant-eating Iguanodon, and the armored Hylaeosaurus. Since that time, hundreds of other known fossil genera belonging to the Dinosauria group, commonly called the dinosaurs (Fig. 1), have been discovered. Paleontologists categorize the dinosaurs into two major lineages, based on their pelvic structure: the Saurischia ("lizard-hipped") and the Ornithischia ("bird-hipped"). These creatures roamed the Earth throughout the Mesozoic Era, initially appearing in the Triassic Period. The dinosaurs lasted for approximately 165 million years, before going extinct in the Cretaceous Period. It should be noted that modern birds are descendants of theropod dinosaurs; thus, in an evolutionary sense, not all dinosaurs are extinct and investigators often refer to the extinct dinosaurs as nonavian dinosaurs. See also: Aves; Fossil; Mesozoic; Ornithischia; Paleontology; Reptilia; Saurischia

The death and disappearance of a species. The extinction of a species usually represents an end point in a long series of population extinctions. During the extinction process, unique evolutionary history is lost at every stage, but the death of the last individual of a species represents the permanent loss of one of life's unique evolutionary and functional forms. Based on the fossil record, the species present today represent only 2–4% of all species that have ever lived. The species comprising the remainder are extinct, with the vast majority having disappeared long before the arrival of humans (Fig. 1). See also: Animal evolution; Evolution; Extinction (paleontology); Fossil; Macroevolution; Plant evolution; Speciation; Species concept

The death and disappearance of a species. The fossil record shows that extinctions have been occurring throughout the history of life. Mass extinctions refer to the loss of a large number of species representing broad taxonomic, ecologic, and geographic ranges in a relatively short period of time. Episodes of mass extinction occur at times of rapid global environmental change; five major events are known from the fossil record of the past 600 million years. Human activity is causing extinctions on a scale comparable to the mass extinctions in the fossil record. See also: Extinction; Fossil; Global climate change; Mass extinctions; Paleontology; Speciation

A record of earlier life buried in rock. Originally meaning any distinctive object that has been dug up (from Latin fodio, dig), the term “fossil” soon came to refer particularly to things resembling animals and plants. These objects were known before it was recognized that life on Earth has a long history. They were generally interpreted as having somehow grown in the rock or been placed there by a creator, or as being remains of organisms living somewhere else at the time in undiscovered seclusion. See also: Paleobotany; Paleontology