adaptations Changes in a lineage of organisms in response to environmental stress. Amniota Vertebrate group that includes the most recent common ancestors of living reptiles, mammals, birds, and all their descendants. amniote A tetrapod that protects the embryo of its offspring within the sealed environment of an amniotic egg; reptiles, mammals, and birds are amniotes.

amphibians Partly aquatic tetrapods that must return to the water to reproduce; amphibians hatch from eggs in the water and live a fully aquatic lifestyle until they reach sexual maturity. Anapsida (diapsids) Amniotes with no temporal fenestrae, including the earliest reptiles. anatomy Term used to describe the basic biological systems of an animal, such as the skeletal and muscular systems. angiosperms The flowering plants.

anterior Directional term indicating the head end of a vertebrate, also known as the cranial end. Archosauria The branch of diapsid reptiles that includes dinosaurs, pterosaurs, crocodiles, birds, and their kin. arthropods (Arthropoda) Animal phylum whose members have a segmented body, body regions dedicated to specific functions, a jointed exoskeleton, and a nervous system on the underside of the body; arthropods include trilobites, crabs, lobsters, brine shrimp, barnacles, insects, spiders, scorpions, and centipedes. basal At the base or earliest level of evolutionary development; a term usually used to refer to an ancestral taxon. biramous Word used to describe double--branched arthropod limbs. buccal pumping Method of breathing used by amphibians, in which air is sucked into the throat and then squeezed down the throat into the lungs by raising the floor of the mouth. clade A group of related organisms including all the descendants of a single common ancestor climate The kind of weather that occurs at a particular place over time. conducting system System of veins in a vascular plant used to transport water and food. continental drift The slow and gradual movement and transformation of the continents due to shifting of the tectonic plates of Earth's crust. coprolite Fossilized animal feces.

costal ventilation Method of breathing in tetrapods in which abdominal muscles and the rib cage expand and contract the lungs, forcing air in and out. craton The large, tectonically stable interior of a continent. cuticle The waxy outer covering of a plant.

derived Term used to describe a trait of an organism that is a departure from the most primitive, or basal, form. dermal Pertaining to the skin, as in dermal armor. desiccation Water loss in the body due largely to evaporation. Diapsida (diapsids) Amniotes with two temporal fenestrae: a lower one like the one seen in synapsids and an upper one just above the lower one and behind the orbit. ecosystem A population of all living organisms and the environment in which they live. ectothermic Word used to describe a cold--blooded creature that regulates its body temperature by absorbing heat from its environment.

endoskeleton An internal skeleton, usually consisting of bones, as is found in vertebrates. endothermy The state of being warm-blooded. era A span of geologic time ranking below the eon; the Archean Eon is divided into four eras dating from more than 4 billion years ago to 2.5 billion years ago; the Proterozoic Eon is divided into three eras dating from 2.5 billion years ago to about 542 million years ago; the Phanerozoic Eon is divided into three eras, the Paleozoic, the Mesozoic, and the Cenozoic; the Paleozoic ("ancient life") Era lasted from 542 million to 251 million years ago; the Mesozoic ("middle life") Era lasted from 251 million to 65 million years ago; the Cenozoic ("recent life") Era began 65 million years ago and continues to the present. Eukarya One of the three domains of living organisms; it includes four kingdoms—Protista, Fungi, Plantae, and Animalia—all of which consist of multicelled organisms with a distinct cell structure whose nucleus contains strands of DNA.

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eukaryotes Multicelled organisms with a distinct cell structure whose nucleus contains strands of DNA. Euryapsida (euryaspids) Amniotes with one temporal fenestra positioned just above and behind the orbit. evolution The natural process that causes species to change gradually over time; evolution is controlled by changes to the genetic code—the DNA—of organisms. exoskeleton A skeleton that forms on the outside of the body, as in invertebrates such as anthropods. extant Term used to describe an organism that is living today. extinction The irreversible elimination of an entire species of plant or animal because the species cannot adapt effectively to changes in its environment. fauna Animals found in a given ecosystem. forelimbs The two front legs of a vertebrate. fossil Any physical trace of prehistoric life.

gene A microscopic unit on a DNA molecule that controls inherited traits.

genera A taxonomic name for one or more closely related organisms; in taxonomic classification, genera rank below families and above species; the singular of the plural genera is genus. gigantism Inherited traits for unusually large growth, made possible through natural selection, for a taxon or taxa of organisms. global chemostat A complex, self--regulatory system by which the ocean naturally maintains its optimum chemical balance. greenhouse effect The trapping of reflected solar radiation by water vapor in clouds, ozone in the lower atmosphere, and atmospheric methane and carbon dioxide (CO2) gas. gymnosperms Seed plants, such as conifers, that have a protective cone or other body for their seed embryos. herbivory The eating of plants and the anatomical adaptations that make this possible. hind limbs The two rear legs of a vertebrate.

index fossils Fossils that are widely distributed and easily recognized but that are restricted to certain geologic strata; these qualities make such fossils useful for dating related stratigraphic layers around the Earth. Lepidosauria A group of diapsid reptiles that includes lizards, snakes, two species of Sphenodon—the lizardlike tuatara of New Zealand— and their extinct kin.

lycopods A taxon of early seedless vascular plants that grew to tree height.

mass extinction An extinction event that kills off more than 25 percent of all species in a million years or less. microbiotic crusts Matlike, terrestrial communities of fungi, algae, and bacteria whose filamentous structures trap small particles of sand and silt to form a soil surface. morphological Pertaining to the body form and structure of an organism.

natural selection One of Charles Darwin's observations regarding the way evolution works; given the complex and changing conditions under which life exists, those individuals with the most favorable combination of inherited traits may survive and reproduce while others may not.

nonvascular plant A plant that lacks vascular or conducting tissue for transporting water and food. notochord A stiff rod running along the back of an organism, found in members of the phylum Chordata. orbit An opening for the eye in the skull. organism Any living plant, animal, bacterium, archaebacterium, protist, or fungus.

ozone A natural filter in Earth's lower atmosphere that prevents most of the deadly ultraviolet radiation from the Sun from striking the surface ground; ozone is made by a reaction that takes place when ultraviolet radiation from the Sun strikes oxygen molecules, splitting them apart and recombining with other elements in the atmosphere to form a protective gaseous band around the planet. paleontologist A scientist who studies prehistoric life, often using fossils. period A span of geologic time ranking below the era; the Phanerozoic Eon is divided into three eras and 11 periods, each covering a span of millions of years; the longest of these periods, including the three in the Mesozoic Era, are sometimes further broken down into smaller divisions of time. photosynthesis A metabolic process in which an organism's cells convert energy from the Sun, carbon dioxide, and water to reproduce their cells; the waste product of photosynthesis is free oxygen released into the atmosphere. physiology The way in which an animal's parts work together and are adapted to help the organism survive.

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population Members of the same species that live in a particular area. predator An animal that actively seeks and feeds on other live animals. progymnosperms Seedless vascular plants that reproduced by spores like seedless ferns but that had a woody bark as a protective covering like typical conifers. seedless vascular plants Vascular plants that reproduce by means of spores instead of seeds. species In classification, the most basic biological unit of living organisms; members of a species can interbreed and produce fertile offspring.

stapes A bone of the middle ear of vertebrates that aids in hearing. stomata A network of tiny pores on the outside surface of a plant for collecting oxygen from the air; the stomata are part of the gas-exchange system of a plant that makes photosynthesis possible. Synapsida (synapsids) Amniotes with one temporal fenestra positioned somewhat behind and below each orbit. taxon (plural: taxa) In classification, a group of related organisms such as a clade, genus, or species. taxonomy The science of classifying living and extinct species of organisms.

temporal fenestrae Openings, or "windows," in the vertebrate skull, just behind the orbit on each side, or temple region, of the skull. tetrapods Vertebrate animals with four limbs, or their evolutionary descendants that have modified or lost limbs, including all amphibians, reptiles, mammals, and birds. Tetrapoda A group that includes the common ancestor of all living tetrapods and all of its descendants. Therapsida (therapsids) A group of synapsid reptiles, including the ancestors of the first mammals. thermoregulation The control of body temperature. trace fossil A type of fossil that preserves evidence of the presence of a prehistoric organism but does not include body parts; fossilized trackways or feces are examples of trace fossils. tracheal system The respiratory system of a terrestrial organism, used to consume oxygen from the air. trackway The fossilized footprints or markings left by a prehistoric animal.

transitional Word used to describe a fossil that represents intermediate conditions in the evolution of a species.

ultraviolet solar radiation Ultraviolet light emitted by the Sun that can be harmful to organisms even if it is not screened by Earth's ozone layer.

uniramous Word used to describe single--branched arthropod limbs. vascular plant A plant that has a conducting system of veins to transport water and food throughout its body.

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