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A diverse group of early jawed fishes from the Paleozoic Era, usually classified as a distinct class of vertebrates. Most acanthodians have tiny body scales with a cross section like that of an onion, the layers reflecting incremental scale growth.…

An order of actinopterygian (ray-finned) fishes represented by the sturgeons and paddlefishes. The order Acipenseriformes, the only extant order of about 10–12 orders of chondrosteans (subclass Chondrostei), probably originated from palaeonisciform…

An order of inarticulated brachiopods. Together with Lingulida and Siphonotretida, Acrotretida is one of the three orders of inarticulated brachiopods currently classified in class Lingulata in subphylum Linguliformea (phylum Brachiopoda).…

A class of teleostom fishes commonly known as the ray-finned fishes, which are distinguished by the structure of the paired fins that are supported by dermal rays. The class Actinopterygii comprises the subclasses Cladistia [Polypteriformes (bichirs…

An order of extremely elongate, limbless fossil amphibians in the subclass Lepospondyli from Permo-Carboniferous rocks of North America and Europe. The order Aistopoda (also spelled Aïstopoda) consists of extremely elongate, limbless fossil…

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. Resins generally are complex mixtures…

An extinct subclass of Cephalopoda (phylum Mollusca). Besides the Ammonoidea (chambered ammonoids), the other subclasses of Cephalopoda are Nautiloidea (chambered nautiloids) and Coleoidea (squid, cuttlefish, octopus, and extinct belemnites), both…

One of four classes (the others are Reptilia, Aves, and Mammalia) composing the superclass Tetrapoda of the subphylum Vertebrata. The living amphibians number over 4700 species and are classified in three orders: Salientia or Anura (frogs and toads,…

Formerly, a group of reptiles that was recognized in rank-based classifications. Amniotes were once divided into subclasses, based on the patterns of temporal fenestrae (openings in the skull roof posterior to the openings for the eyes). Subclass…

A group of fossil jawless fishes, roughly similar in appearance to extant lampreys, known from Early Silurian–Early Devonian deposits (about 410–430 million years ago) that represent marine, coastal environments. Anaspid fossils are known…

The use of live nonhuman animals for scientific research and experimentation. Animal testing has been a cornerstone of biomedical research, having contributed greatly to the understanding of human biology, to the unraveling of the complex underlying…

The phylum comprising the multisegmented, invertebrate wormlike animals, of which the most numerous are the marine bristle worms and the most familiar the terrestrial earthworms. The Annelida (meaning little annuli or rings) include the Polychaeta…

An order of Paleozoic tetrapods that arose Carboniferous (Mississippian) (345 million years ago) and lasted until the Early Triassic (about 245 MYA). The term anthracosaurs (meaning “coal reptiles”) encompasses a range of genera that may not all be…

The smallest order of the class Amphibia, consisting of the wormlike caecilians. The order Apoda is sometimes called Gymnophiona. Its members are known commonly as caecilians, which are wormlike, legless amphibians with indistinct or even hidden…

An order of Paleozoic diapsid reptiles including the families Petrolacosauridae and Araeoscelidae. Members of these families resemble primitive modern lizards, such as the green iguana, in size and most body proportions, but are distinguished by…

An extinct group of mainly Lower Cambrian marine sponges which, although lacking spicules, possessed an intricate, highly porous skeleton of calcite. It was probably a monophyletic group; that is, all representatives were derived from a single…

An order of extinct, free-sporing plants (cryptogams) that lived during the Late Devonian through Early Mississippian. The order includes the genera Archaeopteris (which has been extensively investigated), Actinoxylon, Actinopodium, Svalbardia,…

A genus of fossil birds in the extinct order Archaeopterygiformes, characterized by flight feathers like those of modern birds. Archaeopteryx is generally accepted as the oldest known fossil bird. It represents a unique snapshot of evolution; most…

The most diverse group of early Tertiary mammals are the archaic ungulates (condylarths). Although closely related, they are not a monophyletic group but, along with various extinct and extant mammals, form the taxon Ungulata (Fig. 1). They are…

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…

A diversified group of diapsid reptiles, comprising dinosaurs (and their descendants, birds), crocodylians, pterosaurs (flying reptiles), and various forms closely related to these groups. In the past, the primitive archosaurs from the Triassic…

A heavily armored mammal in the order Cingulata. The order Cingulata (formerly Edentata or Xenarthra) comprises the armadillos, which are divided into two extant families: Chlamyphoridae and Dasypodidae. These creatures are characterized by bony…

The only surviving subclass of the echinoderm class Crinoidea. The Articulata include all living members of the echinoderm class Crinoidea and are generally regarded as the post-Paleozoic crinoids. Crinoids are exclusively marine organisms, as are…

An order comprising the even-toed ungulates (hoofed mammals). There are two main radiations: the predominantly omnivorous Bunodontia, including suoids (such as pigs, peccaries, and hippos); and the more herbivorous Selenodontia, including camels and…

A small order of specialized holostean fishes which are first recorded from Middle Jurassic deposits of Europe. Later records indicate that they were a very successful group in terms of geographic range, and probably had a worldwide distribution in…

A relatively small order of extinct South American ungulates in the infraclass Eutheria, known from the late Paleocene to the late Miocene. Members of the order Astrapotheria are customarily divided into two suborders: the late Paleocene–Eocene…

Any member of an adaptively diverse group of extinct hominin species that inhabited Africa between approximately 4.2 million years ago (MYA) and 1.2 MYA. The term australopith is used in preference to the term australopithecine, which is sometimes…

The class of animals consisting of the birds. Birds (Fig. 1) are vertebrate animals comprising the class Aves. (In some classification schemes, the term Neornithes may also be used when referring to modern birds.) Primarily adapted for flying, the…

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.…

An elasmobranch subdivision of cartilaginous fishes known collectively as the rays, skates, or batoids. Batoidea is one of the two subdivisions of the subclass Elasmobranchii (class Chondrichthyes) and consists of 4 extant orders, 17–20 families,…

An order of extinct 10-armed cephalopod mollusks with an internal shell. The order Belemnitida belongs to the subclass Coleoidea in the class Cephalopoda (phylum Mollusca). Constituent members, which are all extinct, are known as belemnites (Fig. 1)…

A discipline concerned with the application of ethics to biological problems, especially in the field of medicine. Bioethics is a broad term that encompasses the environmental, ethical, legal, and social implications of agriculture, life sciences,…

An extinct group of epifaunal bivalves that constructed reefs during the late Mesozoic. The valves of rudists are coiled to cylindrical in external form, and the shapes and sizes of rudist shells are extreme relative to other members of the class…

One of the seven existing classes in the phylum Mollusca; sometimes known as Pelecypoda. All members of the Bivalvia (bivalves) are aquatic, living at all depths of the sea and in brackish waters and freshwaters. With about 25,000 living species…

A class of extinct Pelmatozoa in the subphylum Blastozoa which arose in the Ordovician and flourished in the Carboniferous and Permian. Blastoids did not survive the Permo-Triassic mass extinction event. Historically, two orders of the Blastoidea…

A phylum of solitary, exclusively marine, coelomate, bivalved animals, with both valves symmetrical about a median longitudinal plane. Brachiopods are typically attached to the substrate by a posteriorly located cuticle-covered stalk called a…

A phylum of sessile aquatic invertebrates (also called Polyzoa) that form colonies of zooids. Each zooid, in its basic form, has a lophophore of ciliated tentacles situated distally on an introvert, a looped gut with the mouth inside the lophophore…

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…

A class of the phylum Porifera, including exclusively marine sponges with a skeleton composed of spicules of calcium carbonate to which can be added a solid basal calcitic skeleton. The sponges that constitute the phylum Porifera are assigned to…

Primitive fossil members of the phylum Chordata with a calcite skeleton of echinoderm type. They occur in marine rocks of Cambrian to Pennsylvanian age (530–300 million years old) and, because of their skeletons, have traditionally been placed in…

An extinct subclass of stalked Crinoidea; a class of the phylum Echinodermata. The Camerata include more than 235 genera confined to the Paleozoic Era, ranging from the Early Ordovician to the Late Permian. Camerate crinoids were one of the four…

An order of crepuscular (active during predawn or twilight hours) or mainly nocturnal birds collectively known as the nightjars or goatsuckers. Members of the order Caprimulgiformes are apparently most closely related to the owls (Strigiformes).…

A moderately coherent group of primitive reptiles constituting an order of the subclass Anapsida. Most members are characterized by a closed cheek (temporal) region. The order is divided into four suborders: Captorhinomorpha, Millerosauria,…

A class in the phylum Mollusca that is exclusively marine and contains the extant members—squids, cuttlefishes, octopuses, and the chambered nautiluses—and the fossil taxa—ammonoids, nautiloids, and belemnoids. The earliest known cephalopod fossils…

An aquatic mammalian order comprising approximately 90 living species of whales, dolphins, and porpoises and their fossil relatives. Like all other mammals, members of the order Cetacea (cetaceans) nurse their young with milk produced by the mother,…

An anapsid order of the class Reptilia, including the turtles, terrapins, and tortoises; also termed Chelonii, Testudinata, and Testudines. The order Chelonia (often referred to as Chelonii, Testudinata, and Testudines) comprises one of the oldest…

The name for several species of small ruminant mammals, also called mouse deer, which constitute the family Tragulidae in the order Artiodactyla. Chevrotains, often known as mouse deer (also spelled mouse-deer), belong to the family Tragulidae in…

The only extant order of the chondrichthyan subclass Holocephali, comprising the ratfishes. Members of the order Chimaeriformes (chimaeriforms or ratfishes; Fig. 1) are the closest living relatives of members of the subclass Elasmobranchii (sharks…

A class of vertebrates comprising the cartilaginous, jawed fishes. Members of the Chondrichthyes (chondrichthyans, or the cartilaginous fishes) are characterized by well-developed jaws and bony teeth; pectoral and pelvic girdles, with each…

An order of sea urchins (phylum Echinodermata, class Echinoidea) in which the 20 columns of plates in the essentially spherical test consist of five pairs of narrow and numerous ambulacral plates and five pairs of broad and few interambulacral…

A diverse phylum of aquatic invertebrate animals, including jellyfish, hydroids, sea anemones, and corals. Members of the phylum Cnidaria (Fig. 1), termed cnidarians, are characterized by the possession of (1) intrinsic nematocysts (stinging…

A class of the phylum Porifera, including sponges with a skeleton of one- to four-rayed siliceous spicules or of spongin fibers, or both. Four taxonomic classes comprise the phylum Porifera—Calcarea, Demospongiae, Hexactinellida, and…

An active and intentional act or practice of putting to death, in a relatively painless way, of persons suffering severely from medical conditions that are incurable, untreatable, or irreversible. Euthanasia is an act by a person, usually a…

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…

Ancient fossilized members of human beings and those species directly ancestral to modern humans. Fossil humans comprise all prehistoric skeletal remains of humans that are archeologically earlier than the Neolithic (necessarily an imprecise limit,…

An organism that has had its genetic material manipulated or altered in a laboratory setting through the use of genetic or genomic engineering techniques. Genetically modified organisms (GMOs) are microorganisms, plants, or animals that have been…

Any member of the extinct mammalian subfamily Glyptodontinae, comprising giant, heavily armored relatives of armadillos. The extinct prehistoric creatures known as glyptodonts (whose name is derived from the Greek term for "grooved tooth") [Fig. 1]…

An extinct order of the subclass Amphidiscophora in the class Hexactinellida. These extinct sponges are distinguished from the order Amphidiscosa (class Hexactinellida, phylum Porifera) in that the birotulates are hemidiscs with asymmetrical ends.…

A class of sponges whose skeletons are made of siliceous hexactine spicules. Members of the class Hexactinellida (phylum Porifera) are commonly called glass sponges (Fig. 1). These exclusively marine sponges are widely distributed in modern oceans.…

The accumulated body of scientific and technical writing published to serve the informational needs of, primarily, scientists, engineers, and research workers, and, to the extent that it can be understood, the general public. This vast literature is…

The Nobel Prizes for 2019 included the following awards for scientific disciplines. Chemistry The chemistry prize was awarded to John B. Goodenough of the University of Texas at Austin, United States; M. Stanley Whittingham of Binghamton University,…

The study of life history as recorded by fossil remains. Paleontology investigates the life of the past through analyses of preserved fossils and their traces (Fig. 1). The term fossil, from the Latin “fossilis” (digging; dug up), originally…

A small extinct class of relatively advanced blastozoan echinoderms containing five genera (three named), ranging from the middle Early Ordovician to the early Late Ordovician in eastern Canada; northeastern, eastern, south-central, and western…

A major group of marine snails, commonly called the true limpets. Patellogastropoda has long been recognized as one of the most primitive groups of living gastropods (class Gastropoda) and was once considered part of the Archaeogastropoda.…

An order of Diadematacea, making up those genera that possess solid spines and a rigid test. The ambulacra show typical diadematoid structure, and the tubercles are noncrenulate. The single known family, Pedinidae, includes 15 genera, ranging from…

A paraphyletic grade of fossil (extinct) synapsids that is ancestral to later forms, such as the therapsids and mammals. (Paraphyletic means not containing all of the descendants of the most recent common ancestor.) Modern taxonomic practice does…

The younger subperiod of the Carboniferous. The Pennsylvanian derives its name from a thick succession of mostly nonmarine, coal-bearing strata in Pennsylvania, United States. Radiometric ages place the beginning of the Pennsylvanian at…

An extinct order of Brachiopoda that lived from the Lower Cambrian to Upper Devonian (540 to 375 million years ago), with a peak during the Silurian Period. Pentamerides (Figs. 1 and Fig. 2) are the largest brachiopods in the Lower Paleozoic, with…

A subclass of Echinoidea lacking stability in the number of columns of plates that make up the ambulacra and interambulacra. The ambulacral columns vary from 2 to 20, the interambulacral from 1 to 14. There are two orders, Bothriocidaroida and…

An order of herbivorous, odd-toed, hoofed mammals, including the living horses, zebras, asses, tapirs, rhinoceroses, and their extinct relatives. They are defined by a number of unique specializations, but the most diagnostic feature is their feet.…

The subfield of philosophy that treats fundamental questions pertaining to science. The philosophy of science explores the fundamental principles, purposes, methodologies, implications, and reliability of the human enterprise known as science. It…

The subfield of anthropology that deals with human and nonhuman primate evolution, the biological bases of human behavior, and human biological variability and its significance. Physical anthropology is a branch of anthropology that investigates the…

A phylum comprising the sponges. Porifera is a phylum of the animal kingdom that includes more than 9000 described species of sponges (Fig. 1). The body plan of sponges is unique among animals. Currents of water are drawn through small pores (ostia)…

A rigid and wave-resistant marine structure that stands above its surroundings. To navigators, a reef is any rocky structure that poses a threat to navigation. Marine biologists and geologists specify that reefs are constructed by organisms that…

An extinct order of the subclass Amphidiscophora in the class Hexactinellida. This is an extinct group of Paleozoic hexactinellids (class Hexactinellida, phylum Porifera) with a branching form. Each branch is provided with dermal, parenchymal, and…

The study of nature and natural phenomena in which facts are observed and classified, and, often, quantitative laws are formulated and verified; involves the application of logical reasoning and data analysis to natural phenomena. Originally, the…

A small monotypic order of flightless, marine swimming birds, comprising the penguins, found predominantly in the colder southern oceans. Penguins (Fig. 1) constitute the order Sphenisciformes in the class Aves. Depending on the classification…

The group of primates most closely related to humans. The primates included in the ape group are the African great apes, the gorillas (Fig. 1) and chimpanzees; the Asian great apes, the orangutans; and the lesser apes from Asia, the gibbons. Humans…

The Nobel Prizes for 2018 included the following awards for scientific disciplines. Chemistry The prize in chemistry was awarded to three scientists. Half of the prize went to Frances H. Arnold from the California Institute of Technology in…

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…

Coral reefs proliferate in ocean waters that are oligotrophic, that is, extremely poor in nutrients. This paradox, first noted by Charles Darwin when he passed through Tahiti in 1842, was difficult to comprehend until symbiotic relationships between…

There are four species of African great apes living today, two chimpanzees [common chimpanzee (Fig. 1) and bonobo (pygmy chimpanzee)] and two gorillas (western gorilla and eastern gorilla), with degrees of subspecies differentiation within them.…

The Nobel Prizes for 2017 included the following awards for scientific disciplines. Chemistry The chemistry prize was awarded to Jacques Dubochet of the University of Lausanne, Switzerland; Joachim Frank of Columbia University, New York, United…

A genus of unicellular shelled foraminifers (family Nummulitidae). Members of the genus Nummulites (termed nummulites) are single-celled protoctists (protozoa) of the Foraminifera taxon (which is typically regarded as a phylum or subphylum,…

An order of coleoid cephalopods, characterized by eight appendages that encircle the mouth, a saclike body, and an internal shell that is much modified or reduced from that of its ancestors. The origins of the order Octopoda (subclass Coleoidea,…

A suborder of trilobites (a class of extinct arthropods) and a diverse clade consisting of roughly 100 distinct species. The trilobites constituting the suborder Olenellina belong to the order Redlichiida in the class Trilobita. These extinct…

Strategies or logical practices, often with iterative steps, used in scientific research. The process of doing science (Fig. 1) is not a "cookbook" set of rules. However, a valid scientific investigation typically involves systematic collection of…

A class of extinct Paleozoic arthropods, occurring in marine rocks of Early Cambrian through Late Permian age. The members of the extinct arthropod class Trilobita are commonly known as trilobites (Fig. 1). About 5000 described genera and 17,000…

Predators actively hunt, kill, and consume their prey for the sake of nutrition. Although the stakes are high for both parties involved, they are more important for the prey item. If the prey loses in the interaction, it loses its life, whereas the…

A food web depicts the feeding relationships among organisms in a biological community. This is an important representation because it indicates the origins, transfer, and loss of energy from the community. It is also intimately related to the…

The interval of geological time, and rocks deposited during that time, from the end of the Devonian Period to the beginning of the Permian Period. The Carboniferous Period spanned from about 359 to 299 million years ago during the Paleozoic Era…

In geological time, the last period of the Mesozoic Era, preceded by the Jurassic Period and followed by the Tertiary Period. The rocks formed during Cretaceous time constitute the Cretaceous System. Omalius d'Halloys first recognized the…

An order of bryozoans in the class Stenolaemata. Members of the order Cyclostomatida (previously termed Cyclostomata or Tubuliporata) belong to the class Stenolaemata in the phylum Bryozoa. Known commonly as cyclostomes (although not to be confused…

The geologic period and system of the Paleozoic Era from the end of the Silurian Period to the beginning of the Carboniferous Period. The Devonian Peroid encompasses an interval of geologic time between 419 and 359 million years ago (MYA) based on…

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)]…

The former name of an order that included the Xenarthra (sloths, anteaters, and armadillos), Pholidota (pangolins), and Tubulidentata (aardvarks). The former order Edentata was so named because of the apparent lack of some or all teeth of its…

The second oldest of the five major worldwide divisions (epochs) of the Tertiary Period; also referred to as the middle epoch of the Paleogene. The Eocene (Fig. 1) extends from 56.0 to 33.9 million years ago (MYA).Tertiary 1 Eocene, the second…

One of two extant orders of brittle stars, comprising the basket stars and snake stars, in the class Ophiuroidea (phylum Echinodermata). The order Euryalida currently includes approximately 190 valid species worldwide, which represents a little…

Tantalizing evidence has emerged indicating that ocean acidification and deoxygenation may have played major roles in several of the mass extinction events that have punctuated the history of life on Earth. These conditions prevailed during…

A superclass of the subphylum Vertebrata consisting of animals that possess jaws. The superclass Gnathostomata comprises the jawed vertebrates (subphylum Vertebrata, phylum Chordata). In other words, gnathostomes are animals with jaws, involving a…