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An adaptive or evolutionary grade among the primates, with representatives divided into Old World and New World groups. The term monkey is not indicative of a taxonomic or phylogenetic relationship. Instead, it is a common name used to describe any…

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 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…

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 evolutionary transition of certain dinosaurs into modern birds has long been an active area of paleontological research. Because the members of the class Aves are living representatives of theropod dinosaurs, paleontologists have sought fossil…

A newly identified reptilian fossil has pushed back the phylogenetic (evolutionary) family tree of Squamata [the dominant order of living reptiles, comprising lizards, snakes, and amphisbaenians (mostly limbless lizards that resemble worms)] by 75…

Scientists have long thought that dinosaur eggs were white in color and that the evolution of colored eggs in birds was strictly an avian innovation that occurred after the demise of the nonavian dinosaurs. However, technical advancements in the…

In early 2018, NASA announced that it had demonstrated a new type of celestial navigation for the first time. Called XNAV, for x-ray pulsar-based navigation, it relies on beams of radiation from pulsars—the cinders of exploded, massive stars. As…

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.…

Recent excavations in Asia unearthed a pristine, three-dimensional fossil of a tiny dinosaur tail with preserved feathers and soft tissues (presumably skin). In 2016, investigators in northern Myanmar (Burma) recovered a piece of Cretaceous-era…

Through study of the fossil record, paleontologists can frequently identify when an ancient species or taxon (a grouping of related organisms) first emerged in evolutionary history. (Paleontologists often prefer the term taxon over species because…

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…

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…

One of the evolutionary lineages of gastropods (class Gastropoda), comprising a very heterogeneous group of snails with forms that resemble typical marine snails with external spiraled shells such as sundial shells (family Architectonicidae), sea…

That portion of geologic time that postdates the latest episode of continental glaciation. The Holocene Epoch is synonymous with the Recent or Postglacial interval of Earth's geologic history and extends from 10,000 years ago to the present day…

A species of fossil human from the Pleistocene representing a specialized side branch in human evolution in Africa and Eurasia. Homo erectus was a common, widespread, and long-surviving ancestor of modern humans (H. sapiens). Fossil remains of this…

An obsolete order of placental insectivorous mammals, now mostly reassigned to the order Eulipotyphla, including the shrews, moles, hedgehogs, and solenodons. The order Insectivora was formerly thought by taxonomists to be a basal placental order,…

The common name for members of the agnathans. The superclass Agnatha (subphylum Vertebrata, phylum Chordata) comprises the jawless vertebrates (agnathans). Members include the cyclostomes (modern lampreys and hagfishes), as well as extinct armored…

The period and system of rocks deposited during the middle part of the Mesozoic Era from the end of the Triassic to the beginning of the Cretaceous. The Jurassic Period encompasses an interval of time between about 201 and 145 million years ago,…

An order of small insectivorous mammals, commonly known as the elephant shrews, containing the single family Macroscelididae. The elephant shrews, also called sengis, constitute the Macroscelididae, the sole family of the order Macroscelidea. They…

A mineral deposit formed by the interactions between microbes and their environment and thought to be one of the oldest ecosystems on Earth. Through the metabolic activities of microbialites, these ancient ecosystems have profoundly affected the…

The single order of the mammalian subclass Prototheria, containing the egg-laying platypus and echidnas. Members of the order Monotremata (monotremes) are egg-laying mammals belonging to the subclass Prototheria (alternatively, in some recent,…

A group of late archaic humans (Homo neanderthalensis) from Europe, southwest Asia, and central Asia that immediately preceded the first modern humans in those regions. The closest evolutionary relatives of modern humans are the Neandertals (also…

A class of the subphylum Asterozoa (phylum Echinodermata), comprising the brittle stars and basket stars, in which the arms are usually clearly demarcated from a central disk and perform whiplike locomotor movements, and the tube feet are…