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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…

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,…