Dictionary of Evolutionary Nomenclature - Letters T and U

tarsier: One of three species of small nocturnal primate belonging to the genus Tarsius, found in Sumatra, Borneo, Celebes, and the Philippines. They have a naked tail measuring 130-270 mm (about 5-11 inches) long, which makes up about half the total length of their bodies (220-460 mm, or 8-19 inches). Tarsiers have enormous eyes, large hairless ears, and gripping pads at the end of their digits. They are mainly arboreal, using both hands to seize insects and small vertebrates such as lizards.

taxon (plural taxa): Any named taxonomic group, such as the family Felidae, or the genus Homo, or the species Homo sapiens. Also, a formally recognized group, as distinct from any other group (such as the group of herbivores, or the group of tree-climbers).

taxonomy: The theory and practice of biological classification.

terrestrial: Living on land.

tetrapod: A member of the group made up of amphibians, reptiles, birds, and mammals.

thecodont: The thecodonts were a diverse group of Triassic reptiles that included large four-legged carnivores, armored herbivores, small, agile two- and our-legged forms, and crocodile-like aquatic reptiles. They gave rise to crocodiles, dinosaurs, and pterosaurs. The term Thecodontia is no longer used, as they are a paraphyletic group. The thecodonts are therefore an evolutionary grade of animals, rather than a clade. Most palaeontologists now use the term "basal archosaur" to refer to thecodonts. As a group, they are defined by certain shared ancestral features, such as teeth in sockets, an archosaurian characteristic that was inherited by the dinosaurs. The name thecodont is actually Latin for "socket-tooth." Members of the group show a general trend toward a more upright, less sprawling stance, with the hind limbs especially being progressively positioned more directly beneath the body, until some could walk upright on two legs.

theory: A well-substantiated explanation of some aspect of the natural world that typically incorporates many confirmed observations, laws, and successfully verified hypotheses.

theropod: The theropod (meaning "beast-footed") dinosaurs are a diverse group o of bipedal dinosaurs. They include the largest terrestrial carnivores ever to have lived, as well as many quite small species. Theropods typically share a number of traits, including hollow, thin-walled bones and modifications of the hands and feet (three main fingers on the hand, and three main (weight-bearing) toes on the foot). Most theropods had sharp, recurved teeth for eating flesh, and claws on the ends of all of the fingers and toes. Some of these characters were lost or modified in some groups later in theropod evolution. Theropod fossils are fairly rare and often fragmentary. Fossils of small theropods are especially rare, since small bones are harder to find and are weathered away easily.

Thiagarajan, Sivasailam: The president of Workshops by Thiagi, Inc., his organization helps people improve their performance through games and simulations.

trait: A characteristic or condition.

transcription: The process by which messenger RNA is read from the DNA forming agene.

transfer RNA (tRNA): A type of RNA that brings the amino acids to the ribosomes to make proteins. There are 20 kinds of transfer RNA molecules, one for each of the 20 main amino acids. A transfer RNA molecule has an amino acid attached to it, and contains the anti-codon corresponding to that amino acid in another part of its structure. In protein synthesis, each codon in the messenger RNA combines
with the appropriate tRNA's anti-codon, and the amino acids are arranged in order to make the protein.

transformism: The evolutionary theory of Lamarck in which changes occur within a lineage of populations, but in which lineages do not split (i.e., no speciation occurs, at least not in the sense of the cladistic species concept) and do not go extinct.

transition: A mutation changing one purine into the other purine, or one pyrimidine into the other pyrimidine (i.e., changes from A to G, or vice versa, or changes from C to T, or vice versa).transitional fossil: A fossil or group of fossils representing a series of similar species, genera, or families, that link an older group of organisms to a younger group. Often, transitional fossils combine some traits of older, ancestral species with traits of more recent species (for instance, a series of transitional fossils documents the evolution of fully aquatic whales from terrestrial ancestors).translation: The process by which a protein is manufactured at a ribosome, using messenger RNA code and transfer RNA to supply the amino acids.

transversion: A mutation changing a purine into a pyrimidine, or vice versa(i.e., changes from A or G to C or T and changes from C or T to A or G).

trilobite: An extinct marine arthropod common from the Cambrian to Permian eras(570-245 million years ago). Trilobite fossils are abundant in rocks of this period. Trilobites were 10-675 mm long, and their flattened oval bodies were divided into three lobes by two longitudinal furrows. They had a single head shield, which bore a pair of antennae and, in many species, insect-like compound eyes. This was followed by more than 20 short body segments, each with a pair of forked appendages. Many trilobites apparently burrowed in sand or mud, preying on other animals or scavenging.

tuberculosis: An infection of the lungs, accompanied by fever and a loss of appetite, caused by the bacillus Mycobacterium tuberculosis.

tunicate: A group of simple chordates, including sea squirts (class Ascidacea)that live attached to rocks, and the salps (class Thaliacea) that float in the sea. Tunicates are small marine animals, cylindrical, spherical, or irregular in shape, ranging from several millimetres to over 30 cm in size. They have a saclike cellulose tunic covering the body; water is drawn in through a siphon and food particles are filtered out. The free-swimming tadpole-like larvae show the major characteristics of all chordates. They subsequently undergo metamorphosis, losing their chordate features and becoming adults. One group (class Larvacea) retain their larval characteristics throughout life.

typology: (1) The definition of classificatory groups by phenetic similarity to a "type" specimen. A species, for example, might be defined as all individuals less than x phenetic units from the species' type. (2) The theory that distinct "types" exist in nature, perhaps because they are part of some plan of nature.(See also idealism.) The type of the species is then the most important form of it, and variants around that type are noise, or "mistakes." Neo-Darwinism opposes typology because in a gene pool no one variant is any more important than any others.

unequal crossing-over: A crossing-over in which the two chromosomes do not exchange equal lengths of DNA; one receives more than the other.