ACTINOPTERYGIANS: group of vertebrates that possess fins with well-developed rays (lepidotrichia) and, generally, a diminutive fin endoskeleton. It contains more than 24,000 species, including teleosts (trout, pike, herring, swordfish, etc.), sturgeons, and the paddlefish.
ADAPTATION: character that was selected to fulfill a function that it retains, or the evolutionary process that gives rise to this character.
AMNIOTES: group of vertebrates that includes mammals and reptiles (these include birds). Most are terrestrial. A key feature of this group is the “amniotic” egg, characterized by extra-embryonic membranes unique to this group, such as the amnios and the chorion.
AMPHIBIANS: group of vertebrates that includes lissamphibians (Anura, composed of frogs and toads; Urodela, which includes salamanders and newts; and Gymnophiona, also known as caecilians or apodans) and several extinct taxa (aïstopodes, adelogyrinids, nectridiens, lysorophiens, etc.) that are more closely related to lissamphibians than to amniotes. Amphibians include several aquatic, amphibious, and terrestrial species.
ANALOGY: relationship between two organs that fulfill the same function but which have different evolutionary origins. For instance, the gills of sharks and tetrapod lungs are both respiratory structures, but have distinct origins; the lung is not a transformed gill. The wings of bats and those of birds are also analogous (they represent two independent modifications of a walking forelimb).
ANURA: taxon that includes frogs and toads; it is one of the three main lissamphibian taxa. The vernacular term “anuran” is equivalent.
APOMORPHY: derived (recent) character; it is an evolutionary innovation.
AUTAPOMORPHY: derived (recent) character unique to a taxon. For instance, mammary glands are an autapomorphy of the taxon Mammalia (which includes mammals).
AUTOTROPHIC (ADJ.): capable of producing organic substances (often sugars) from simpler substances, using an energy source, like sunlight in photosynthesis. A few examples of autotrophic organisms include cyanobacteria, red and brown alga, and green plants.
BATRACHOMORPHA: the smallest clade that includes tetrapods and seymouriamorphs.
BRANCH: segment of an evolutionary tree that separates two nodes (terminal taxa, at the tip of the tree, are also nodes). Usually, branch length reflects evolutionary time, but it can also represent observed evolutionary change (e.g., in nucleotide sequences or in morphological characters).
CHONDRICHTHYANS: group of aquatic vertebrates with an entirely cartilaginous endoskeleton. It includes chimera (ratfish) and elasmobranchs (sharks, skates, and rays; the latter two are sharks that acquired a flattened body shape).
CLADE: monophyletic group, such as birds, mammals, or tetrapods.
CLADISTICS: technique that can be used to reconstruct evolutionary relationships by using shared derived (evolutionarily unique) characters.
CLADOGRAM: diagram showing the evolutionary relationships between taxa. It shows only a topology (branch lengths are not illustrated). Cladograms do not show the absolute age of taxa.
CROWN GROUP: smallest clade that includes two extant taxa. For instance, amniotes form a crown group composed of the smallest clade that includes mammals and reptiles.
DIPNOANS: group of primitively aquatic vertebrates (also known as lungfishes); they possess paired fins, gills, and lungs. Dipnoans are the closest extant relatives of tetrapods.
EURAMERICA: continent that included Laurentia (which itself corresponds with most of North America), Baltica (plate that included the territory now occupied by Baltic countries), and other plates that represent much of Europa. It formed in the Devonian through the collision between Laurentia and Baltica) and became integrated into Pangea in the Permian. Euramerica is also known as Laurussia or the Old Red Sandstone Continent.
EXAPTATION: character that was selected for a function that it no longer has, or that is no longer its main function.
FOSSIL: remains (usually of a mineralized structure, bone, shell, etc.) or trace of activity (tracway, burrow, etc.) of ancient organisms (generally older than 12,000 years).
GNATHOSTOMES: jawed vertebrates. This taxon includes, among others, chondrichthyans and osteichthyans.
GYMNOPHIONA: taxon that includes extant limbless amphibians. They are tropical and poorly known because many species are fossorial (i.e., subterranean; the others are aquatic).
HOMOLOGY: relationship between two organs or structures that have a common evolutionary origin, even though their function and aspect may differ (for instance, the tetrapod lung and the teleost swim bladder).
LEPIDOTRICHIA: dermal fin rays. They are composed of long, slender bony elements, primitively segmented, branched, and paired, although in some taxa, they may fuse to each other to form spiny structures (as in the perch) that are unsegmented, not ramified, and median (not paired).
LISSAMPHIBIA: taxon (whose members are called “lissamphibians”) that includes the three large clades of extant amphibians (anurans, urodeles, and gymnophionans) but not Paleozoic amphibians.
METAPTERYGIAL AXIS: axis that appears in embryonic fins and along which structures homologous with arm and leg bones develop.
MONOPHYLETIC (ADJ.): includes all the descendants of an ancestor (for instance, the taxon Mammalia, which includes all extant mammals, is monophyletic).
MONOTYPIC (ADJ.): redundant. Applies to a taxon (under rank-based nomen clature) that includes a single taxon of the immediately lower rank. For instance, a family that includes a single genus, or a genus that includes a single species, is monotypic.
NEOTENY: retention of larval or juvenile characters in adults (defined as individuals that have reached sexual maturity).
NEURAL ARCH: part of the vertebra that surrounds the spinal chord.
NODE: either end of a branch of a phylogenetic tree. Internal nodes are most often discussed; these occur at the meeting point of two branches, but the tips of terminal branches (which may represent species) are also nodes.
OSTEICHTYANS: group of vertebrates whose internal skeleton is largely ossified. It includes aquatic (actinopterygians, dipnoans) and terrestrial (tetrapods) taxa.
OUTGROUP: taxon related to a group whose phylogeny we want to study (the latter is the ingroup). Generally, the outgroup must be closely related to the ingroup, but it must not be part of it. Thus, to study tetrapod evolution, dipnoans are the most appropriate extant outgroup.
PARAPHYLETIC (ADJ.): includes only a subset of the descendants of the last common ancestor of its members (for instance, the taxon Reptilia under rank-based nomenclature, when excluding birds).
PHYLOGENY: diagram that represents the evolutionary relationships between taxa (or more rarely, genes or organisms). It includes a topology (which indicates only a nested or hierarchical pattern of relationships) and branch lengths (which represent the evolutionary distances between taxa).
REPTILIOMORPHS: taxon that includes amniotes (mammals and reptiles) and all extinct groups (such as diadectomorphs) that are more closely related to amniotes than to lissamphibians (extant amphibians).
SARCOPTERYGIANS: taxon that includes the coelacanth, dipnoans, and tetrapods, in addition to various extinct taxa.
SQUAMATES: taxon that includes “lizards” (a paraphyletic group), snakes, and amphisbaenians (small squamates with reduced limbs, if any).
STEGOCEPHALIANS: taxon that includes all limbed vertebrates and a few closely related species which may have retained paired fins.
STEM GROUP: paraphyletic group that includes all extinct species which are more closely related to one crown group than to another crown group. For instance, stem tetrapods include extinct species that are more closely related to tetrapods (a crown group) than to dipnoans (the most closely related crown group). It is a purely phylogenetic concept. Thus, some stem tetrapods had digits, such as Acanthostega, whereas others had paired fins, such as Eusthenopteron.
STEM TETRAPODS: paraphyletic group that includes all organisms which are more closely related to tetrapods (without being part of the crown group) than to their closest extant relatives (dipnoans). This group includes animals with digits, such as Acanthostega, and others with paired fins, such as Eusthenopteron.
STROMATOLITE: a layered, pillow-shaped structure formed by the trapping and binding of sedimentary particles by the biofilms of various microorganisms, especially of cyanobacteria (photosynthetic bacteria).
SYNAPOMORPHY: derived (recent) character (an evolutionary novelty) shared by at least two taxa. Synapomorphies are usually taken as evidence of close relationships. For instance, digits are present in most tetrapods and show that they are closely related, whereas the coelacanth, salmon, and sharks, which retain fins (an older character), are more distantly related to each other and to tetrapods.
SYSTEMATICS: science that studies the diversity and evolution of biological organisms. It includes taxonomy.
TAXON: group of closely related biological organisms.
TAXONOMY: field in systematics concerned with the recognition, naming, and delineation of taxa. Taxonomy may also designate a classification of biological organisms.
TELEOSTS: group of primitively aquatic vertebrates (a few amphibious species exist) that possess fins with dermal rays (lepidotrichia) and a swim bladder. It includes several commercially important species (trout, herring, salmon, tuna, swordfish, etc.).
TETRAPODA: smallest clade that includes lissamphibians (frogs, salamanders, and gymnophionans) and amniotes (mammals and reptiles).
TETRAPODOMORPHS: group that includes tetrapods and stem tetrapods (all extinct sarcopterygians that are more closely related to tetrapods than to dipnoans). This taxon includes all stegocephalians as well as some finned sarcopterygians, such as Eusthenopteron.
TOPOLOGY: relative position of taxa in a phylogeny as shown by a cladogram or a similar diagram. A topology includes data only on the relative kinship of taxa, not their evolutionary distance.
URODELA: taxon (whose members are called “urodeles”) that includes salamanders and newts; this is one of the three main lissamphibian groups.
VERTEBRAL CENTRUM: the part of the vertebra located under the spinal chord, which partly replaces and surrounds the notochord.