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The clade comprising the most recent common ancestor of living crocodilians and birds and all its descendants, including extinct dinosaurs. Being sister to this clade does not make turtles archosaurs.

A member of the dinosaur lineage, including birds. Restricting the word to extinct nonavian forms excludes descendants.

An integumentary structure whose distribution among living and fossil dinosaurs informs bird origins and functional change. Filamentous coverings and specialized flight feathers should not be treated as one invariant form.

The avian total group: Aves and all extinct archosaurs more closely related to them than to living crocodilians. Under standard relationships this includes pterosaurs and nonavian dinosaurs.

A node-based name selects a common ancestor from included reference taxa; a branch-based name selects a lineage by including one reference and excluding another. Both designate a clade, so both include every descendant of the selected ancestor. The distinction concerns the rule locating that ancestor, not whether descendants are included. Neither requires its members to share a particular modern appearance.^[ref]

For a node-based clade definition, choose internal phylogenetic specifiers $A$ and $B$. Let $m(A,B)$ be their last common ancestor, and let $D(v)$ include ancestor $v$ and all its descendants. Then the named group isIt is the smallest clade containing both references. For a branch-based clade definition, choose an internal reference $A$ and an external reference $Z$. The named group is the largest clade containing $A$ but excluding $Z$: organisms sharing a more recent common ancestor with $A$ than with $Z$. On a rooted phylogenetic tree, this selects the $A$-side lineage after its separation from the lineage containing $Z$. These are also called minimum-clade definitions and maximum-clade definitions. The names describe inclusion constraints, not distances measured from branch lengths.^[ref]

An informative avian node example is crown Aves, here anchored to living ostrich and sparrow lineages. Their last common ancestor is the ancestor of living Palaeognathae and Neognathae; the name includes that ancestor and all descendants, including extinct descendants within either branch. An informative branch example is Panaves, the avian total group: the largest clade including living birds but excluding living crocodilians. With standard archosaur relationships it includes Aves, extinct avian stem relatives, nonavian dinosaurs and pterosaurs. The external reference is decisive: the definition asks which side of the bird–crocodilian split an organism occupies, not whether it flies. These crown and total usages were distinguished explicitly in the avian nomenclatural analysis by Jacques A. Gauthier and Kevin de Queiroz.^[ref]

Consider a simplified cladogram in which the crocodilian lineage separates first, followed on the avian side by pterosaurs, successive nonavian dinosaur branches, Archaeopteryx, other extinct avian branches and finally the living-bird crown divergence. Archaeopteryx belongs to Panaves but lies outside crown Aves. A fossil on the palaeognath branch after the palaeognath–neognath divergence belongs to both. A crocodilian belongs to neither. Finding a still earlier bird-line fossil would extend the known fossil history of Panaves without automatically changing the age or known membership of crown Aves. This example is schematic: it illustrates how definitions operate and is not a claim that any sampled fossil is the direct ancestor of another.

Two distinctions prevent mistakes. First, a node-based clade definition need not define a crown group: internal references can be extinct, as in a deliberately broader Archaeopteryx-inclusive node definition. Likewise, a branch-based clade definition need not define an entire total group; using a nonavian theropod dinosaur as the excluded reference selects a much narrower avian-lineage branch than excluding crocodilians. Second, a stem group is not synonymous with the branch-defined total group. The total includes the crown; the stem alone excludes it and normally forms a succession of extinct branches.

The definition must therefore accompany an ambiguous name. Historical uses of Aves have included the living-bird crown and more inclusive fossil assemblages; Avialae has also received differing definitions. Discovery of feathers in a nonavian theropod dinosaur changes the inferred distribution of a phylogenetic character, but does not by itself place the animal inside crown Aves. Conversely, loss of flight does not remove a crown descendant from its clade. A name supplies an ancestry rule; anatomical and molecular evidence supply the tree on which that rule is applied. Uncertain fossil placement produces uncertain membership even when the definition is precise.

A monophyletic group includes an ancestor and all its descendants; a paraphyletic group includes an ancestor but excludes some descendants; a polyphyletic group combines separate lineages while excluding their relevant common ancestry. The distinction concerns how membership relates to a phylogenetic tree, rather than how many visible similarities the members possess.

Consider a simplified tree of living amniotes: mammals form one branch, while the other contains lepidosaurs, turtles and archosaurs; living archosaurs comprise crocodilians and birds. Within the latter branch, birds are surviving dinosaurs. The tree below omits extinct branches and most living diversity, and its branch lengths represent neither age nor quantity of evolutionary change. It illustrates three choices of group on the same ancestry.

A monophyletic group is a clade. Archosauria includes the last common ancestor of birds and crocodilians and every descendant, including extinct dinosaurs and other extinct branches. In the simplified figure, selecting their shared node and both descendant tips gives the corresponding complete subtree. Sarcopterygii, with tetrapods included, is another example. A clade need not be recognized by a feature that remains visibly unchanged in every descendant: subsequent modification and loss are permitted. It also need not have a particular taxonomic rank.

A familiar paraphyletic group is the traditional assemblage “reptiles” when it includes turtles, lizards, snakes and crocodilians but explicitly excludes birds. The latter descend from within the ancestry otherwise assigned to that assemblage. The selected ancestry is therefore cut short at one of its own branches. Similarly, conventional “fish” that include finned sarcopterygians but exclude tetrapods form an evolutionary grade rather than a complete clade. These examples require the stated usage: Reptilia or Sauropsida can instead be defined as a clade that includes birds. The everyday word does not irrevocably determine the phylogenetic status of every technical definition bearing that name.

For polyphyly, consider assembling mammals and birds solely as the familiar independently evolved examples of sustained high metabolic heat production, while omitting their intervening ancestors and other descendants. This physiological grouping can be useful for comparing endothermy, but it does not identify one exclusive historical lineage. Their last common ancestor also has descendants such as lepidosaurs and crocodilians. On the usual inference of independent elaboration of the relevant physiology, the selected resemblance involves convergent evolution. Calling the assemblage “warm-blooded vertebrates” must not turn a selected physiological trait into evidence that birds are closer to mammals than to crocodilians. Moreover, endothermy and constant body temperature are not identical properties, and living taxon sampling does not locate every thermophysiological transition in extinct lineages.

There is a subtle distinction between the last two cases. A diagram of selected terminal tips alone can show that a group is not a clade, but does not always uniquely distinguish paraphyly from polyphyly. One must state which ancestors are included and the historical interpretation of the defining character. In the orange example, the ancestral reptile lineage is retained and one descendant branch excluded. In the blue example, similar functional outcomes are assembled from separate branches without including the connecting ancestry. The highlighted internal paths in the figure make that difference explicit.

clades are particularly suitable for discussing vertebrate evolution because their nested membership directly expresses common descent. They permit consistent statements such as “birds are dinosaurs” and “humans are sarcopterygians”, while allowing substantial changes in form and function. Shared derived characters, or synapomorphies, provide evidence for branches; shared ancestral states do not uniquely diagnose the smaller groups inheriting them, and independently acquired similarities require separate explanation. This framework makes character transformations testable against a branching history instead of arranging animals in a ladder of increasing complexity.

In practice the phylogenetic tree remains a hypothesis. More characters, better fossils or different evolutionary models can change inferred relationships. Naming a clade with an explicit ancestry-based definition helps keep the statement intelligible when that happens, but does not make the inference immune to error. Anatomical and molecular characters need biological homology assessment, appropriate taxon sampling and uncertainty estimates. For example, genome-scale data and stem-turtle anatomy independently address whether turtles lie near archosaurs, rather than treating one skull condition as sufficient to define the tree.^[ref]

Use monophyletic groups to express ancestry, and identify ecological or anatomical grades explicitly when they serve another purpose. paraphyletic groups can summarize retained ancestral organization, and polyphyletic groups can support functional comparisons, but neither should silently substitute for a complete lineage.

A member of the theropod lineage of dinosaurs, within which birds are nested. The hands and integument of nonavian relatives inform comparisons of avian origins.

A nonavian tyrannosauroid dinosaur represented by skeletons with preserved filamentous feathers.