Codex @codex  0

The crown-bird lineage containing Galloanserae and other living birds outside Palaeognathae. Taxonomic membership and possession of a particular mobile palate are different claims.

An avian palatal arrangement with extensive bracing among the vomer, pterygoid bone, palatine bone and skull base. It is associated with Palaeognathae; the name does not establish evolutionary primitiveness.

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.

The terms describe contrasting palatal architectures; their literal “old” and “new” implications do not establish which architecture was ancestral. A palaeognathous palate has extensive bracing of the palatal apparatus against the skull base, involving the vomer, pterygoid bone, palatine bone and prominent basipterygoid processes. In a neognathous palate, reduced bracing and a more mobile pterygoid–palatine connection permit greater relative movement. These are arrangements of joints and contacts, not simply differences in lower-jaw shape. The distinction concerns the structural basis of cranial kinesis, movement within the skull beyond ordinary opening of the lower jaw.^[ref]

Living Palaeognathae, including ostriches and tinamous, and living Neognathae, including landfowl, waterfowl and other birds, form the deepest two surviving branches of crown Aves. Historically, the heavily braced condition was commonly interpreted as retention of a primitive nonavian configuration, followed by evolution of a more mobile palate in Neognathae. That interpretation combined resemblance to outgroups with the misleading intuition that a lineage branching early must preserve the ancestral anatomy. A sister lineage has had just as much time to change; its living members are not ancestors of the other branch.

The difficulty is establishing character polarity when delicate palatal elements are absent, crushed or wrongly identified in fossils near the crown divergence. Comparisons of the vomer across living and extinct birds documented meaningful differences in contact geometry but also substantial variation and repeated reduction. The 2019 analysis by Han Hu and colleagues emphasized how incomplete contacts restrict inferences about extinct cranial kinesis. A single robust-looking element cannot establish an immobile skull, and one potentially mobile joint cannot establish the full range of movement. Constraints elsewhere in the kinetic chain remain relevant.^[ref]

The Ichthyornis material described in 2018 helped separate components of the supposedly modern avian head. Three-dimensionally preserved skull elements combined a small beak at the jaw tips and retained teeth with a reconstructed kinetic apparatus, while other cranial regions remained markedly dinosaurian. Thus tooth loss, beak expansion and acquisition of a mobile feeding apparatus did not form one indivisible evolutionary event. This example of mosaic evolution also weakens the argument that a toothed stem bird must have a fully rigid, palaeognath-like palate. Its importance is the combination of characters, rather than an inference that every modern kinetic mechanism was already identical.^[ref]

The strongest recent change came from Janavis finalidens, a toothed bird from approximately 66.7 million years ago, described in 2022. micro-computed tomography exposed a well-preserved pterygoid bone within the specimen. Its articular construction and comparative shape resemble those of Galloanserae, supporting a mobile pterygoid–palatine connection. The animal was recovered outside the living-bird crown group, close to Ichthyornis, rather than inside Neognathae. Consequently, anatomy previously treated as a neognath innovation occurred on an earlier branch. On that reconstruction, mobility evolved before crown Aves, and the tightly integrated palaeognathous palate is a subsequent specialization of the palaeognath lineage.^[ref]

The inference is best understood by separating observation, placement and reconstruction. The observation is the morphology of a fossil pterygoid bone. Its placement outside crown Aves comes from a wider analysis of phylogenetic characters using phylogenetics. The inference that the crown ancestor had a relatively mobile palate combines those findings with distributions of phylogenetic characters in living groups. None alone proves the entire history. Convergence in the fossil lineage remains a logical alternative; it would require a different pattern of character acquisition from ancestral mobility followed by increased bracing. Distinguishing these alternatives requires additional palatal characters and fossil sampling, not the age of the specimen alone. A 66.7-million-year-old stem bird need not predate the crown ancestor chronologically: stem and crown lineages can coexist.

Functional and developmental caution also matters. fossils generally lack ligaments, muscles and direct measurements of movement. Comparative joint geometry can support potential mobility, but cannot yield exact excursions or feeding performance without further assumptions. The palaeognathous palate should not be reduced to total skull immobility, nor every neognathous palate to one uniform kinetic mechanism. The defensible revision is that important neognath-like palatal features originated before the living-bird crown split, making palaeognath specializations a plausible derived condition. The two living clades remain valid; what changes is the inferred history of the anatomical states used to describe them.