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\b[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>.<Evolution of the vomer and its implications for cranial kinesis in Paraves by Han Hu and others>[Results and Discussion; vomer contacts, morphological disparity and limits of functional reconstruction]{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.<Evolution of the vomer and its implications for cranial kinesis in Paraves by Han Hu and others>[Results and Discussion; vomer contacts, morphological disparity and limits of functional reconstruction]{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.<Complete Ichthyornis skull illuminates mosaic assembly of the avian head by Daniel J. Field and others>[Nature 557, pp. 96–100; mosaic cranial construction and inferred kinetic apparatus]{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.<Cretaceous ornithurine supports a neognathous crown bird ancestor by Juan Benito and others>[Nature 612, pp. 100–105; Janavis pterygoid, comparative shape and ancestral-palate interpretation]{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. \b[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.
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