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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.

The frog skeleton evolved through mosaic evolution: reorganization of the head and pelvis preceded completion of the compact, tailless, long-limbed jumping apparatus. Its principal transformations are shortening and stiffening of the axial skeleton, conversion of the posterior body into a sacrum–pelvis–urostyle system, and specialization of the appendicular skeleton for propulsion and landing. These are connected mechanically, but need not have evolved simultaneously or initially served the same function.

In the derived adult frog, the trunk is short, most species having eight or nine presacral vertebrae, and the ribs are strongly reduced or absent. The adult tail no longer consists of a long freely jointed series. Instead, the urostyle forms a posterior axial rod from caudal vertebral and associated developmental components. This increases the integration of the posterior body with the pelvic girdle. The ilium has an elongated, anteriorly directed shaft, placing much of the pelvic apparatus posterior to its sacral connection. Changes in the sacroiliac joint and adjacent musculature permit transmission of bilateral hindlimb thrust to the trunk. The urostyle is therefore more than a tail that simply shrank: it is part of a reorganized force-transmission system.

The limbs change in both proportion and construction. Elongate hindlimbs increase the possible extension stroke, and elongated proximal ankle elements, the tibiale and fibulare, add another effective segment to the propulsive chain. Union of tibia and fibula into a tibiofibula and of radius bone and ulna into a radioulna stiffens the lower limb segments. The forelimbs and pectoral girdle participate in support and absorption of landing forces, while the frog head is typically broad, short and flattened, with reduction and fusion of cranial elements, including integration of frontal and parietal components into a frontoparietal bone. These are generalizations about the characteristic skeleton, not a claim that every living frog is an obligate jumper: swimming, burrowing, climbing and walking lineages modify the inherited apparatus.

The decisive stem example is Triadobatrachus massinoti, from the Early Triassic of Madagascar, about 250 million years ago. It belongs to the frog total group, Salientia, outside crown Anura. Its phylogenetic importance is the association of recognizable frog-lineage features with retained ancestral construction. An anteriorly elongated ilium and frog-like cranial organization were already present, while a short series of separate caudal vertebrae persisted. The forearm and lower-leg pairs were not united into the modern radioulna and tibiofibula, and the hindlimbs lacked the full proportional specialization of powerful living jumpers. Thus pelvic reorganization did not require that all the later limb and tail modifications had already evolved.

The 2016 micro-computed tomography redescription is especially important because it revises the evidence rather than merely repeating an iconic reconstruction. It recognized 15 presacral vertebrae, with an atlas vertebra–axis vertebra complex, replacing the traditional interpretation of 14 with a divided atlas. Its trunk was therefore substantially less abbreviated in vertebral count than that of a typical living frog. Reconstructions under alternative amphibian relationships suggested that some proportional trunk reduction may already have occurred before the frog–salamander split, preceding the iliac elongation and tail shortening seen in this fossil. That inference depends on the comparative tree and model; vertebral number and actual trunk proportions are different characters. A count alone cannot demonstrate exactly how flexibly the animal moved.^[ref]

Locomotor inference requires an additional test. In 2016, Andrés I. Lires and colleagues compared major limb-bone lengths across 188 living limbed amphibians and lizards, relating their proportions to observed locomotor categories while considering shared ancestry. Triadobatrachus massinoti was better supported as using salamander-like lateral undulatory locomotion than as a specialized jumper. This is evidence against assuming that each frog-like skeletal feature originated as an adaptation for saltation. The limitations are that bone proportions are proxies for performance, living comparison sets may omit extinct combinations, and soft tissues and direct behaviour remain unknown. Failure to support specialized jumping does not exclude every short hop or vigorous hindlimb movement.^[ref]

Comparison with Prosalirus bitis, from the Early Jurassic, provides a later anatomical checkpoint. Its caudopelvic construction includes a urostyle, rod-like iliac organization and elongate hindlimbs, and was interpreted as supporting jumping through changes in pelvic orientation and iliosacral mobility. This comparison reinforces a sequence in which early frog-lineage pelvic and cranial modifications preceded completion of the more specialized locomotor system. It does not turn the named fossils into a direct ancestor–descendant chain, nor require every phylogenetic character to have followed exactly the same history in every branch.^[ref]

Triadobatrachus massinoti separates frog ancestry from fully developed frog jumping. It documents a combination that comparison of living frogs and salamanders alone could not predict reliably. The appropriate evolutionary explanation allows early changes in body support or movement to become exaptations for later saltation, followed by stronger specialization of the limbs and caudopelvic apparatus. Its retained tail and incompletely fused appendicular elements demonstrate staged assembly, while the revised vertebral interpretation shows why the ordering must remain anchored to observed anatomy.

Living cartilaginous fishes are specialized descendants, not a surviving anatomical stage before the origin of bony vertebrates. fossil discoveries have shifted several apparently “bony-fish” features toward the ancestry of all living jawed vertebrates. Their absence in living Chondrichthyes can consequently represent character loss, while other shark features remain ancestral or are new specializations. The change is a revision of character polarity, not a claim that every aspect of shark anatomy is reduced.

A simple comparison of living sharks with Osteichthyes can suggest a one-way sequence: an ancestor with a cartilaginous internal skeleton, followed by origin of extensive bone on the bony-vertebrate branch. That inference is insecure because the two living sister lineages can both have changed, and living jawless outgroups can also have lost structures. The fossil record samples extinct combinations near the relevant divergences. To determine whether a structure is a gain or loss, one must place those combinations on a rooted phylogenetic tree and distinguish anatomical absence from poor preservation.

The early gnathostome problem involves four familiar assemblages: placoderms, acanthodians, Chondrichthyes and Osteichthyes. Treating all four as separate complete clades can conceal transitions occurring along the stems of surviving groups. In particular, many newer analyses place acanthodians as successive branches on the chondrichthyan stem. Their name “spiny sharks” is not itself evidence for that placement; cranial, scale, fin and other characters must support it. If this topology is correct, the large difference between living sharks and early armoured jawed fishes includes a history of reduction within the cartilaginous-fish lineage.

The 2012 redescription of the Acanthodes braincase illustrates why anatomical interpretation and tree position must be kept separate. It revealed a mixture of conditions previously assigned to different living groups and argued for shark-like aspects of the last common ancestor of modern jawed vertebrates. Its analysis still placed Acanthodes on the osteichthyan stem, rather than treating every acanthodian as a stem chondrichthyan. Thus a resemblance initially interpreted as an osteichthyan specialization could instead be an ancestral feature retained in more than one lineage. Later hypotheses should not be projected retrospectively onto that study.^[ref]

Entelognathus primordialis, described in 2013 from the Silurian of China, supplied a particularly important association: an armoured placoderm-like body and skull with large marginal jaw bones identifiable as premaxilla, maxilla and dentary. Such bones had been treated as characteristic innovations of Osteichthyes. Placement of this animal near the top of the jawed-vertebrate stem, together with recovery of acanthodians on the chondrichthyan stem, instead supported a macromeric, extensively ossified dermal ancestor. On that reconstruction, the small-scale-dominated covering and absence of those large jaw bones in living Chondrichthyes reflect reduction and reorganization rather than simple retention of a primitively unossified condition.^[ref]

Janusiscus schultzei, described in 2015 from the Early Devonian, reinforced the point using a different anatomical region. Its combination of a bony dermal skull roof and informative internal cranial conditions supported a position outside crown gnathostomes. Braincase anatomy therefore did not align neatly with the traditional picture of a primitive shark head versus a derived bony-fish head. Together, these fossils move apparently osteichthyan-like states closer to the common ancestry of both surviving groups. Their significance comes from character combinations and placement, not from choosing whichever living animal looks most similar.^[ref]

A separate question concerns internal skeletal tissue. dermal bone, perichondral bone, endochondral bone and calcified cartilage must not be conflated. endochondral bone forms during replacement of a cartilaginous template; perichondral bone forms around it; calcified cartilage remains a different tissue despite being mineralized. Therefore a bony dermal skull roof alone cannot establish endochondral ossification in the same animal.

The 2020 description of Minjinia turgenensis, from the Early Devonian of Mongolia, addressed that distinction. micro-computed tomography revealed extensive internal trabecular tissue in the braincase, interpreted as endochondral bone, rather than just an external bony shell. Its recovery close to the crown divergence permits endochondral ossification to be reconstructed before the separation of Chondrichthyes and Osteichthyes, followed by loss or marked suppression along the chondrichthyan lineage. This conclusion depends on identification of the tissue and the animal's position; the absence of a more complete skeleton and uncertainty among early jawed-vertebrate trees constrain how strongly it can be generalized.^[ref]

The resulting inference is conditional but testable. If well-supported stem relatives and the bony-vertebrate branch possess a particular homologous tissue, while living cartilaginous fishes lack it, an ancestral presence followed by loss can fit better than several independent origins. Moving a crucial fossil to another branch, changing a tissue identification or allowing different transition rates can change the ancestral reconstruction. maximum parsimony is useful but is not proof that losses and gains have equal biological probability. Character coding should record unknown preservation as unknown, rather than use an unobserved delicate feature as evidence of absence.

The major revision is from a ladder of increasing ossification to a branching history of gains, retention and losses. It concerns particular tissues and anatomical elements: reduction of large dermal bones need not coincide exactly with loss of endochondral ossification. fossils reveal that apparently simple modern anatomy can be secondarily simplified, and that major living-group differences may reflect transformations on both sides of a divergence. Continued uncertainty over placoderm relationships and individual tissue homologies defines the comparisons still needed; it does not restore living sharks as unchanged proxies for the jawed-vertebrate ancestor.