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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.

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.

The frog total group, including Anura and extinct stem relatives such as Triadobatrachus massinoti.