= Solution
\b[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 \b[<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 \b[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.<Triadobatrachus massinoti, the earliest known lissamphibian (Vertebrata: Tetrapoda) re-examined by μCT scan, and the evolution of trunk length in batrachians by Eduardo Ascarrunz and others>[Contributions to Zoology 85, pp. 201–234; revised vertebral count, axial reconstruction and trunk-length analysis]{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.<Walk before you jump: new insights on early frog locomotion from the oldest known salientian by Andrés I. Lires and others>[Paleobiology 42, pp. 612–623; limb proportions of 188 living amphibians and lizards and fossil locomotor inference]{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.<An Early Jurassic jumping frog by Neil H. Shubin and Farish A. Jenkins Jr.>[Nature 377, pp. 49–52; Prosalirus pelvis, urostyle and reconstruction of saltation]{ref}
\b[<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>.
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