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Used here for the living-frog crown group, including the common ancestor of living frogs and every descendant.

A chemical synapse transmits a signal by releasing neurotransmitter from a presynaptic neuron to act on postsynaptic receptors across a synaptic cleft. The two cell membranes remain separate. A typical fast transmission sequence is:

See Chemical Synapses and 3.1 Synapses. The fast pathway uses synaptic vesicles prepared near release sites; it does not require a distant synaptic vesicle to travel across the terminal after every action potential. See Basic Neurochemistry: calcium entry and vesicles at release sites. Some chemical synapses release neurotransmitter in response to graded presynaptic membrane potential changes rather than action potentials.

Neurotransmitter clearance limits how long released neurotransmitter remains available. Depending on the neurotransmitter and location, diffusion away, uptake through membrane transport proteins into the sending or neighboring biological cells, and breakdown by enzymes contribute. Receptor binding and unbinding also affect response duration. Recovering synaptic vesicle membrane is a separate recycling task; uptake of released neurotransmitter is not the return of an intact synaptic vesicle. See General Principles of Cell Communication.

This electrical-to-chemical-to-electrical sequence offers several places to adjust a signal: synaptic release probability, receptor availability and neurotransmitter clearance can change its strength or duration. It helps explain why synaptic plasticity can modify communication between neurons, and why an arriving action potential need not cause a postsynaptic action potential.

Otto Loewi's 1936 Nobel lecture describes his 1921 experiment: fluid from a frog heart whose nerves had been stimulated produced the corresponding response when transferred to another heart. The transferable substance established chemical mediation in that preparation. This was evidence for released chemical signaling, not yet an identification of synaptic vesicle fusion and not a claim that every synapse is chemical. See Loewi, The Chemical Transmission of Nerve Action, Nobel lecture, 12 December 1936.

A skull-roof bone formed by integration of frontal and parietal components, characteristic of frogs and some other vertebrates.

Metamorphosis is a developmental transformation between substantially different life stages, such as a tadpole becoming a frog or a caterpillar becoming an adult butterfly. It can reorganize feeding structures, movement, habitat use and internal tissues, rather than merely increasing size. Regulated cell differentiation, tissue remodelling and cell death contribute to the transition.

An early experiment showed that feeding tadpoles thyroid tissue accelerated differentiation relative to comparison diets. ^[ref] The intervention connected a secretory organ to the timing of a whole-body transformation. It did not isolate a pure hormone or establish that growth and differentiation are the same process. Modern thyroid hormone biology explains this amphibian example; insect metamorphosis uses a different hormonal system.

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.

An Early Jurassic frog relative with a derived caudopelvic apparatus interpreted as consistent with jumping.

The compound forearm element formed by union of the radius bone and ulna in adult frogs.

A skeletal element associated with the vertebral axis. Reduction of free ribs is part of the characteristic frog body organization.

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

A common name for several burrowing frogs. Their tadpoles include systems used to investigate environmentally induced resource-use phenotypes.

A proximal ankle element that is elongated in the derived frog hindlimb.

The compound lower-leg element formed by union of the tibia and fibula in adult frogs.

An Early Triassic stem frog from Madagascar with an anteriorly elongated ilium, a retained short tail and incompletely transformed limbs. CT redescription identified 15 presacral vertebrae rather than the traditional count of 14.