The Lower Triassic amphibians Triadobatrachus massinoti and Czatkobatrachus polonicus are universally regarded as stem anurans. However, there is still uncertainty about whether or not they were capable of jumping like true anurans as their postcranial features have so far provided only equivocal evidence. Although previous work has concentrated on the anatomy of the hind limb, here we examine the anatomy of the forelimb, comparing stem and crown-group anurans to other amphibians. The forelimb and pectoral girdle of Triadobatrachus share several features with modern frogs, including a frog-like deltoid attachment of the scapula. Though the radius and ulna are unfused, the humerus is similar to those of modern anurans. The deltopectoral crest is slightly elongated and a lateral deflection of the ventral edge of the crest creates a concavity on the lateral face of the humerus. These features are uncommon in most tetrapod groups characterized by a sprawling stance but are typical of modern anurans, in which the anterior chest musculature is enlarged. Our findings indicate that the importance of the deltoid had increased relative to that of the pectoralis muscle in Triadobatrachus. Czatkobatrachus is somewhat less similar to modern jumping anurans in its deltopectoral crest and pectoral girdle. Most anurans extend the forelimbs forward during a jump and land on their forefeet, perhaps accounting for the enlargement of the deltoid and the orientation of the deltopectoral crest. This type of behavior during landing is not seen in Ascaphus and Leiopelma but several features of these two genera, such as the fusion of the radio-ulna, the forearm musculature, and the degree of medial rotation of the manus, indicate that their landing behavior may be derived rather than primitive. Overall, Triadobatrachus was certainly not as capable of long jumps as some modern anurans, yet its anatomy does suggest that jumping or hopping was part of its locomotor repertoire.
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