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The earliest known salientian is †Triadobatrachus massinoti, from the Early Triassic of Madagascar; it is known from a single fossil. This "proto-frog" is about 250 million years old. It is a "proto-frog" because it had not yet quite evolved to the combination of features that we think of as being associated with frogs.
†Triadobatrachus had 14 vertebrae in front of the sacrum, and had a short tail, with the tail vertebrae separate from each other. In Triadobatrachus the radius and ulna (bones of the forearm) were not fused to each other; neither were the tibia and fibula (bones of the shank). The tibiale and fibulare of †Triadobatrachus may be slightly elongate, but not nearly to the degree seen in frogs. However, features that clearly link †Triadobatrachus to frogs are an anteriorly directed ilium, reduction in number of presacral vertebrae (to 14), fusion of the frontal and parietal into a frontoparietal bone, and a toothless dentary bone in the lower jaw. There is a large morphological gap between frogs and salamanders, and †Triadobatrachus has many of the features that you would expect from an organism that is intermediate between the two. This taxon is known from a single specimen. It was re-studied by Rage and Rocek (1989).