WITH nearly 4,000 living species1, frogs are numerically the most successful of modern amphibian groups. Their distinctive anatomy, which exhibits numerous unique features in both the axial and appendicular skeletons2á¤-6, represents a major departure from the body plan of Palaeozoic amphibians. We report here the discovery of the earliest known frog, associated with caecilians that retained limbs7, that exhibits primitive characters but shares with later anurans such features as fusion of the caudal vertebrae (urostyle), a rod-like ilium, and elongate hind limbs. The evolution of saltation in anurans entailed translocation of the ilium below the sacral rib, recruitment of the primitive tail musculature to transmit force from the hind limb to the axial skeleton during a jump, and development of iliosacral mobility. We reinterpret the caudopelvic transition from Palaeozoic amphibians to modern frogs with reference to Triadobatrachm massinoti, an Early Triassic amphibian that possesses some frog-like features8. The Early Jurassic age and Laurasian provenance of the new frog support the hypothesis4 that the widespread occurrence of primitive extant anurans reflects an ancient Pangaean distribution.