1. ^* Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
2. ^† Department of Organismic and Evolutionary Biology, and Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138, USA

Abstract

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 skeletons^2á¤-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.