Vieraella is an extinct genus of frog from the Lower Jurassic (Early Pliensbachian to Toarcian) Roca Blanca Formation of Argentina, and one of the oldest true frogs known. This genus is known by a single excepcionally well preserved specimen, P.V.L. 2188, with at least eight presacrals vertebrae, free ribs, ulna and radius not fused, bony skull with some discoglossid characters.
Description
Despite living around 188 million years ago, Vieraella was anatomically very similar to modern frogs. For example, its hind legs were adapted for jumping, and the skull already possessed the lattice-like form found in modern species. It was, however, an unusually small frog, measuring only in length. Although older frog-like creatures are known, such as Triadobatrachus, these possessed many primitive characteristics, and cannot be said to be "true" frogs.
References
Further reading
Estes, R., and O. A. Reig. 1973. The early fossil record of frogs: a review of the evidence. pp. 11–63 In J. L. Vial (Ed.), Evolutionary Biology of the Anurans: Contemporary Research on Major Problems. University of Missouri Press, Columbia.