Triadobatrachus massinoti (Piveteau 1936; Ascarrunz et al. 2016) Early Triassic ~250 mya, ~10 cm in length, is considered the oldest frog known. Although distinct from all living frogs and toads, Triadobatrachus is considered a frog due to its reduced vertebral count, the relative length of its hind legs, the presence of elongated ankle bones (tibiale and fibulare) and the anterior extension of its ilium. Derived from a sister to Gerobatrachus, Triadobatrachus phylogenetically preceded modern frogs like Rana.

Distinct from Gerobatrachus, the skull of Triadobatrachus was relatively smaller with a narrower skull roof and a larger orbit that extended into the cheek where the bones were reduced. The frontal and parietal were fused together, but note the parietals were not completely fused to each other! The intertemporal was fused to the parietal. A large postparietal shelf was absent. The lower jaw was toothless.

The presacral vertebral count was reduced to 14. The tail was reduced to a nub of at least six vertebrae. The ilium included a larger anterior process, but note the thigh muscles did not extend to the anterior tip of the ilium as in reptiles and mammals. The femur, tibia and fibula were elongated as were two proximal ankle bones, the tibiale and fibulare. Unfortunately the hands and feet remain unknown.

Despite the much longer hind legs, Triadobatrachus was considered not an effective jumper.

Ascaphus truei (Stejneger 1899) is the extant tailed frog. The so-called tail is only found in males and is an extension of the cloaca for internal fertilization, the primitive state. More dorsal and lumbar vertebrae are present than in Rana (below). As expected, Ascaphus nests with Rana in the LRT.

Rana catesbeiana (Linneaus 1758, Shaw 1802) extant, bull frog. Derived from a sister to Triadobatrachus, Rana is the last of this lineage and includes some 90 species.

Distinct from Triadobatrachus, the skull of Rana is more spade-shaped in dorsal view. The nasals extend laterally to the maxilla. The squamosal is reduced to a frame for the eardrum.

The presacral count is further reduced to 9. The tail vertebrae are fused into a single long bone, the urostyle, following the sacral vertebrae, but note that while the urostyle is long, it lies completely between the ilia and does not extend beyond the ischium.

A cartilaginous element, the suprascapula, extends dorsal to the scapula and roofs over the first few dorsal ribs. The radius and ulna fuse. The hand is larger. The thumb is absent. Only a vestigial stub sometimes remains of metacarpal 1. Digit 2 of the manus has one fewer phalanx probably by fusion. Digits 3 and 4 have two fewer phalanges.

The ilium is elongated to half the torso length. The femur is gracile and elongated. The tibia and fibula are fused. The tibiale (astragalus) and fibulare (calcaneum) further elongate the foot. No trace of pedal 2.1 is present. A small medial carpal spur is present.

Ascarrunz E, Rage J-C, Legreneur P and Laurin M 2016. Triadobatrachus massinoti, the earliest known lissamphibian (Vertebrata: Tetrapoda) re-examined by μCT scan, and the evolution of trunk length in batrachians. Contributions to Zoology 85(2):201–234.

Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.

Piveteau J. 1936. Une forme ancestrale des amphibiens anoures dans le Trias inférieur de Madagascar. Comptes Rendus hebdomadaires des séances de l'Académie des Sciences 202:1607–1608.

Rage J-C and Rocek Z 1989. Redescription of Triadobatrachus massinoti (Piveteau, 1936) an anuran amphibian from the early Triassic. Palaeontographica Abt. A 206(1-3):1-16. online pdf

Stejneger LH 1899. Type species: Ascaphus truei. Proceedings of the US National Museuem 21:899.

wiki/Triadobatrachus
wiki/bull frog
wiki/Rana
wiki/Tailed_frog