Evolution

The origins and evolutionary relationships between the three main groups of amphibians are hotly debated. A molecular phylogeny based on rDNA analysis dating from 2005 suggests salamanders and caecilians are more closely related to each other than they are to frogs and the divergence of the three groups took place in the Paleozoic or early Mesozoic before the breakup of the supercontinent Pangaea and soon after their divergence from the lobe-finned fishes. This would help account for the relative scarcity of amphibian fossils from the period before the groups split. Another molecular phylogenetic analysis conducted about the same time concluded the lissamphibians first appeared about 330 million years ago and that the temnospondyl-origin hypothesis is more credible than other theories. The neobatrachians seemed to have originated in Africa/India, the salamanders in East Asia and the caecilians in tropical Pangaea. Other researchers, while agreeing with the main thrust of this study, questioned the choice of calibration points used to synchronise the data. They proposed that the date of lissamphibian diversification be put in the Permian, rather less than 300 million years ago, a date in better agreement with the palaeontological data. A further study in 2011 using both extinct and living taxa sampled for morphological, as well as molecular data, came to the conclusion that Lissamphibia is monophyletic and that it should be nested within Lepospondyli rather than within Temnospondyli. The study postulated the Lissamphibia originated no earlier than the late Carboniferous, some 290 to 305 million years ago. The split between Anura and Caudata was estimated as taking place 292 million years ago, rather later than most molecular studies suggest, with the caecilians splitting off 239 million years ago.

A fossilized frog from the Czech Republic, possibly Palaeobatrachus gigas
In 2008, Gerobatrachus hottoni, a temnospondyl with many frog- and salamander-like characteristics, was discovered in Texas. It dated back 290 million years and was hailed as a missing link, a stem batrachian close to the common ancestor of frogs and salamanders, consistent with the widely accepted hypothesis that frogs and salamanders are more closely related to each other (forming a clade called Batrachia) than they are to caecilians. The earliest known salientians (see below), closer to the extant frogs than to the extant salamanders, are Triadobatrachus massinoti, from the Early Triassic of Madagascar (about 250 million years ago), and the fragmentary Czatkobatrachus polonicus from the Early Triassic of Poland (about the same age as Triadobatrachus). The skull of Triadobatrachus is frog-like, being broad with large eye sockets, but the fossil has features diverging from modern frogs. These include a longer body with more vertebrae. The tail has separate vertebrae unlike the fused urostyle or coccyx found in modern frogs. The tibia and fibula bones are also separate, making it probable that Triadobatrachus was not an efficient leaper.

Salientia (Latin salere (salio), "to jump") is a stem group including modern frogs in the order Anura and their close fossil relatives the "proto-frogs" (e.g., Triadobatrachus and Czatkobatrachus). The common features possessed by the "proto-frogs" in the Salientia group include 14 presacral vertebrae (modern frogs have eight or 9), a long and forward-sloping ilium in the pelvis, the presence of a frontoparietal bone, and a lower jaw without teeth. The earliest frog fossil that falls into the anuran lineage proper, Prosalirus bitis, lived in the early Jurassic. It was discovered in 1995 in the Kayenta Formation of Arizona and dates back to the Early Jurassic epoch (199.6 to 175 million years ago), making Prosalirus somewhat more recent than Triadobatrachus. Like the latter, Prosalirus did not have greatly enlarged legs, but had the typical three-pronged pelvic structure of modern frogs. Unlike Triadobatrachus, Prosalirus had already lost nearly all of its tail and was well adapted for jumping.

The earliest known "true frog" is Vieraella herbsti, from the Early Jurassic. It is known only from the dorsal and ventral impressions of a single animal and was estimated to be 33 mm (1.3 in) from snout to vent. Notobatrachus degiustoi from the middle Jurassic is slightly younger, about 155–170 million years old. The main evolutionary changes in this species involved the shortening of the body and the loss of the tail. The evolution of modern Anura likely was complete by the Jurassic period. Since then, evolutionary changes in chromosome numbers have taken place about 20 times faster in mammals than in frogs, which means speciation is occurring more rapidly in mammals.

Frog fossils have been found on all continents except Antarctica, but biogeographic evidence suggests they also inhabited Antarctica in an earlier era when the climate was warmer.