Frog - Evolution

Evolution

The origins and evolutionary relationships between the three main groups of amphibians is hotly debated. A molecular phylogeny based on rDNA analysis dating from 2005 suggests that salamanders and caecilians are more closely related to each other than they are to frogs and that 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 fish. 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 that 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 that is 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 that 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.

In 2008, a fossil of Gerobatrachus hottoni, a stem anuran with many salamander-like characteristics, was discovered in Texas. It dated back 290 million years and was hailed as a missing link, a common ancestor of frogs and salamanders. It seemed to indicate that frogs and salamanders are more closely related to each other than they are to caecilians. Before that, the earliest known proto-frog was Triadobatrachus massinoti, from the 250 million-year-old early Triassic of Madagascar. Its skull is frog-like, being broad with large eye sockets, but the fossil has features diverging from modern frogs. These include a longer body and 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 fourteen presacral vertebrae (modern frogs have eight or nine), 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. It is likely that the evolution of modern Anura was complete by the Jurassic period. Since then, evolutionary changes in chromosome numbers have taken place about twenty times faster in mammals than in frogs, which means that speciation is occurring more rapidly in mammals.

An early, well-preserved fossil of Sanyanlichan, which lived 125 million years ago, was found in China in 2001. It had all the characteristics of modern frogs but there were nine presacral vertebrae in its backbone instead of the eight found in present-day species. It is believed to be the ancestor of modern discoglossid frogs such as the midwife toad (Alytes) and the fire-bellied toad (Bombina). Frog fossils have been found on all continents except Antarctica but biogeographic evidence suggests they also inhabited that continent in an earlier era when the climate was warmer.

A cladogram showing the relationships of the different families of frogs in the clade Anura can be seen by clicking "Show" on the right. This diagram, in the form of a tree, shows how each frog family is related to other families, with each node representing a point of common ancestry. It is based on Frost et al. (2006) and Heinicke et al. (2009).