A frog is any member of a primarily carnivorous and diverse group of tailless, short-bodied amphibians composing the order Anura (which literally means without tail in Early Greek). The earliest fossil “proto-frog” arrived in the ancient Triassic of Madagascar, but molecular clock dating hints their origins may stretch further back to the Permian, over 265 million years ago. Frogs are broadly distributed, extending from the tropics to subarctic regions, but the greatest species diversity is in tropical rainforests. There are approximately 7,300 recorded species, which account for about 88% of extant amphibian species. They are also one of the five most distinct vertebrate orders. Warty frog species tend to be called toads, but the difference between frogs and toads is informal, not from evolutionary history or taxonomy.
Complete Evolution of Frogs
The evolutionary and origins relationships between the 3 principal groups of amphibians are disputed. A molecular phylogeny based on an rDNA study dating from 2005 CE suggests that caecilians and salamanders are more closely related to each other than they are to frogs, and the separation of the three groups took place in the early Mesozoic or Paleozoic before the split of the supercontinent Pangaea and soon after they diverged from the lobe-finned fishes. This would help account for the corresponding scarcity of amphibian fossils from the time before the groups split. Another molecular phylogenetic study carried about the same time settled that lissamphibians first emerged about 330 million years ago and that the temnospondyl-origin theory is more credible than other theories.
- The neobatrachians seemed to have introduced in India/Africa,
- the salamanders in South-East Asia,
- And the later caecilians in tropical regions of supercontinent Pangaea.
Salientia (Latin word which means “to jump”) is the name of the whole group that combines modern frogs in the order Anura as well as their fossil relatives, the “stem-frogs” or “proto-frogs.” The common features possessed by these proto-frogs include:
- Fourteen presacral vertebrae (modern frogs have around nine).
- A lower jaw without teeth.
- The bearing of a frontoparietal bone.
- A forward-sloping and long ilium in the pelvis.
The oldest known amphibians that were more intimately related to frogs than to salamanders are Triadobatrachus massinoti, from the Triassic period of Madagascar (around 250 million years ago), and Czatkobatrachus polonicus, from the Early Triassic of Poland.
According to the recent genetic studies, the families Hyloidea, Microhylidae, and the clade Natatanura (including about 88% of living frogs) expanded together some 66 million years ago, around the time of Cretaceous–Paleogene extinction event, which is linked to the Chicxulub impactor. All origins of arboreality (e.g., in Natatanura and Hyloidea) follow from that time and the resurgence of forest that transpired afterward.