What truly defines a frog are the morphological features present in all, including a maximum of 9 vertebrae in the front sacrum, such that the posterior three or four vertebrae are fused into a urostyle (Duellman & Trueb, 1986). Furthermore, frogs do not possess tails into adulthood, and possess a radioulna, which is a fused radius and ulna, and a fused tibiofibula, which is a fused tibia and fibula (Larson, 2004). The hind legs of most species are far longer than the front legs, by means of elongated tibiale and fibulare, an adaptation for leaping (Duellman & Trueb, 1986). This, of course, is a reduced characteristic in those species that have adapted to lifestyles that do not require great leaping, such as the fossorial species. 

Synapomorphies that deifine all taxa in Salientia include 14 presacral vertebrae, elongate and anteriorly directed ilium, the presence of a frotoparietal, and a the lack of tail and teeth (Milner, 1988).

Another important distinction among the frog order is the tadpole stage. The majority of frogs are biphasic, hatching from eggs, and experiencing an aquatic larval stage prior to metamorphosis. In frogs, the aquatic larval stage, or tadpole stage, is very distinct in that tadpoles always fully metamorphose. Contrast this to the caudates, where neoteny or paedomorphism is observed in nine of the ten families, with four families comprised fully of species that retain larval characteristics into adulthood. In addition, anuran larvae always absorb their tails during metamorphosis, whereas the tail is retained in caudates. Tadpoles are highly specialized, compared to caudate and caecilian larvae, and are equipped to consume as much food as possible during this stage. The more food consumed, the faster the tadpole will grow and metamorphose.

The earliest known anuran is the fossil Triadobatrachus massinoti from Madagascar, which existed during the Early Triassic, approximately 250 million years ago (Larson, 2004). This fossil species had not yet evolved all of the features that define the modern frogs, but was a stepping stone of the living amphibians today. The earliest true frog is Vieraella herbsti, from the the Early Jurassic, about 188-213 million years ago (Larson, 2004). Modern frogs existed prior to the major groups of dinosaurs. 

There are more than 4,500 recognized anuran species, making the order anura the largest of the three. Frogs also have the widest distribution, which includes just about every habitat imaginable, with high concentrations in the tropics. The distribution map at right shows the approximate global range of anurans. The largest of the thirty anuran families is Leptodactylidae, comprised of over 1,100 species, followed by Hylidae at more than 800 species, and Ranidae at more than 700 species. Allophrynidae, Nasikobatrachidae, and Rhinophrynidae are the smallest families, all comprised of only one species. For more information about each anuran family, see the Anuran Families and the Taxonomic Model section.

Current anuran taxonomy is rather chaotic at times. There are often disagreements as to the legitimacy of certain taxon, from subspecies level all the way up to the families. In addition, new species are discovered quite frequently in the wild, at least compared to many other animal orders. Our abilities to analyze species at the molecular level are becoming sharper as time goes on, which often results in the re-classification of existing species.  

Milner, A.R. 1988. The relationships and origins of living amphibians. Pp. 59-102 In M. J. Benton (Ed.), The Phylogeny and Classification of the Tetrapods. 1. Amphibians, Reptiles, Birds. Oxford University Press, Oxford.