Frog - Etymology and Taxonomy

Etymology and Taxonomy

The name frog derives from Old English frogga, abbreviated to frox, forsc and frosc, probably deriving from Proto-Indo-European preu = "to jump". Approximately 88% of amphibian species are classified in the order Anura. These include around 4,810 species in 33 families, of which the Leptodactylidae (1,100 spp.), Hylidae (800 spp.) and Ranidae (750 spp.) are the richest in species.

The use of the common names "frog" and "toad" has no taxonomic justification. From a classification perspective, all members of the order Anura are frogs, but only members of the family Bufonidae are considered "true toads". The use of the term "frog" in common names usually refers to species that are aquatic or semi-aquatic and have smooth, moist skins; the term "toad" generally refers to species that are terrestrial with dry, warty skins. There are numerous exceptions to this rule. The European fire-bellied toad (Bombina bombina) has a slightly warty skin and prefers a watery habitat whereas the Panamanian golden frog (Atelopus zeteki) is in the toad family Bufonidae and has a smooth skin.

Anura includes all modern frogs and any fossil species that fit within the anuran definition. The characteristics of anuran adults include: nine or fewer presacral vertebrae, a long and forward sloping ilium, the presence of a urostyle, no tail, shorter forelimb than hindlimb, radius and ulna fused, tibia and fibula fused, elongate ankle bones, absence of a prefrontal bone, presence of a hyoid plate, a lower jaw without teeth, an unsupported tongue, lymph spaces underneath the skin and a muscle, the protractor lentis, attached to the lens of the eye. The anuran larva or tadpole has a single central respiratory spiracle and mouthparts consisting of keratinous beaks and denticles.

Frogs and toads are broadly classified into three suborders: Archaeobatrachia, which includes four families of primitive frogs; Mesobatrachia, which includes five families of more evolutionary intermediate frogs; and Neobatrachia, by far the largest group, which contains the remaining 24 families of modern frogs, including most common species found throughout the world. Neobatrachia is further divided into the two superfamilies Hyloidea and Ranoidea. This classification is based on such morphological features as the number of vertebrae, the structure of the pectoral girdle and the morphology of tadpoles. While this classification is largely accepted, relationships among families of frogs are still debated.

Some species of anurans hybridize readily. For instance, the edible frog (Pelophylax esculentus) is a hybrid between the pool frog (Pelophylax lessonae) and the marsh frog (Pelophylax ridibundus). The fire-bellied toads Bombina bombina and Bombina variegata are similar in forming hybrids. These are less fertile than their parents, giving rise to a hybrid zone where the hybrids are prevalent.