Introduction
The North American bullfrog, Rana catesbeiana, is a member of the Anura, which together with the Caudata (salamanders and newts) and the Gymnophiona (caecilians or apodans) form the Lissamphibia, the only living group of amphibians.
Frogs have a long fossil history, with the Early Triassic Triadobatrachus massinoti recognized as the earliest frog. This primitive frog is included with the Anura in the Salienta (although some authorities consider the Salienta and Anura as equivalent). As with lissamphibians generally, anurans tend to have permeable, scaleless skin, kept moist by numerous mucous glands, that allows for considerable cutaneous respiration. Frogs have highly specialized locomotory features that make them instantly recognizable. Most obvious perhaps are that the body is rigid, short, and wide, the hind limbs are long and, familiarly though not exclusively, used for jumping, and the tail is absent. It is to the last feature that the group owes its name: Anuran is derived from the ancient Greek words meaning “without” (an) and “tail” (oura).These specializations, among others, provide ample proof of the risks involved in viewing living vertebrates as primitive or somehow intermediate between other vertebrate grades. They (and the other lissamphibians) are in fact highly derived tetrapods. Anurans are the most successful lissamphibians, including more than 4,000 species, living on all continents except Antarctica. They have diversified into numerous and markedly different ecological types, even within families.
Anurans generally have an aquatic tadpole or larval stage and undergo metamorphosis to produce the radically different adult form, but different reproductive strategies have evolved. Some, such as some members of the Pipidae, a group of specialized aquatic frogs, produce eggs that develop directly into juvenile frogs, whereas other pipids have aquatic larvae. Some species of Nectophrynoides (Bufonidae, the true toads) are viviparous, and in Gastrotheca (Hylidae) the juvenile frogs develop directly in pouches in the female’s skin.
In the species Rhinoderma darwini (Rhinodermatidae) the tadpoles complete their development in the vocal sacs of the male.Long hind limbs used for jumping is the stereotyped frog locomotory behavior. Again, considerable specialization exists among anurans in this regard. For example, pipids are specialized aquatic frogs with webbed feet used for propulsion through water. Several clades (e.g., Centrolenidae, Hylidae, Rhacophoridae) include arboreal frogs, which can move by quadrupedal walking or climbing as well as by leaping. The Hemisotidae include burrowing frogs that dig headforemost, a behavior reflected by their heavily ossified skulls. Several terrestrial frogs tend to hop or walk rather than jump, such as the Bufonidae, which tend to have heavy or robust bodies with relatively short legs.
Although frogs are commonly used in vertebrate dissection courses, it is worth remembering that, as vertebrates, they are neither primitive nor typical—they are just readily available. However, their frequent use, particularly for some species, has been a factor in their decline in many areas. Rana catesbeiana belongs to the Ranidae, although the systematics of this group are not resolved and it may be paraphyletic. R. catesbeiana is a native North American frog with a fairly wide natural distribution and has been introduced in Asia, South America, and parts of Europe. Bullfrogs vary considerably in size, but length tends to be between 10-17 cm, although many will be 20 cm in length. They live in water, so are found near lakes, ponds, and rivers.