11.16–11.20: All terrestrial vertebrates are tetrapods.

Capybara (Hydrochoerus hydrochaeris ) rodents found in South America. They can reach more than 200 pounds (91kg)!

The ancestors of all vertebrates that live on land had lungs and four legs. Some modern vertebrates, such as whales and snakes, have evolved so that their limbs are reduced to a few shrunken and unused bones. Whales and some snakes have returned to living in water, and some salamanders have lost lungs and breathe through their skin, but these are recent changes, occurring in species whose ancestors had lungs and four legs. Taxonomically, all terrestrial vertebrates are tetrapods (tetra = four; poda = feet).

Terrestrial vertebrates are divided into two main groups: (1) animals (called non-amniotes) such as amphibians that reproduce in water and do not have desiccation-proof amniotic eggs; and (2) animals (called amniotes) such as reptiles, birds, and mammals that have amniotic eggs. We discuss amphibians here, then reptiles and mammals in the next two sections.

The first terrestrial vertebrates were amphibians, from the Greek word amphibios, meaning “living a double life.” There are two stages in the life of most amphibians: a water-breathing juvenile form and an air-breathing adult form. The adults of many amphibian species lay their eggs in water (FIGURE 11-26). Amphibian eggs lack the waterproof layers that allow amniotes to lay eggs on land, so most of the 6,000 species in this group are still significantly tied to life in the water—because they must always be near water to lay their eggs, which are simple structures, not unlike fish eggs. Frogs and toads make up the vast majority of amphibians (about 5,400 species). Other amphibians include salamanders (550 species) and caecilians, a group of legless, burrowing animals (170 species).

Figure 11.26: From egg to larva to adult: the amphibian life cycle.

The juvenile stage of frogs, called a tadpole, lives in water, lacks legs, and eats algae. A few weeks (or up to many months in some species) after hatching, metamorphosis occurs, and the tadpole develops legs, lungs, and a digestive system fit for its adult life as a carnivore. Adult frogs have thin, moist skin through which gas (oxygen and carbon dioxide) exchange with the air can take place. When a frog is underwater, for example, oxygen diffuses directly from the water into the blood.

The past two decades have seen a stunning decline in amphibian populations throughout the world, and almost a third of all amphibian species are endangered or threatened. This situation is attributable to a combination of causes: climate change, habitat degradation, fungal diseases, and increased pollution.