Quiz 25: Early Tetrapods and Modern Amphibians

Characteristic differences between the aquatic and terrestrial environments that influenced the early evolution of tetrapods are: • Oxygen content, which can be obtained far easily by terrestrials than aquatic ones, by adaptation of lungs and other respiratory structures. • Density, which is 1000 times less buoyant density than water, of air and 50 times less viscous. This led to the development of strong limbed terrestrial tetrapods with adequate structural support. • Temperature regulation needs to be done as air fluctuates in temperature more than in water. • Habitat diversity is present on land with different types of shelters which are absent in the water

The different modes of respiration used by amphibians are: • Cutaneous respiration [through skin] • Respiration through gills and lungs. Amphiumas and terrestrial plethodontids show a paradox in their mode of respiration. Amphiumas being aquatic lost their gills before adulthood. They primarily breathe by lungs and get air by pointing their nostrils above surface of water. The plethodontids are strictly terrestrial but do not have lungs and respire through cutaneous respiration. The epidermis is penetrated with capillary network and air is pumped through the mouth where respiratory gases are exchanged through vascularized membranes of buccal cavity.

The supposed sequence of the evolution of the tetrapod limb is: