Amphibians are cold-blooded (ectothermic) vertebrates. Their
body temperature stays at or near the temperature of the ground
or air. All the amphibian species you will learn about here, will
grow from an egg to a juvenile while underwater. While they
are in that aquatic form, they have no lungs; they must get
oxygen from the water. When they change form
(metamorphose) into an adult, they acquire lungs that can
get oxygen from the air.
Many amphibians must keep their skin moist even after they
migrate away from the vernal pools as adults. Mammal burrows
offer a damp place to hide and hunt for spiders, insects and
other prey. What reptiles might also be hiding and hunting for
For 2 billion years, bacteria were the only creatures on Earth.
Long before the dinosaurs, a special type of bacteria slowly
increased the level of oxygen in the Earth’s air to 20 percent.
Without this oxygen other plants and animals could not have
evolved, including us.