Tetrapods are vertebrates that either have four limbs or are descended from vertebrates with four limbs.
The name tetrapod is Greek for "four feet".
Amphibians, reptiles, birds and mammals are tetrapods.
Tetrapods have lungs for breathing air.
Most tetrapods can walk on land for part or all of their lives.
Some tetrapods, such as whales, dolphins and manatees, live in water for their entire lives, but they descended from tetrapods that lived on land.
Snakes are tetrapods. Although they have no legs, they descended from tetrapods with four legs.
Tetrapods can be found all over the world.
There are tetrapods that can fly - birds and bats.
Tetrapods range in size from tiny frogs that measure about 1/3 of an inch long to the blue whale, which can be over 100 feet long.
There are two main groups of tetrapods - amphibians and amniotes.
Amphibians lay eggs in water. The young live in water and breathe through gills, then undergo metamorphosis to become adults.
An adult amphibian usually lives on land and breathes air.
Amniotes include reptiles, mammals and birds.
An amniote embryo develops inside a cavity that is filled with fluid, known as amniotic fluid, and surrounded by a protective membrane.
This cavity may be outside the mother's body - in an egg - or it may be inside the body of the mother.
With this arrangement, the embryo is provided with a liquid environment, so that it can survive on land. Thus, amniotes evolved to have the ability to spend their entire lives on land.
Tetrapods descended from lobe-finned fish. These fish had round, muscular paired fins (pectoral and pelvic fins) with fleshy lobes at their bases.
They could swivel their fins about in shoulder and hip sockets in the same way that tetrapods can swivel their limbs.
Lobe-finned fish could use their fins to pull themselves along the bottom of shallow bodies of water and up onto shores.
Some lobe-finned fish had lungs and could breathe air.
Most of the species of lobe-finned fish that once lived on Earth are now extinct.
The only lobe-finned fish that are alive today are lungfish and the coelacanth.
The first tetrapods lived along coasts and in shallow waters and swamps.
395 million-year-old footprints that were discovered in the Swietokrzyskie Mountains (Holy Cross Mountains) in Poland are believed to be the earliest evidence of tetrapods.
The oldest fossilized tetrapod bones that we know of are about 377 million years old.