What were some of the earliest species that led to amphibians?
Several fossils of pre-amphibian species have been uncovered. For example, one of the earliest known steps to amphibians is the Acanthostega, which evolved about 360 million years ago during late Devonian. It was one of the first to have feet; it also had toes (eight per limb), no fin rays, a large, load-bearing pelvis, and may have retained its gills into adulthood. Another early amphibian fossil is an intermediate between fish and amphibian: the Ichthyostega, with the best fossils having been uncovered in Greenland. This early amphibian lived in the swamps of the Late Devonian period, enjoying mild, warm climates. By this time, too, insects had evolved on land, providing food for the slow-moving amphibian. The Ichthyostega was a three-foot- (one-meter-) long animal with four limbs and a fin on its tail a combination of amphibian and fish features that allowed it to swim and to climb on land.
What were some of the problems amphibians faced in moving from water to land?
The early amphibians main problem was support. In the water, a body is virtually weightless because it is supported by the buoyancy of water. But on land, an amphibians body had to be held up from the ground, and the internal organs needed to be protected from being crushed by gravity; thus, a strong ribcage was essential. The backbone, ligaments, and muscles also had to be strong, supporting not only the weight of the body between the front and hind legs, but also the head. The limbs and limb muscles had to change design to allow walking. Hind limbs became attached to a supportive pelvis, and the skeleton as a whole was made stronger.
Another problem was adapting to breathing on land. Early amphibians had to modify their respiratory system (changing from gills to lungs), as lungs took over more and more of the breathing. The reproductive system, water balance, and senses also had to adapt to the new life in and out of the water. For example, the first amphibians probably spent much of their time in the water, giving birth to totally aquatic young (tadpoles) that would eventually be able to live both in and out of water. Amphibians water dependency adapted to allow them to live out of water as long as they at least stayed damp. Their senses also had to adapt their sight, smell, and hearing taking on more important roles. For instance, amphibian eardrums developed to enable the semi-land dwelling animals to hear sounds in the air. Their eyes had to modify in order to see in air instead of water; protective eyelids developed; and tear ducts evolved, allowing their eyes to be continually moistened with tears.
Even after all these changes, amphibians still were tied to ponds, lakes, or the edges of the oceans, especially since the eggs still had to be laid and hatched in water. Evolution did not change the amphibians too much modern amphibians are still tied to water.
When did reptiles first evolve from amphibians?
It is thought that during the Carboniferous period, a group of amphibians gave rise to the reptiles. The first reptiles were small, lizard-sized animals, but they had many differences from their amphibian ancestors, including waterproof skin and thick-shelled eggs. This made it unnecessary for the reptiles to stay near water, to keep moist, or to lay their eggs in water. In fact, the evolution of the reptiles, geologically speaking, occurred very rapidly. Within about 40 million years, the reptiles had produced thousands of different species.
What was one of the most important changes that enabled reptiles to become true land-dwelling animals?
The development of the amniote egg freed the reptiles completely from life in the water by allowing them to fully reproduce on land. Unlike the young of amphibians, who had to go through a larval stage in the water before metamorphosing into an adult, the amniote egg acted as a sort of private pond for the young reptiles.
The egg itself had a hard shell, which contained numerous small pores. These pores allowed air to enter, but the shell prevented the inside from drying up as long as the surroundings were humid. The eggs were fertilized inside the mothers body before being laid. There were three very thin bags inside the shell itself, each of which had a specific function. The first bag held the developing young and a liquid (which took the place of the pond or stream); this area was called the amnion, from which the egg gets its name. The next bag contained the yolk, the source of food for the developing embryo. The third bag was in contact with the air diffusing in through the shell. Thus, the young reptiles had food, air, protection from predators, and an aquatic environment in which to grow. The young would eventually hatch into a miniature version of its parents and was able to fend for itself. Because of the egg, the reptiles no longer had to have a source of water to reproduce and could spread out, populating the land as well as hunting for prey well away from water.