Rise of the Reptiles

Complex, multicellular life evolved in the oceans, some 3 billion years after the appearance of the first bacteria. Invertebrates resembling modern sea squirts eventually gave rise to primitive fish, which had bodies supported by internal skeletons. During the Late Devonian Period, some 370 million years ago, some fish developed simple lungs and limblike fins that enabled them to crawl out of the water onto land. They evolved into amphibians—thin-skinned animals such as salamanders and frogs that live in moist places and breed in water. About 290 million years ago, a branch of the amphibians acquired better waterproofing and became reptiles.


Amphibians such as frogs have thin, delicate skins that must be kept moist. They spawn like fish, laying jelly-covered eggs in water or damp places, and fertilizing them externally. The eggs of most species hatch as aquatic larvae, which eventually metamorphose into land-dwelling adults.

stage 1

The eggs must be fertilized in the water after being laid, so they do not have tough protective shells.

Adult fROG hunts on land, but must take care that it does not dry out

Frog eggs hatch as tiny legless tadpoles, which have feathery gills for breathing in water.

sTAGE 3 d

As they grow, the tadpoles develop gills and sprout hind legs, but still swim with their tails.

Each tadpole grows front legs and changes shape Its tail shrinks, and it hops out of the water as a tiny frog.

Deep, heavy tail would have been an impediment on land, but was well adapted for swimming sTAGE 4 ►

Deep, heavy tail would have been an impediment on land, but was well adapted for swimming d amphíbían píoneer

An early four-legged amphibian, Acanthostega was about 2 ft (60 cm) long, with a flattened body and a long tail. It seems to have lived rather like a modern crocodile, feeding largely on fish in the water, but its strong vertebrate skeleton would have enabled it to roam freely through the Devonian swamps.


During the Carboniferous Period, the amphibians diversified into a wide variety of forms. Some were tiny, newtlike creatures, while later species such as Eryops were massively built predators.


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