Page 10 - DINOSOUR ATLAS

dinosaur atlas
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 d amphibian life cycle
internal skeletons. During the Late Devonian Period, some 370 million years Amphibians such as frogs have
ago, some fish developed simple lungs and limblike fins that enabled them to thin, delicate skins that must be
crawl out of the water onto land. They evolved into amphibians—thin-skinned kept moist. They spawn like fish,
animals such as salamanders and frogs that live in moist places laying jelly-covered eggs in water
or damp places, and fertilizing
and breed in water. About 290 million years ago, them externally. The eggs of most
a branch of the amphibians acquired better species hatch as aquatic larvae,
waterproofing and became reptiles. 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 Adult frog hunts
protective shells. on land, but must
take care that it
does not dry out

stage 4 .
stage 2 . Each tadpole grows front
stage 3 d
Frog eggs hatch as legs and changes shape.
tiny legless As they grow, the tadpoles Its tail shrinks, and it
tadpoles, which develop gills and sprout hind hops out of the water
have feathery gills legs, but still swim with as a tiny frog.
for breathing in water. their tails.

deep, heAvy tAil
would have been an
impediment on land,
but was well adapted
for swimming
d amphibian pioneer
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.