Biology (Holt)

Reptiles

Reptiles evolved from amphibian ancestors about 340 million years

ago. Modern reptiles include snakes, lizards, turtles, and crocodiles.

Reptiles are better suited to dry land than amphibians because rep-

tiles’ watertight skin slows the loss of moisture. Reptiles also have

a watertight egg, such as the one shown in Figure 16. Unlike

amphibians, reptiles can lay their eggs on dry land. Amphibians

must lay their eggs in water or in very moist soil because their eggs

are unable to retain enough water to remain alive.

Mammals and Birds

Birds apparently evolved from feathered dinosaurs during or after

the Jurassic period. Therapsids, reptiles with complex teeth and legs

positioned beneath their body, gave rise to mammals about the same

time dinosaurs evolved, during the Triassic period. Sixty-five million

years ago, during the fifth mass extinction, most species disappeared

forever. All of the dinosaurs except for the ancestors of birds became

extinct. The smaller reptiles, mammals, and birds survived. Although

many resources were available to the surviving animals, the world’s

climate was no longer largely dry. Thus the reptiles’ advantages in dry

climates were not so important. Birds and mammals then became

the dominant vertebrates on land.

Both extinctions and continental drift played important roles in

evolution. is the movement of Earth’s land masses

over Earth’s surface through geologic time. Continental drift resulted

in the present-day position of the continents. The movement of con-

tinents helps explain why there are a large number of marsupial

(pouched) mammal species in both Australia and South America,

continents that were once connected.

Continental drift

268 CHAPTER 12History of Life on Earth

Figure 16 Reptiles.
Reptiles, such as this
crocodile, were the largest
group of land-dwelling
organisms until the end of
the Cretaceous period.

Age(in Millions of Years Ago)

Birds and mammals spread Mammals abundant on land

70 ••••••• 60 50 40 30 20 10 .5•Today•

CRETACEOUS PERIOD TERTIARY PERIOD QUATERNARY PERIOD



Fifth mass

extinction



Major mammal

groups evolve



Appearance of

Australopithecus



First

Homo sapiens

Summarize why ozone was important in

enabling organisms to live on land. 7B

Namethe first multicellular organisms that

colonized land. 7B

Identify the first kinds of animals to live on land.

Describethe first kinds of vertebrates that

inhabited land. 7B

Critical Thinking Defendthe argument that

invasion of land could not have happened until

well after the evolution of cyanobacteria. 7B

Mycorrhizae are mutualistic

relationships between the roots of plants and

Aamphibians. Ccyanobacteria.

Binsects. Dfungi.

TAKS Test PrepTAKS Test Prep

Section 3 Review

8B 12B

7B