LOS DINOSAURIOS

This chapter examines the evolution of the reptiles during the Permian

period and covers the greatest mass extinction in Earth history. The

Permian, from 280 to 250 million years ago, was named for a wellexposed

sequence of marine rocks and terrestrial redbeds on the western side

of the Ural Mountains in the Russian district of Perm. Rocks of Permian age

are distinct in western North America, particularly Texas, Nevada, and Utah

(Fig. 119). Important reserves of oil and natural gas reside in the Permian

Basin of Texas and Oklahoma. Extensive coal deposits of Permian age exist in

Asia, Africa, Australia, and North America.

During the Permian, all major continents were combined into the

supercontinent Pangaea, where widespread mountain building and extensive

volcanism were prevalent. The interior of Pangaea was largely desert,

causing the decline of the amphibians in favor of the reptiles. At the end of

the Permian, perhaps the greatest extinction Earth has ever known eliminated

more than 95 percent of all species, paving the way for the ascension

of the dinosaurs.

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PERMIAN REPTILES

THE AGE OF DESERT INHABITANTS

9

THE REPTILIAN ERA

The reptile age, which had its beginning in the Permian and lasted 200 million

years, witnessed the evolution of some 20 orders of reptilian families.

Amphibians, which were prominent in the Carboniferous, declined considerably

in the Permian because of a preference for life in the water. When the

Carboniferous swamps dried out and were largely replaced with deserts, the

amphibians gave way to the reptiles, which were well adapted to drier climates.

In the latter part of the Permian, the reptiles succeeded the amphibians

and became the dominant land-dwelling animals of the Mesozoic era. The

generally warm climate of the Mesozoic was advantageous to the reptiles and

aided them in colonizing the land.

The increase in the number of reptilian fossil footprints in Carboniferous

and Permian sediments shows the rise of the reptiles at the expense of the

Figure 119 Fremont

Canyon with the Henry

Mountains in the

background,Wayne and

Garfield Counties, Utah.

(Photo by J. R. Stacy,

courtesy USGS)

160

Historical Geology

amphibians.The superiority of the reptiles was largely due to their more efficient

mode of locomotion. Even at an early age, 290 million years ago, small

reptiles were bipedaled and ran on two legs (Fig. 120), the fastest way to travel.

They needed this speed not only to run down prey but also to escape a variety

of meat-eating reptiles, including dimetrodon, a fierce Permian carnivore.

The most compelling evidence for bipedalism is that the length of the

hind limbs are much longer than the forelimbs and would therefore make walking

on four legs awkward.These were members of the diapsid group, one of the

most primitive reptile lineages. It gave rise to dinosaurs, birds, and most living

reptiles, including crocodiles, lizards, and snakes. The reptiles were also better

suited to a full-time life on dry land. In contrast, the amphibians depended on

a local source of water for moistening their skins and for reproduction.

The reptilian foot was a major improvement over that of the amphibian,

with changes in the form of the digits, the addition of a thumblike fifth digit,

and the appearance of claws. In some reptiles, the tracks narrowed and the

stride lengthened. Others maintained a four-footed walking gait and ran

Figure 120 The small

plant eater camptosaur, an

ancestor of many later

dinosaurs, ran on its hind

legs for speed and agility.

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PERMIAN REPTILES

reared up on their hind legs.Although most reptiles walked or ran on all fours,

by the late Permian, some smaller reptiles often stood on their hind legs when

they required speed and agility.The body pivoted at the hips, and a long tail

counterbalanced the nearly erect trunk. This stance freed the forelimbs for

attacking small prey and completing other useful tasks.

Reptiles have scales that retain the animal’s bodily fluids. In contrast,

amphibians have a permeable skin that must be moistened frequently.Another

major advancement over the amphibians was the reptiles’ mode of reproduction.

Like fish, amphibians laid their eggs in water. After hatching, the young

fended for themselves, often becoming prey for predators. The reptiles’ eggs

...