Rise of the mammallike reptiles

The earliest synapsid reptiles to evolve were the pelycosaurs (see pp. 186, 188). From little creatures like Archaeothyris a variety of large reptiles evolved, both carnivores and herbivores. Their heyday was during the early part of the Permian period, when they made up some 70 percent of the terrestrial fauna. Their remains are particularly common in Texas. Some 280 million years ago, the warm, humid, delta floodplains of this region would have swarmed with a variety of early amphibians and fishes, all prey to the powerful jaws of the dominant predators of the day, the pelycosaurs.

Toward the middle of the Permian, another group of mammal-like reptiles arose from the pelycosaurs, and replaced them. These were the therapsids, the direct ancestors of the mammals. They are first found in the rocks of European Russia; their sudden appearance in the fossil record suggests that they may have evolved in more upland areas (where fossilization is less likely).

During the Late Permian, these therapsids spread to the great southern land-mass of Gondwanaland. Many types are found in the Karroo rocks, laid down in the lowland basin of southern Africa. Others are found in European Russia, in the sediments eroded from the newly formed Ural Mountains. Later still, in the Early Triassic, they spread to Asia, South America, India and even Antarctica, all of which were joined together at the time into the supercontinent Pan-gaea (see pp. 10-11).

The therapsids dominated the land until the middle of the Triassic period, and successfully adopted the lifestyles of carnivores, herbivores and insecti-vores. Then, 2 new groups of terrestrial reptile surged into their niches — the early, carnivorous ruling reptiles (see pp. 94-97) and the herbivorous rhynch-osaurs (see p. 89).

Thereafter, the therapsids began their slow decline; 55 million years later, by Mid-Jurassic times, the last group (the tritylodont cynodonts, see p. 193) became extinct. But not before its members had given rise to the first mammals — tiny, shrewlike creatures that were to wait for more than 150 million years before they entered the limelight of vertebrate history.

Through them, the synapsid reptiles had the last ghostly laugh over their dinosaur usurpers, when those great creatures lay dead at the end of the Cretaceous, and the mammal-like reptiles lived on to inherit the earth in the form of their mammalian descendants.

Vital steps toward warm-bloodedness

The main advance of mammals over reptiles is their ability to control their body temperature. Mammals are warmblooded endotherms; reptiles are coldblooded ectotherms (see p. 93). But there is little doubt that the early mammallike reptiles were still cold-blooded, relying on the sun as their prime source of body heat. This is strongly suggested in early synapsids, such as the sphenaco-

dont pelycosaurs. Some of these animals had great skin-covered sails on their backs, the function of which must have been to control body temperature — to absorb heat when cold, and radiate it when warm (see Dimetrodon, p. 188).

Pelycosaurs without such obvious devices, and early therapsids such as the dinocephalians, probably controlled their body temperature simply by increasing their body size. A larger, bulk-

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