sought out a shaded area or exposed itself to the wind. This appendage might
have been a crude forerunner of the temperature control system in mammals.
As the climate warmed, the pelycosaurs lost their sails and perhaps
gained some degree of internal thermal control. They thrived for about 50
million years.Then they gave way to their descendants, the mammal-like rep-
tiles called the therapsids. The first therapsids retained many characteristics of
the pelycosaurs, with legs better adapted for much higher running speeds.
They ranged in size from as small as a mouse to as large as a hippopotamus.
The early members invaded the southern continents at the beginning of
the Permian when those lands were recovering from the Carboniferous
glaciation.This suggests the animals were sufficiently warm-blooded to with-
stand the cold. They had probably undergone some physiological adaptations
to enable them to feed and travel through the snows of the cold winters.They
were apparently too large to hibernate, as shown by the lack of growth rings
in their bones similar to tree rings that mark alternating seasons of growth.
The development of fur appeared in the more advanced therapsids as they
migrated into colder climates. Therapsids might also have operated at lower
body temperatures than most living mammals to conserve energy.
The family of mammal-like reptiles clearly shows a transition from rep-
tile to mammal. Mammals evolved from the mammal-like reptiles over a
period of more than 100 million years. During that time, the animals adapted
so as to function better in a terrestrial environment. Teeth evolved from sim-
ple cones replaced repeatedly during the animal’s lifetime to more complex
shapes replaced only once upon maturity. However, the mammalian jaw and
other parts of the skull still shared many similarities with reptiles.
Mammals are fully warm-blooded. The advantages of being warm-
blooded are tremendous. A stable body temperature finely tuned to operate
within a narrow thermal range provides a high rate of metabolism indepen-
dent of the outside temperature.Therefore, the work output of legs, heart, and
lungs increases enormously, giving mammals the ability to out performand
out endure reptiles. The principle of heat loss for large reptiles, by which a
large body radiates more thermal energy than a small one, applies to large rep-
tiles as well as mammals. In addition, mammals have a coat of insulation
including an outer layer of fat and fur to prevent the escape of body heat dur-
ing cold weather.
The therapsids appear to have reproduced like reptiles by laying eggs.They
might have protected and incubated the eggs and fed their young.This, in turn,
might have resulted in longer egg retention in the female and led to live births.
The therapsids dominated animal life for more than 40 million years until the
middle Tr iassic.Then for unknown reasons, they lost out to the dinosaurs. From
then on, primitive mammals were relegated to the role of a shrewlike nocturnal
hunter of insects until the dinosaurs finally became extinct.Historical Geology