amphibians.The superiority of the reptiles was largely due to their more effi-
cient 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 vari-
ety 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 walk-
ing 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 120The small
plant eater camptosaur, an
ancestor of many later
dinosaurs, ran on its hind
legs for speed and agility.PERMIAN REPTILES