lacked holes in the sides of their skulls, were once regarded as outsiders among
modern reptiles because living reptiles and birds termed diapsids have two
holes in the sides of their skulls.
MAMMAL-LIKE REPTILES
The mammal-like reptiles were animals in transition from reptiles to mam-
mals. Fossilized bones of a mammal-like reptile with large down-pointing
tusks called Lystrosaurus were discovered in the Transantarctic Range of
Antarctica. Its presence provides strong evidence that the continent had once
been joined with southern Africa and India. As an ancestor to mammals,
lystrosaurus was the most common vertebrate on land and was found
throughout Pangaea. Mammal-like reptiles called dicynodonts also had two
caninelike tusks and fed on small animals along riverbanks.
The pelycosaurs were the first animals to depart from the basic reptilian
stock some 300 million years ago.They were distinguished from other reptiles by
their large size and varied diet. The earliest predators could kill relatively large
prey, including other reptiles.A pelycosaur called dimetrodon (Fig. 123) obtained
a length of about 11 feet. It had a large dorsal sail composed of webs of mem-
brane well supplied with blood, stretching across bony protruding spines and
probably used for temperature control. When the animal was cold, it turned its
body broadside to the sun to absorb more sunlight.When the animal was hot,it
Figure 123Dimetrodon
used the huge sail on its
back to regulate its body
temperature.PERMIAN REPTILES