Joanna Patterson, age 12, of Lemon Grove, California, for her question:

Did other reptiles live with the dinosaurs?

We tend to think that the fantastic dinosaurs had the whole world to themselves. Actually they shared it with many other reptiles, some of which survived to modern times. The snakes did not arrive until much later. But in the days of the dinosaurs, there were ancestral turtles, lizards and crocodiles. There were weird flying reptiles and an assortment of bulky reptiles in the seas. There also were mammal like reptiles and bird type reptiles.

The Age of Reptiles dates back more than 300 million years    and the dinosaurs existed through less than half of this period. The story began in the Carboniferous Period, when the world was mild and moist. Scorpions and a few insects were at home in the ferny, coal making forests, and huge salamanders wallowed in the soggy swamps. There were no birds and no mammals, no flowers and no modern trees.

During this strange scene of long ago, the first reptilian ancestors left the ancient seas and established their future on the land. Later, some of them returned to the sea. Some of the early land reptiles of about 280 million years ago resembled large squat lizards, with or with¬out fantastic crests down their spines.

The most remarkable reptile of the Permian Period was cotylosaurus, a squat,, round nosed lizard type. He is called "the stem reptile" because some of the world's most fantastic creatures stemmed from his line. One branch of his clan led to ancestral turtles, another to lizards which eventually led to the snakes. But the most original offshoots of the stem reptile family tree were the thecondonts.

These fellows were shaped like chickens and ostriches. They appeared in great numbers when the warm, humid Triassic Period began about 230 million years ago. The thecondonts themselves survived a relatively short time, but their descendants branched out into the most fantastic assortment of animals that ever appeared on this earth.

One branch of this clan gave birth to a weird assortment of flying reptiles, another to ancestral crocodiles. Two main branches became the two groups of fantastic dinosaurs that dominated life on earth for more than 100 million years. All the dinosaurs, large and small, were either saurichians, with weak lizard type hipbones, or ornithischians, with stronger, better developed hipbones. In each group, there were carnivores and herbivores, bipeds and four footed dinosaurs.

Through the Jurassic and Cretaceous Periods, the astounding dinosaurs shared the world with reptilian turtles, crocodiles and lizards, with strange flying reptiles, mammal like reptiles and an assortment of lumber¬ing sea going reptiles. Their most astonishing story ended with life's most astonishing mystery. The sea going reptiles and the flying reptiles disappeared and the last of dinosaurs departed about 63 million years ago. And nobody has the faintest idea why, after so many ages of success, all these reptilian creatures perished from the earth.