Patricia Tucker, age 11, of Bridgeport, Connecticut, for her question:

When did the earliest dinosaurs live?

The Age of Reptiles was the Mesozoic Era of geological history. This dramatic chapter of the past began about 800 million years ago. It was the long, long day of the dinosaurs, when these astounding reptiles dominated the earth. The earliest reptile ancestors had arrived on the scene during a previous age and shared the world with the last of the giant amphibians. But the dinosaur branch of the reptilian clan did not appear until the Triassic Period which opened the Mesozoic Era. At least, we have no fossil or other evidence that they did.

The earliest dinosaurs of 200 million years ago were not much like the lumbering giants and thundering monsters that came later. The first arrivals were chicken sized lizards, striding around on long, strong hind legs. Their short fore legs were used for grabbing and tearing apart their food, which was mostly vegetation. They thrived and the family branched out and an astounding variety of dinosaurs appeared to enjoy the balmy world climate and the lush, green land. Some 50 million years later, giant dinosaurs were the lords of the earth.