These young adult dinosaursDinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago, although the exact origin and timing of the evolution of dinosaurs is the subject of active research. They became the dominant terrestrial v… mated then, and the female dinosaur laid new eggs in a nest. The eggs hatched, and more dinosaurs were born . This is called a life cycle. New members of a group are born; they grow to mature creatures; they mate and have babies; then they get old and die.