Keith  Hilliard, age 13, of High Point, North Carolina, for his question:

When was the Mesozoic Era?

Two billion years seems like an impossibly large helping of history to digest. The trick, of course, is to survey it in chronological sections    somewhat like the chapters of a book. It so happens that the earth has kept a sort of diary of her doings and recorded in durable layers of crustal rocks. The events lead logically from one to the next and the age old story divides itself naturally into five major chapters. One of them is the fascinating Mesozoic Era.

An era of geological history lasts 100 million years or more and the long chapter is subdivided into chapters called periods. The Mesozoic Era spanned approximately 125 million years and closed about 60 million years ago. Its logical sequence of events subdivide themselves naturally into three major periods    the Triassic, the Jurassic and the Cretaceous. Our word “zoo” is borrowed from an older word meaning animal. The same word is used to coin the scientific names for the five eras of the earth’s history. It denotes that these chapters record the fabulous story of life.

The term Mesozoic means middle life. However, it is the fourth of the eras and the record of previous life forms goes back at least another half a billion years. It began with small and simple creatures, too fragile to leave fossils in the rocky pages of the earth’s diary. The patient struggle to larger and more complex plants and animals progressed slowly, slowly through the first and second chapters    the Archeozoic and Proterozoic Eras. The successes were inherited by the Paleozoic Era, which recorded life’s astounding conquest of dry land. When this era closed, there were ferny forests populated with wide winged insects, giant salamanders, and small primitive reptiles. About 185 million years ago, these promising achievements were inherited by the Mesozoic Era.

This chapter recorded the Age of Reptiles    the amazing success story of the dinosaurs. In the Triassic Period, they began branching out in various shapes and sizes and some types returned to the sea. The first small, primitive mammals arrived. The Jurassic Period saw the dinosaurs rise to dominate life on land and sea. Some took to the air and became the first feathered creatures. The fabulous success story continued on through the long Cretaceous Period. The insects advanced and so did the little mammals. But meantime the earth made some major changes. There was massive mountain making, the climate cooled and grew drier. Through more than 100 million years, the fabulous dinosaurs had adapted to keep pace with the time. Now, for reasons still unknown, they failed. The last of them became extinct when the Cretaceous Period and the Mesozoic Era closed some 60 million years ago.

The logical sequence of events is recorded in the earth’s rocks but earth scientists can give them only approximate dates. All we can say is that the Mesozoic Era began about 185 million years ago and ended about 60 million years ago. When it ended, the furry mammals were all set to stake their claim to the current Cenozoic Era.