Introduction

The story of the evolution of dinosaurs, which began in the late Triassic and Early Jurassic Epochs, continued in the Middle and Late Jurassic Epochs and is told in Time of the Giants. Herein is the story of an astounding and geologically stable period in the Earth's history, during which dinosaurs seemed to grow to unfettered sizes and succeeded in spreading their domination to all parts of the planet.

Time of the Giants investigates those dinosaurs that arose during the Middle and Late Jurassic Epochs, a span of 30 million years that lasted from about 175 million to 145 million years ago. This was the time of the largest land animals ever to walk the Earth—the sauropods. These giants were not alone, however; other kinds of dinosaurs were diversifying rapidly as well. Most notably, predatory dinosaurs began their own trend towards gigantism, and plated and armored herbivores developed prolifically, filling ecological spaces not tread upon by the long--necked giants.

Another book in this series, Dawn of the Dinosaur Age, defines and introduces the dinosaurs, tracing their origins and their early evolution. After exploring the classification of dinosaurs into two major groups—the Saurischia, or "lizard-hipped" dinosaurs, and the Ornithischia, or "bird-hipped" dinosaurs—Dawn of the Dinosaur Age discusses the traits and lifestyles of the early dinosaurs, especially the theropods and sauropodomorphs.

This book, Time of the Giants, expands on those two lines of dinosaurs by introducing the members of each group that were part of the great Jurassic radiation of dinosaurs. The ornithischians— barely known at all from the Late Triassic and Early Jurassic—play a more significant role as the Jurassic Period continues and are

represented by increasingly diverse groups of highly specialized plant eaters.

OVERVIEW OF TIME OF THE GIANTS

Time of the Giants begins by looking at the geological and ecological conditions that created opportunities for the expansion of dinosaurs in the Middle and Late Jurassic Epochs. Chapter 1 describes widespread changes to ocean and land environments, including worldwide climate changes that served as catalysts for the spread of dinosaurs. One factor that deserves a close look is the way the evolution of plants interacted with the evolution of dinosaurs to create a variety of ecological niches for dinosaurian herbivores.

Chapter 2 explores the broad range of the largest of all land animals, the sauropods. Discussions of sauropod anatomical traits, feeding habits, nesting practices, locomotion, and metabolism bring together many lines of scientific evidence to portray these giants as living, breathing creatures. This chapter also tells the story of the first sauropod discoveries and describes the changing scientific images of sauropods over the past 150 years.

Chapter 3 concentrates on the theropods, or carnivorous dinosaurs. It continues the story of predatory dinosaurs by examining widely diverse theropods of all sizes that first appeared during the latter Jurassic Period. Among these were the first evolutionary experiments in gigantic carnivores, including Allosaurus. The chapter explores the attack and feeding styles of predatory dinosaurs and tackles the subject of dinosaur intelligence. The chapter introduces major families of theropods, including the coelurosaurs; among the coelurosaurs were some small predators that led to the evolution of birds. The first bird, Archaeopteryx, is also a part of the story of Jurassic theropods. In Chapter 3, its significance in the study of bird origins is explored.

Chapter 4 introduces the first well-known ornithischian dinosaurs, the armored and plated herbivores. Among them are Stego-sauria and Ankylosauria. The origins, traits, diversity, and lifestyles of these animals are detailed, along with a review of the kinds of plants that enabled them to define an ecological niche for themselves in the world of giant herbivores.

Each chapter uses an abundance of tables, maps, figures, and photos to depict the conditions, habitats, and changing evolutionary patterns that affected the lives of the early dinosaurs and their kin. Several chapters also include "Think About It" sidebars that focus on interesting issues, people, history, and discoveries related to Mesozoic life.

Time of the Giants builds on the same foundational principles of geology, fossils, and the study of life that are introduced in other volumes of The Prehistoric Earth. Readers who want to refresh their knowledge of certain basic terms and principles in the study of past life may wish to consult the glossary that begins on page 131 of Time of the Giants. Perhaps most important to keep in mind are the basic rules governing evolution: that the process of evolution is set in motion first by the traits inherited by individuals and then by the interaction of a population of a species with those traits with its habitat. Changes that enable the population to survive accumulate generation after generation, often producing and allowing species to adapt to changing conditions in the world around them. As Charles Darwin (1809-1882) explained, "The small differences distinguishing varieties of the same species steadily tend to increase, till they equal the greater differences between species of the same genus, or even of distinct genera." These are the rules of nature that served to stoke the engine of evolution during the Paleozoic and that gave rise to forms of life whose descendants still populate the Earth.

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