The Cambrian Period of the Paleozoic Era is known for its remarkable explosion of multicelled organisms with hard shells and exo-skeletons. The evolution of early life during the Cambrian set into motion an arms race between predator and prey that has continued to escalate over the ages with each new species of organism.
From the roots of life in the Cambrian arose the first animals with backbones—the vertebrates. In the vast Paleozoic oceans, fishes of many kinds became the focus of a biological drama that led to their dominance of the world's saltwater and freshwater habitats. Several kinds of Paleozoic fishes such as the sharks and the ray-finned fishes have prospered since their origins more than 400 million years ago. Interestingly, it was a less successful group of fishes, the lobe-finned fishes, that gave rise to the first land vertebrates some 360 million years ago.
when life first explored new opportunities on land
During the late part of the Devonian Period, from about 380 million to 360 million years ago, something remarkable happened. A certain group of fishes developed legs and other key features that would eventually enable them to leave the water to become the first land vertebrates. It was an extraordinary turning point in the history of life on Earth because it triggered the evolution of all land vertebrates and led to the eventual appearance of a curious species known as Homo sapiens. Vertebrate life out of water was founded entirely on the adaptations that arose from a group of big, lazy fish that liked to wallow about in shallow water. The human species is standing on the shoulders of ancestral lobe-finned fishes, and those fishes had the forelimbs to prove it.
March Onto Land tells the story of the Paleozoic animals that conquered the challenge of living out of the water. After having gained a toehold on dry land, terrestrial vertebrates grew in numbers and diversity to become the most important large-bodied organisms on the planet. All 45,000 species of living vertebrates, including humans, have origins rooted deep in the Paleozoic past.
Making the move to land was a change that required many specialized adaptations over millions of years. How fish led to animals with legs, how those early land animals adapted to breathing out of water, and how land animals diversified as the terrestrial experiment of evolution unfolded is the story of March Onto Land. That story leads, in time, to the appearance of the first dinosaurs and their reptilian kin in the Mesozoic Era and to the rise of the mammals, including humans, in the Cenozoic Era.
overview of MARCH ONTO LAND
March Onto Land begins with a look at the dramatic geological and climatic conditions of the Paleozoic Era that made living on land possible for vertebrates. This first section is called, fittingly, The Greening of the Paleozoic World. Chapter 1 describes widespread changes to ocean and land environments, including worldwide climates, that served as catalysts for the spread of plants and invertebrates and then of vertebrates to terrestrial habitats the world over.
Chapter 2 examines the evolution from oceanborne algae of the first land plants and the gradual greening of terrestrial world habitats. Chapter 3 tells the story of the first land animals—invertebrates such as scorpions, giant millipedes, spiders, and insects. The presence on land of plants and invertebrates made the spread of vertebrates to the land possible by providing abundant oxygen (a plant byproduct), stable habitats formed by the rooting of trees and other plants, and life-sustaining food in the form of plants as well as invertebrates.
Section Two, Vertebrates on Land, consists of two chapters that trace the adaptation of backboned animals for life on land. Chapter 4 explains the various anatomical adaptations required for survival out of water and describes how certain lines of fish evolved into the first four-limbed vertebrates, the tetrapods. Chapter 5 takes a close look at specific families of fishes and early tetrapods to illustrate the transition of vertebrates from water to land. As March Onto Land was being written, Dr. Ted Daeschler, the scientific consultant for this volume, made headlines when he and his colleagues Neil Shu-bin and Farish Jenkins announced the discovery of an important new "missing link" in the fossil history of the fish-tetrapod transition. With Ted Daeschler's help, this book features a close look at the discovery—Tiktaalik—a key fossil specimen discovered in Late Devonian rocks of the Canadian Arctic.
Section Three of this book is The Evolution of Early Amniotes. In Chapter 6, the story of early land vertebrates expands into the evolution of reptiles and their kin. One key anatomical adaptation that freed many vertebrates from oceans, lakes, and streams was the ability to lay their eggs on land. These animals developed a new kind of semipermeable, amniotic egg that could protect the developing embryo from its surroundings. Modern-day egg-bearing vertebrates—the amniotes—have their roots in the Paleozoic Era. Mammals, including humans, are also amniotes, although their eggs are developed within the body.
Chapter 7 introduces the major classes of amniotes, including the Anapsida (basal reptiles and turtles), Synapsida (ancestral mammals), Diapsida (lizards, crocodiles, snakes, and extinct dinosaurs and their kin), and the Euryapsida (extinct marine reptiles and their kin). The evolution and adaptations of these animals are traced through the close of the Paleozoic Era and into the beginning of the Triassic Period. It was also during this span of time that all life on Earth faced the most devastating mass extinction event in the fossil record. The Permian-Triassic extinction is examined in March Onto Land from the standpoint of its effect on terrestrial animals and plants and the ways in which some branches of the vertebrate family tree persisted while others perished.
Each chapter in March Onto Land uses an abundance of tables, maps, figures, and photos to depict the life, habitat, and changing evolutionary patterns that affected Paleozoic organisms. Many chapters also include "Think About It" boxes that elaborate on interesting issues, people, and discoveries related to Paleozoic life.
March Onto Land builds on foundational principles of geology, fossils, and the study of life. Readers who would like to refresh their knowledge of certain basic terms and principles in the study of past life may want to consult the Glossary in the back of March Onto Land. Perhaps most important to keep in mind are the basic rules that govern evolution: that the direction of evolution is set in motion first by the traits inherited by individuals, or arising from mutations, and then by the interaction of that individual with its habitat. These changes accumulate, generation after generation, and so allow 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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