Only in hindsight can we recognize a series of evolutionary changes as truly significant in the history of life. For example, only by looking back in time from the perspective of what we know today can we comprehend that the ability of plants and animals to live on land was hugely consequential to the diversity and distribution of life on Earth. In crossing the barriers of living in water to living on land, plants and animals took a step that would open a major new chapter in the history of life. If the cascade of evolutionary changes that enabled life to invade land had not taken place during the Silurian and Devonian Periods, life on earth may have remained only in the seas, where it had been for the preceding 2 billion years or more.

In Thom Holmes's March Onto Land, we see the power of evolution to adapt living things for the new ecological opportunities in semiterrestrial or terrestrial habitats. The evolution of life onto land is also an excellent example of the interconnectedness of life: As one group evolves to better fit new conditions, it creates new opportunities for other groups. Thus, these coevolutionary processes are constantly at work, creating connections between all parts of the ecosystem. Holmes develops this idea within the first section of this book, The Greening of the Paleozoic World, with his examination of plants and arthropods, the true pioneers of terrestrialization. In large part, these organisms created the physical and biological setting that were the crucible for the origin of limbed vertebrates during the latter part of the Paleozoic.

Many discoveries of the earliest limbed vertebrates (early tet-rapods) and their closest ancestors among the lobe-finned fishes have been made in recent years. Thom Holmes has endeavored

_Foreword 13

to present the latest discoveries and the most recent analyses of the growing body of fossil evidence concerning the evolutionary transition from fish to tetrapod that occurred in the late part of the Devonian Period. Holmes discusses recent fossils bearing on this question that have been discovered from locations around the world, including my own experiences in the wonderfully rugged terrain on Canada's Nunavut Territory. You will learn that many of the features we associate with limbed animals, including the basic skeletal structures within limbs, first appeared within the lobe-finned fish. In fact, although it seems counterintuitive, the earliest limbed animals were primarily aquatic, suggesting that rudimentary limbs evolved for use in shallow water and swampy habitats. Only after many millions of years, during which these early tetra-pods developed specialized structures in the limbs and other parts of the body, did they free themselves from dependence on watery habitats.

In Section Three of March Onto Land, Thom Holmes surveys the early amniotes, fully terrestrial tetrapods from the Carboniferous (Pennsylvania and Mississippian) and Permian Periods. This diverse assemblage demonstrates a great range of specializations to living on land—a classic adaptive radiation in which a variety of innovations such as the amniote egg and improved limb structure opened up myriad new ecological opportunities in growing terrestrial habitats. Finally, with a review of the major amniote lineages, March onto Land lays the foundation for any future discussion of the reptilian groups that dominate terrestrial and marine habitats during the Mesozoic.

As one who specializes in the study of Paleozoic vertebrates, I often hear the opinion that "nothing interesting has happened since the Paleozoic." Indeed, vertebrate life made fundamental strides during the Paleozoic. I hope that this book introduces you to many fascinating fossils and helps make clear how and why life evolved as it has. The principles and processes that determined the early evolution of life continued into the Mesozoic and Cenozoic Eras and to the present day, making the history of life on Earth a great odyssey that we share with all other living things. No wonder every fossil is so interesting!

—Dr. Ted Daeschler Academy of Natural Sciences Philadelphia, Pennsylvania

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