When the Invasion of Land Failed

When the Invasion of Land Failed: The Legacy of the Devonian Extinctions

George R. McGhee
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  • Book Info
    When the Invasion of Land Failed
    Book Description:

    The invasion of land by ocean-dwelling plants and animals was one of the most revolutionary events in the evolution of life on Earth, yet the animal invasion almost failed -- twice -- because of the twin mass extinctions of the Late Devonian Epoch. Some 359 to 375 million years ago, these catastrophic events dealt our ancestors a blow that almost drove them back into the sea. If those extinctions had been just a bit more severe, spiders and insects -- instead of vertebrates -- might have become the ecologically dominant forms of animal life on land.

    This book examines the profound evolutionary consequences of the Late Devonian extinctions and the various theories proposed to explain their occurrence. Only one group of four-limbed vertebrates exists on Earth, while other tetrapod-like fishes are extinct. This gap is why the idea of "fish with feet" seems so peculiar to us, yet such animals were once a vital part of our world, and if the Devonian extinctions had not happened, members of these species, like the famousAcanthostegaandIchthyostega, might have continued to live in our rivers and lakes. Synthesizing decades of research and including a wealth of new discoveries, this accessible, comprehensive text explores the causes of the Devonian extinctions, the reasons vertebrates were so severely affected, and the potential evolution of the modern world if the extinctions had never taken place.

    eISBN: 978-0-231-53636-3
    Subjects: General Science

Table of Contents

  1. Front Matter
    (pp. i-vi)
  2. Table of Contents
    (pp. vii-viii)
  3. Preface
    (pp. ix-xiv)
  4. CHAPTER 1 The Evolution of Life on Land
    (pp. 1-26)

    The planet Earth is 4,560 million years old. For over 3,000 million years, life on Earth was confined to the oceans, and the land areas of the planet looked much like the rocky landscapes of Mars today. Complex life on land, and the terrestrial ecosystem, is a relatively new phenomenon in the history of the Earth. The oldest simple terrestrial ecosystems of macroscopic organisms—plants, fungi, and animals—can be traced back only to the middle of the Ordovician Period of geologic time (for the geologic timescale, see table 1.1),¹ some 468 million years ago. That is, only in the...

  5. CHAPTER 2 The Plants Establish a Beachhead
    (pp. 27-52)

    Plants are photoautotrophic organisms; that is, they have the ability to synthesize complex hydrocarbons like sugars—food—from simple molecules and water by using energy obtained from sunlight. Autotrophic organisms form the base of the trophic pyramid in any ecosystem, whether it be marine or terrestrial, and are themselves used as a source of food by the heterotrophic organisms in the ecosystem—the animals—who cannot synthesize food for themselves. Thus, in order for animals to leave the oceans and invade the terrestrial realm, plants must first establish a beachhead on dry land.

    Land plants, the Embryophyta (table 2.1), evolved...

  6. CHAPTER 3 The First Animal Invasion
    (pp. 53-98)

    In the previous chapter, we considered what we know about the land plants that built the foundation of the trophic pyramid of the terrestrial ecosystem. In this chapter, we will consider what we know about the construction of the higher levels of the trophic pyramid: the evolution of the terrestrial herbivorous animals, which live by eating the plants in the foundation of the trophic pyramid, and the evolution of the terrestrial carnivorous animals, which live by eating the herbivores.

    At first glance, the construction of the terrestrial trophic pyramid would seem to be a straightforward evolutionary progression: the plants evolve...

  7. Color plates
    (pp. None)
  8. CHAPTER 4 The First Catastrophe and Retreat
    (pp. 99-158)

    The Late Devonian biodiversity crisis struck the Earth.¹ In the waning years of the Frasnian, the vertebrates and arthropods on land began to die in greater numbers. Reproduction rates in the living fell, and fewer and fewer young vertebrates and arthropods were born into terrestrial habitats. Population sizes inexorably declined, generation after generation, as land animals produced fewer matings and fewer offspring. With the deaths of their last members, vertebrate and arthropod species began to vanish from the land. By the end of the Frasnian, all of the known Frasnian tetrapod species had perished. Gone.

    They were not alone. In...

  9. CHAPTER 5 The Second Animal Invasion
    (pp. 159-178)

    An evolutionary or genetic bottleneck is produced when the population sizes of a given species shrink almost to the critical minimum level from which a species cannot recover. Such a critical reduction in species’ population sizes can be produced either by the abrupt death of the majority of the individuals in the populations or by the failure of the individuals in one generation of the populations to reproduce enough offspring to ensure the survival of the subsequent generation—or by both processes acting together simultaneously.

    One of the immediately observable consequences of a species surviving an evolutionary bottleneck is the...

  10. CHAPTER 6 The Second Catastrophe and Retreat
    (pp. 179-212)

    The end-Devonian biodiversity crisis struck the Earth.¹ In the waning years of the Famennian, vertebrates and arthropods once again began to die in greater numbers. Population sizes once again dwindled, became smaller and smaller, and vanished. By the end of the Famennian, all of the known Famennian tetrapod species had perished.

    On land, the huge forests of the Earth began to die. The trunks of the great lignophyte treeArchaeopteris hibernica(color plate 5) still towered 30 meters (100 feet) into the skies—but their leaves were gone, they were dead. One by one, the great trunks fell. Only a...

  11. CHAPTER 7 Victory at Last
    (pp. 213-262)

    Just as in the End-Frasnian Bottleneck,¹ the same classic evolutionary bottleneck phenomena of reduction in morphological variance and geographic range are seen in the tetrapod species appearing in the fossil record in and after the Tournaisian Gap, and I refer to this effect as the “End-Famennian Bottleneck.” McGill University paleontologist and tetrapod specialist Robert Carroll comments on this reversal of the fortunes of the numerous Famennian tetrapods and the peculiar nature of the amphibians that are found in the Early Carboniferous:

    Fossils from the very end of the Devonian appear on the verge of a new way of life, lacking...

  12. CHAPTER 8 The Legacy of the Devonian Extinctions
    (pp. 263-276)

    At the very least, the End-Frasnian Bottleneck delayed the course of vertebrate evolution by some seven million years. It took that long for the harsh climates of the Famennian Gap to ameliorate, and for the Earth to once again become more hospitable to life.

    However, the effect of that bottleneck did not merely delay vertebrate evolution; it also altered the direction of vertebrate evolution. Tetrapods suffered a sharp reduction in their previously morphologically diverse forms. In particular, the diversity of sizes seen in Frasnian tetrapods was lost, as only the mid-sized forms survived the bottleneck to re-diversify in the Famennian...

  13. Notes
    (pp. 277-294)
  14. References
    (pp. 295-312)
  15. Index
    (pp. 313-318)