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Chapter 14 Epilogue
391
CHAPTER 14
Epilogue that grand subject, that almost keystone of the laws of creation, geographical distribution. Charles Darwin, in a letter to J. D. Hooker, 1845.
When a new species evolves, if it does not immediately become extinct, it will proceed to enlarge its territory. As it does so, it produces a geographic pattern. This pattern will change and persist through time as long as that species, and the phyletic line formed by it, lasts. If the pattern in its entirety could be recovered, it would reveal the origin, the expansion, the contraction, and end of that evolutionary line. This biogeographic ideal cannot be realized because the fossil record is never that complete. For phyletic lines represented by living species, it is possible to gain insight into the historical pattern by working out the phylogeny and comparing it to the present distribution. If the living group has a fossil record, the chances of working out its geographic history may be enhanced. Recent progress in paleontology, geophysics, climatology, and in the systematics of various groups of animals and plants, has provided us with a new perspective of biogeography. The emerging view is one of continuous change, a history of dynamic shifts in the biota in response to alterations in the physical environment. The changes in patterns of dispersal and evolution remain hazy and uncertain for much of the Paleozoic, but become sharper and more informative in the Mesozoic and Cenozoic. Two related series of physical events have had profound effects and are, in large part, responsible for the biogeography of today's world. These are the making and breaking of land and sea barriers and global sea-level fluctuations. The first momentous event was the fragmentation of the (theoretical) Proterozoic supercontinent called Rodinia. This tectonic activity may have caused a sea-level rise, flooding of the continental shelves, and a climatic amelioration. These changes possibly set the stage for the most important evolutionary radiation in the history of earth's metazoan biota. As many as 100 different phyla may have evolved during the Cambrian Period. The Paleozoic continents remained separated until the latest Silurian, about 410 Ma ago. This marked a consolidation between Baltica and Laurentia which formed the new continent of Laurussia. At the same time, some continental terranes became detached from Gondwana and moved northward. The next significant collision was that between Gondwana and Laurussia which took place in late Devonian about 365 Ma ago. During the Carboniferous and early Permian, the principal continents of Gondwana, Laurussia,
392
Epilogue
Siberia, and Kazakhstania moved closer together. By the late Permian, the formation of Pangaea was virtually complete. The breakup of Pangaea began, in about the mid-Jurassic, with the separation of Madagascar from Africa and the creation of the Turgai Sea which separated Euramerica from Asia. By the late Jurassic-early Cretaceous, Africa was separating from Euramerica, South America, and Australia-Antarctica. The latter pair may have already moved apart. By the mid-Cretaceous, rising sea level had formed the great Mid-continental Sea which divided Euramerica. In the late Cretaceous, Westamerica became connected to Asia and an isthmus attached South America to Central and North America. By the end of the Cretaceous, the Mid-continental Sea had dried up. The early Central American isthmus may have persisted into the Paleocene. In the early Eocene, the North Atlantic Ocean expanded, breaking the link between Europe and North America. In the Paleocene-Eocene, a filter bridge (probably an archipelago) became available between South America and Australia-New Zealand via Antarctica. India became firmly fused with Asia by the Eocene. With the desiccation of the Turgai Sea in the Oligocene, Europe was attached to Asia. By the early Miocene, Africa became connected to Asia via the Arabian Peninsula. This marked the end of the Tethys Sea and the beginning of the Mediterranean. With the flooding of Beringia about 3.5 Ma ago, a marine Transarctic Biotic Interchange took place. Some 0.5 Ma later, the formation of the Panamanian isthmus permitted the terrestrial Great American Biotic Interchange. All of these continental breakages and linkages created dispersal corridors in one environment and, at the same time, dispersal barriers in the other. The series of historic extinction events had important biogeographic and evolutionary consequences. We now live in an era of neocatastrophism in which it is widely believed that the great extinctions were sudden, catastrophic events caused by extraterrestrial impacts or volcanic eruptions or both. On the other hand, the fossils of many organisms tell us that the extinction episodes developed gradually over periods of one to several Ma. The recovery periods tended to take even longer. Throughout its history, the earth has been struck by many missiles from outer space and numerous volcanic eruptions have taken place. But these sudden, high-energy events are not consistent with the tempo of the global extinctions. The one event that is consistent is the regression in sea level. As Hallam (1992) has pointed out, the most important cause of sea-level change involves alteration in the volume of the oceanic ridges. These structures may vary in both spreading rate and overall length. The changes may take place at a rate of about 1 cm per 103 years. Continental compression can cause the sea level to fall at a comparably slow rate. Glacioeustatic changes can take place much more rapidly but their importance throughout the Phanerozoic is controversial because of limited evidence for the existence of major ice sheets. Ocean-trench subsidence and the rise of mantle plumes appear to be of negligible importance. Neither oceanic sedimentation nor the desiccation of isolated basins is likely to have played a major role. The volume of ocean water has probably remained constant through the Phanerozoic, the addition of juvenile water being more or less counteracted by the subtraction of pore water in sediments by subduction. The implication of sea-level regressions as the primary cause of the historic extinctions is only part of the story. The regressions themselves did not have much direct ef-
Chapter 14
393
fect, except on the marine biota of the continental shelves. As the sea level dropped, the continents became larger and higher. These changes caused the continental climates to become colder and dryer. The cessation of tectonic-plate movement produced less volcanism and a drop in the production of CO2 from that source. Increased weathering of the exposed land absorbed greater amounts of CO2. The drop in atmospheric CO2 meant more heat lost to space by radiation. The result was a decrease in global temperature related to the extent of the regression. The larger, higher continents caused severe weather patterns which affected the entire globe. So, sea-level regressions, depending on their magnitude, caused climatic changes which had a detrimental effect on diversity on land and in the sea. Conversely, sea-level rises had the opposite effect. Long-term sea level cycles, called supercycles, appear to be the proximal causes of the historic alteration between greenhouse and icehouse conditions. The relative positions of the continents, considering their effects on the major oceanic currents, are also important factors in the global climatic cycles. There has been considerable speculation about the evolutionary effects of extinction episodes. These provide a consistent message which gives the impression that a "wiping out of the old forms to make way for the new" has long-term evolutionary benefits. But, the extinction episodes have had a much greater effect on the tropical organisms than on those at higher latitudes. When this happens, the advanced tropical species tend to be replaced by more primitive forms from higher latitudes or other refuges. It has been argued that the historical extinction episodes were disastrous interruptions to evolutionary progress. They set back the clock of evolutionary time and destroyed communities that took millions of years to reassemble. In the preface, it was noted that a complete biogeography should offer a prognosis for the future. This is a painful task, for we are in the midst of the most-rapid decrease in species diversity ever recorded. It means that most species now living on earth will be lost within the next 200 years. This will nullify all the diversity increase of the past 65 Ma, leaving a depauperate world to be studied by future biogeographers. As the species diversity declines, it will become increasingly difficult to reconstruct historic trends in biogeography and evolution. Although many causes of this catastrophic decline have been identified, the primary one is seldom mentioned. This is the effect of the exploding human population which increases by 95 million each year, or about 260 000 per day. Religious opposition has made human population control a taboo subject. Most conservation groups have avoided the problem. It is the missing agenda (Meffe et al., 1993). This work is primarily an examination of historic changes in the marine and terrestrial biotas. The scope of the subject is so large and the literature so voluminous, that one cannot possibly find every publication of value and give it the recognition it deserves. Also, every biologist interested in biogeography has his or her own concept of how the subject should be addressed. There are very few generalists, so that each person will look at biogeography from a different perspective, one that has been shaped by specialty and experience. While this work is a book of fact, as opposed to fiction, my interpretation of the facts may be quite different than the conclusions of others. As John Steinbeck observed, "The design of a book is the pattern of a reality controlled and shaped by the mind of the writer. This is completely understood about poetry or fiction, but it is too seldom realized about books of fact."
CHAPTER 14
Epilogue that grand subject, that almost keystone of the laws of creation, geographical distribution. Charles Darwin, in a letter to J. D. Hooker, 1845.
When a new species evolves, if it does not immediately become extinct, it will proceed to enlarge its territory. As it does so, it produces a geographic pattern. This pattern will change and persist through time as long as that species, and the phyletic line formed by it, lasts. If the pattern in its entirety could be recovered, it would reveal the origin, the expansion, the contraction, and end of that evolutionary line. This biogeographic ideal cannot be realized because the fossil record is never that complete. For phyletic lines represented by living species, it is possible to gain insight into the historical pattern by working out the phylogeny and comparing it to the present distribution. If the living group has a fossil record, the chances of working out its geographic history may be enhanced. Recent progress in paleontology, geophysics, climatology, and in the systematics of various groups of animals and plants, has provided us with a new perspective of biogeography. The emerging view is one of continuous change, a history of dynamic shifts in the biota in response to alterations in the physical environment. The changes in patterns of dispersal and evolution remain hazy and uncertain for much of the Paleozoic, but become sharper and more informative in the Mesozoic and Cenozoic. Two related series of physical events have had profound effects and are, in large part, responsible for the biogeography of today's world. These are the making and breaking of land and sea barriers and global sea-level fluctuations. The first momentous event was the fragmentation of the (theoretical) Proterozoic supercontinent called Rodinia. This tectonic activity may have caused a sea-level rise, flooding of the continental shelves, and a climatic amelioration. These changes possibly set the stage for the most important evolutionary radiation in the history of earth's metazoan biota. As many as 100 different phyla may have evolved during the Cambrian Period. The Paleozoic continents remained separated until the latest Silurian, about 410 Ma ago. This marked a consolidation between Baltica and Laurentia which formed the new continent of Laurussia. At the same time, some continental terranes became detached from Gondwana and moved northward. The next significant collision was that between Gondwana and Laurussia which took place in late Devonian about 365 Ma ago. During the Carboniferous and early Permian, the principal continents of Gondwana, Laurussia,
392
Epilogue
Siberia, and Kazakhstania moved closer together. By the late Permian, the formation of Pangaea was virtually complete. The breakup of Pangaea began, in about the mid-Jurassic, with the separation of Madagascar from Africa and the creation of the Turgai Sea which separated Euramerica from Asia. By the late Jurassic-early Cretaceous, Africa was separating from Euramerica, South America, and Australia-Antarctica. The latter pair may have already moved apart. By the mid-Cretaceous, rising sea level had formed the great Mid-continental Sea which divided Euramerica. In the late Cretaceous, Westamerica became connected to Asia and an isthmus attached South America to Central and North America. By the end of the Cretaceous, the Mid-continental Sea had dried up. The early Central American isthmus may have persisted into the Paleocene. In the early Eocene, the North Atlantic Ocean expanded, breaking the link between Europe and North America. In the Paleocene-Eocene, a filter bridge (probably an archipelago) became available between South America and Australia-New Zealand via Antarctica. India became firmly fused with Asia by the Eocene. With the desiccation of the Turgai Sea in the Oligocene, Europe was attached to Asia. By the early Miocene, Africa became connected to Asia via the Arabian Peninsula. This marked the end of the Tethys Sea and the beginning of the Mediterranean. With the flooding of Beringia about 3.5 Ma ago, a marine Transarctic Biotic Interchange took place. Some 0.5 Ma later, the formation of the Panamanian isthmus permitted the terrestrial Great American Biotic Interchange. All of these continental breakages and linkages created dispersal corridors in one environment and, at the same time, dispersal barriers in the other. The series of historic extinction events had important biogeographic and evolutionary consequences. We now live in an era of neocatastrophism in which it is widely believed that the great extinctions were sudden, catastrophic events caused by extraterrestrial impacts or volcanic eruptions or both. On the other hand, the fossils of many organisms tell us that the extinction episodes developed gradually over periods of one to several Ma. The recovery periods tended to take even longer. Throughout its history, the earth has been struck by many missiles from outer space and numerous volcanic eruptions have taken place. But these sudden, high-energy events are not consistent with the tempo of the global extinctions. The one event that is consistent is the regression in sea level. As Hallam (1992) has pointed out, the most important cause of sea-level change involves alteration in the volume of the oceanic ridges. These structures may vary in both spreading rate and overall length. The changes may take place at a rate of about 1 cm per 103 years. Continental compression can cause the sea level to fall at a comparably slow rate. Glacioeustatic changes can take place much more rapidly but their importance throughout the Phanerozoic is controversial because of limited evidence for the existence of major ice sheets. Ocean-trench subsidence and the rise of mantle plumes appear to be of negligible importance. Neither oceanic sedimentation nor the desiccation of isolated basins is likely to have played a major role. The volume of ocean water has probably remained constant through the Phanerozoic, the addition of juvenile water being more or less counteracted by the subtraction of pore water in sediments by subduction. The implication of sea-level regressions as the primary cause of the historic extinctions is only part of the story. The regressions themselves did not have much direct ef-
Chapter 14
393
fect, except on the marine biota of the continental shelves. As the sea level dropped, the continents became larger and higher. These changes caused the continental climates to become colder and dryer. The cessation of tectonic-plate movement produced less volcanism and a drop in the production of CO2 from that source. Increased weathering of the exposed land absorbed greater amounts of CO2. The drop in atmospheric CO2 meant more heat lost to space by radiation. The result was a decrease in global temperature related to the extent of the regression. The larger, higher continents caused severe weather patterns which affected the entire globe. So, sea-level regressions, depending on their magnitude, caused climatic changes which had a detrimental effect on diversity on land and in the sea. Conversely, sea-level rises had the opposite effect. Long-term sea level cycles, called supercycles, appear to be the proximal causes of the historic alteration between greenhouse and icehouse conditions. The relative positions of the continents, considering their effects on the major oceanic currents, are also important factors in the global climatic cycles. There has been considerable speculation about the evolutionary effects of extinction episodes. These provide a consistent message which gives the impression that a "wiping out of the old forms to make way for the new" has long-term evolutionary benefits. But, the extinction episodes have had a much greater effect on the tropical organisms than on those at higher latitudes. When this happens, the advanced tropical species tend to be replaced by more primitive forms from higher latitudes or other refuges. It has been argued that the historical extinction episodes were disastrous interruptions to evolutionary progress. They set back the clock of evolutionary time and destroyed communities that took millions of years to reassemble. In the preface, it was noted that a complete biogeography should offer a prognosis for the future. This is a painful task, for we are in the midst of the most-rapid decrease in species diversity ever recorded. It means that most species now living on earth will be lost within the next 200 years. This will nullify all the diversity increase of the past 65 Ma, leaving a depauperate world to be studied by future biogeographers. As the species diversity declines, it will become increasingly difficult to reconstruct historic trends in biogeography and evolution. Although many causes of this catastrophic decline have been identified, the primary one is seldom mentioned. This is the effect of the exploding human population which increases by 95 million each year, or about 260 000 per day. Religious opposition has made human population control a taboo subject. Most conservation groups have avoided the problem. It is the missing agenda (Meffe et al., 1993). This work is primarily an examination of historic changes in the marine and terrestrial biotas. The scope of the subject is so large and the literature so voluminous, that one cannot possibly find every publication of value and give it the recognition it deserves. Also, every biologist interested in biogeography has his or her own concept of how the subject should be addressed. There are very few generalists, so that each person will look at biogeography from a different perspective, one that has been shaped by specialty and experience. While this work is a book of fact, as opposed to fiction, my interpretation of the facts may be quite different than the conclusions of others. As John Steinbeck observed, "The design of a book is the pattern of a reality controlled and shaped by the mind of the writer. This is completely understood about poetry or fiction, but it is too seldom realized about books of fact."