With millions of years of Earth history to study, it is interesting that so much attention is devoted to the rare and relatively short lived time intervals that represent Earth’s major mass extinctions. Perhaps this interest is twofold. On the one hand, there is a fair degree of self-interest in studying extinction considering the present biodiversity crisis we now face. On the other hand, these periods of time have had an incredible effect on life history. These cataclysmic times represent periods of environmental and ecological abnormality amidst millions of years of relative stability.
As such, these mass extinctions are times of incredible change, which can be studied both evolutionarily as well as ecologically. When viewed through an evolutionaryframework, mass extinction events represent unique time periods in the history of life. These ecological crises prune the tree of life, removing families and killing off entire lineages at random (Raup 1981). Those lineages lucky enough to survive the catastrophe continue and diversify. Often, it is by this seemingly random removal of organisms that large scale evolutionary changes can take place. Take, for instance, the present state of our world, which is primarily dominated by large mammals. Had the non-avian dinosaurs not met with an untimely demise at the end of the Cretaceous, mammals would probably never have been able to diversify into the numerous forms that we see today. It is for this reason that the study of mass extinction events is incredibly important to evolutionary biology. Mass extinctions are essentially historical “turning points” that affect the evolution of all of the Earth’s biota on a gross scale.
The Earth had already been revolving around the Sun for nearly four billion years when Life entered a major new stage. For more than two billion years, the only life forms had been isolated cells floating in the worldwide ocean. But now these cells began to associate with one another, becoming the first multicellular organisms.
This was some 700 million years ago. It would take only another 100 Ma for certain organisms to develop a skeleton: hard parts that could be preserved in rock long after the organisms died. What we know of the past forms of Life on Earth is largely based on these fossils: they have given us a far more accurate picture of the past 600 Ma than we have of the billions of years that went before.
Another 100 Ma, and the seas are now populated with fish. Yet another 100, and their descendants can lay sturdy eggs; now equipped with lungs, they grow bolder, abandon the water, and conquer the continents, as yet uninhabited. Then, 260 Ma ago comes the “invention” of warm blood, and the first proto-mammals begin to prosper. Here, at the end of the Paleozoic Era , the abundant and varied fauna and flora bear every mark of success, both in the ocean depths and on the emergent land. Yet almost all at once, 250 Ma ago, a catastrophe causes 90% of all species to vanish forever.
2 For an entire species to disappear, every individual it comprises must die without descendants. When 90% of all species die out, the populations of the remaining 10% will certainly be hard hit as well: in fact, perhaps 99% of all animals living at the end of the Paleozoic perished. This is the most extensive of all mass extinctions known today. But not all died, and the survivors set out to reconquer the space so unexpectedly swept clear for them. This start of the Mesozoic Era is dominated by pig-sized plant-eaters called Lystrosaurus. They have large amphibians for company, along with other reptiles who will soon give rise to the first true mammals and the first dinosaurs. A new catastrophe, less violent than the first, arrives to decimate the last proto-mammals, the great amphibians, and (in the oceans) almost all species of ammonoids. Small, hiding in the trees and living on insects, our mammal ancestors were anything but conspicuous. You might almost say they encouraged the world to forget they were there. For this, in fact, was the real beginning of the age of dinosaurs. Recent paleontologic research has given us a whole new perspective on these beasts. Some may have been warm-blooded. The great long-necked, plant-eating sauropods, like the celebrated Diplodocus, gradually gave way to animals sporting horns and duckbills, grazing no longeron the treetops but on grass and bushes. Their predators were those great carnivores, colorful and agile, who for decades have delighted children and made film producers’ fortunes. A few minutes of Jurassic Park and The Lost World (the movies) give a very fine view of them.die out, the populations of the remaining 10% will certainly be hard hit as well: in fact, perhaps 99% of all animals living at the end of the Paleozoic perished. This is the most extensive of all mass extinctions known today.
But not all died, and the survivors set out to reconquer the space so unexpectedly swept clear for them. This start of the Mesozoic Era is dominated by pig-sized plant-eaters called Lystrosaurus. They have large amphibians for company, along with other reptiles who will soon give rise to the first true mammals and the first dinosaurs.
A new catastrophe, less violent than the first, arrives to decimate the last proto-mammals, the great amphibians, and (in the oceans) almost all species of ammonoids. Small, hiding in the trees and living on insects, our mammal ancestors were anything but conspicuous. You might almost say they encouraged the world to forget they were there. For this, in fact, was the real beginning of the age of dinosaurs. Recent paleontologic research has given us a whole new perspective on these beasts. Some may have been warm-blooded. The great long-necked, plant-eating sauropods, like the celebrated Diplodocus, gradually gave way to animals sporting horns and duckbills, grazing no longer on the treetops but on grass and bushes. Their predators were those great carnivores, colorful and agile, who for decades have delighted children and made film producers’ fortunes. A few minutes of Jurassic Park and The Lost World (the movies) give a very fine view of them. Then, 65 Ma ago, a huge catastrophe once again ravaged this world, which had seemed so perfectly adapted and balanced. This was the end of the dinosaurs and many mammals, but also of a great many other terrestrial and marine species, including the wellknown ammonites and a considerable number of smaller and less familiar organisms that constituted the marine plankton. In all, twothirds of the species then living (and possibly 80% of all individuals) were wiped out. This is the second great mass extinction.
The Earth had already been revolving around the Sun for nearly four billion years when Life entered a major new stage. For more than two billion years, the only life forms had been isolated cells floating in the worldwide ocean. But now these cells began to associate with one another, becoming the first multicellular organisms.
This was some 700 million years ago. It would take only another 100 Ma for certain organisms to develop a skeleton: hard parts that could be preserved in rock long after the organisms died. What we know of the past forms of Life on Earth is largely based on these fossils: they have given us a far more accurate picture of the past 600 Ma than we have of the billions of years that went before.
Another 100 Ma, and the seas are now populated with fish. Yet another 100, and their descendants can lay sturdy eggs; now equipped with lungs, they grow bolder, abandon the water, and conquer the continents, as yet uninhabited. Then, 260 Ma ago comes the “invention” of warm blood, and the first proto-mammals begin to prosper. Here, at the end of the Paleozoic Era , the abundant and varied fauna and flora bear every mark of success, both in the ocean depths and on the emergent land. Yet almost all at once, 250 Ma ago, a catastrophe causes 90% of all species to vanish forever.
2 For an entire species to disappear, every individual it comprises must die without descendants. When 90% of all species die out, the populations of the remaining 10% will certainly be hard hit as well: in fact, perhaps 99% of all animals living at the end of the Paleozoic perished. This is the most extensive of all mass extinctions known today. But not all died, and the survivors set out to reconquer the space so unexpectedly swept clear for them. This start of the Mesozoic Era is dominated by pig-sized plant-eaters called Lystrosaurus. They have large amphibians for company, along with other reptiles who will soon give rise to the first true mammals and the first dinosaurs. A new catastrophe, less violent than the first, arrives to decimate the last proto-mammals, the great amphibians, and (in the oceans) almost all species of ammonoids. Small, hiding in the trees and living on insects, our mammal ancestors were anything but conspicuous. You might almost say they encouraged the world to forget they were there. For this, in fact, was the real beginning of the age of dinosaurs. Recent paleontologic research has given us a whole new perspective on these beasts. Some may have been warm-blooded. The great long-necked, plant-eating sauropods, like the celebrated Diplodocus, gradually gave way to animals sporting horns and duckbills, grazing no longeron the treetops but on grass and bushes. Their predators were those great carnivores, colorful and agile, who for decades have delighted children and made film producers’ fortunes. A few minutes of Jurassic Park and The Lost World (the movies) give a very fine view of them.die out, the populations of the remaining 10% will certainly be hard hit as well: in fact, perhaps 99% of all animals living at the end of the Paleozoic perished. This is the most extensive of all mass extinctions known today.
But not all died, and the survivors set out to reconquer the space so unexpectedly swept clear for them. This start of the Mesozoic Era is dominated by pig-sized plant-eaters called Lystrosaurus. They have large amphibians for company, along with other reptiles who will soon give rise to the first true mammals and the first dinosaurs.
A new catastrophe, less violent than the first, arrives to decimate the last proto-mammals, the great amphibians, and (in the oceans) almost all species of ammonoids. Small, hiding in the trees and living on insects, our mammal ancestors were anything but conspicuous. You might almost say they encouraged the world to forget they were there. For this, in fact, was the real beginning of the age of dinosaurs. Recent paleontologic research has given us a whole new perspective on these beasts. Some may have been warm-blooded. The great long-necked, plant-eating sauropods, like the celebrated Diplodocus, gradually gave way to animals sporting horns and duckbills, grazing no longer on the treetops but on grass and bushes. Their predators were those great carnivores, colorful and agile, who for decades have delighted children and made film producers’ fortunes. A few minutes of Jurassic Park and The Lost World (the movies) give a very fine view of them. Then, 65 Ma ago, a huge catastrophe once again ravaged this world, which had seemed so perfectly adapted and balanced. This was the end of the dinosaurs and many mammals, but also of a great many other terrestrial and marine species, including the wellknown ammonites and a considerable number of smaller and less familiar organisms that constituted the marine plankton. In all, twothirds of the species then living (and possibly 80% of all individuals) were wiped out. This is the second great mass extinction.
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