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Age of Animals


Vendian Period, 635 - 541 MYA
Cambrian Period, 541 - 485.4 MYA
Ordovician Period, 485.4 - 443.4 MYA
Silurian Period, 443.4 - 419.2 MYA
Devonian Period, 419.2 - 358.9 MYA
Carboniferous Period, 358.9 - 298.9 MYA
Permian Period, 298.9 - 251.9 MYA
Triassic Period, 251.9 - 201.3 MYA
Jurassic Period, 201.3 - 145.0 MYA
Cretaceous Period, 145.0 - 66.0 MYA
Tertiary Period, 66.0 - 2.588 MYA
Quaternary Period, 2.588 MYA - present

The Geologic time scale is adopted from the 30 December 2011 version by the International Union of Geological Sciences (IUGS).
MYA = million years ago, FA = first appearance.

See "Prehistoric Time Line" or "Museum of Paleontology" for a peek into ancient life, and "Encyclopedia of Life (EOL)" for information about life on Earth today.

Vendian Period, 635 - 541 MYA

For most of the nearly 4 billion years that life has existed on Earth, evolution produced little beyond bacteria, plankton, and multi-celled algae. But beginning about 600 million years ago in the Precambrian, the fossil record speaks of more rapid change. First, there was the rise and fall of mysterious creatures of the "Vendian biota" or "Ediacara fauna" (see Figure 01a), named for the fossil site in Australia where they were first discovered. The question of what these fossils are is still not settled to everyone's satisfaction; at various times they have been considered algae, lichens, giant protozoans, or even a separate kingdom of life unrelated to anything living today. Some of
Vendian Period these fossils are simple blobs that are hard to interpret and could represent almost anything. Some are most like cnidarians, worms, or soft-bodied relatives of the arthropods. Others are less easy to interpret and may belong to extinct phyla. But besides the fossils of soft bodies, Vendian rocks contain trace fossils, probably made by wormlike animals slithering over mud. The Vendian rocks thus give us a good look at the first animals to live on Earth. The Ediacaran hey-day predates by a distinct interval of perhaps 20 million years or more, the so-called "Cambrian Explosion". Although some scientists believe that many of these Ediacara fauna might have survived into the Cambrian period, they had vanished without a trace from later fossil records. Other scientists have suggested that the Ediacaran fauna were "failed experiments" in the evolution of multicellular animals. Unlike the Cambrian organisms, these odd designs left no descendants. A novel explanation suggests that the Ediacaran fossils weren't animals at all. Rather, they were probably lichens. Whatever the interpretation, it seems that the appearance of the Ediacaran fauna and the Cambrian biota are two separate events, and both flourished suddenly in a "complete state".

Figure 01a Ediacara Fauna [view large image]

Ediacarans For much of the past 20 years the debate has been polarized between those who believe that the Ediacarans were a dead-end experiment in evolution and those who maintain that the Ediacarans are the "long fuse" of the Cambrian explosion. As more fossils were discovered in Newfoundland, ... (Avalon assemblage - the oldest), the White Sea region of Russia (White Sea assemblage including those from Ediacara Hills, ...), and Namibia, ... (Nama assemblage), it turns out that both camps are, to some extent, right. As shown in Figure 01b, the Avalon assemblage consists of primitive type of animal living in deep sea with fungus-like traits that left no descendants. The other group from the White Sea and Nama assemblages lived in shallow-water. One of these, Parvancorina, bears a close resemblance to a recently discovered early Cambrian arthropod. Another, Arkarua, looks a lot like a Cambrian echinoderm. It is now thought that a handful of Ediacarans did cross over into the early Cambrian. The overwhelming majority did not make it, though; the few that did vanished within 5 million years. The first experiment in complex, multicellular life was over. But it laid the foundation for everything that followed. It is suggested that the sudden precambrian boom was triggered by massive increase in deep-sea oxygen levels, and plenty of organic matter from the melting glaciers.

Figure 01b Ediacarans
[view large image]

The experimental method was to create large body from small units through fractal repetition.

Oxygen Level Recent measurement of oxygen level over the past 600 million years suggests that oxygen may be the driving force for evolution. Figure 01c shows that periods of lower oxygen level have coincided with all the major mass extinctions, whereas land colonisation occurred with rising levels. The importance of oxygen can be illustrated by the lack of it. It is well known that animals need to feed, drink, reproduce and respire. The first three requirements can usually be put off for days or even years, but for the vast majority of animals respiration can be put off only for a few minutes. Evolution is prodded by natural selection, which is an euphemism for variable rates of death. And nothing kills quicker than lack of oxygen.

Figure 01c Oxygen Level
[view large image]

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