Records of the diversification of animal life
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Information to know |
The basis for the information |
Explanatory comments |
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Late in the Proterozoic Eon the Earth experienced significant glaciation – so much so that some believe the Earth was covered in ice even in the tropics. |
Glaciers leave very distinctive kinds of sedimentary deposits and other characteristics. These have been found in many places in rocks of this age. Chemistry of the sediments also suggest something strange happened at this point in time. |
The “snowball Earth” hypothesis is just a few years old. Some have suggested that, since the event came just before the diversification of animals, that maybe the event somehow stimulated this evolution, but no convincing arguments have made for how these events would be linked. |
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The oldest known body fossils of something apparently multicellular occurs shortly before the Cambrian Period, about 0.6 Ga[1] |
A couple dozen shapes of mystery “animals” appear nearly worldwide; none are skeletonized, yet they seem to preserve well in sandy environments; they are known as the “Ediacaran” or “Vendian fauna” |
Some people have tried to link all of these forms with modern taxonomic groups; others have suggested that they are all some form of organism that thrived briefly, but died out soon after; some think some combination |
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The oldest known trace[2] fossils are about 0.6 Ga, but some have claimed to have found trace fossils as far back as 1 Ga or more |
Trace fossils are found in many rocks in the world. |
Trace fossils bigger than microscopic are thought to have been formed by a multicellular organism. |
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A few types of shell fossils of unknown affinity occur before the Cambrian boundary, then a number of mysterious “small shelly fossils” occur in the early Cambrian. |
These occur through the world in sediments of this age. |
It is now believed that some or all of the “small shelly fossils” are sclerites – each is just one piece of a shelly armor that covered some kind of early Cambrian animals. |
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Animal life after the Cambrian[3] boundary is much more diverse, and/or the animals are better skeletonized. |
The fossil record becomes much more extensive after the Cambrian boundary. |
There has been some debate about whether the key factor at this boundary was the origin of most animal phyla, or the emergence of skeletons among many of the phyla. |
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Most animal phyla[4] with a good fossil record seem to show up in the early-mid Cambrian Period, or slightly before the Cambrian. |
The fossil records of most skeletonized phyla and a few soft-bodied forms from exceptionally preserved fossils. |
- Very small soft-bodied phyla have little or no fossil record. - This interval of time is known as the Cambrian radiation or Cambrian revolution. It is considered by many to be the most important event in the history of animal life. |
When you look at a book on the history of life, it will usually gloss over the first 3 billion years of life in a couple paragraphs, then begin in more detail in the Cambrian.
Many feel that it is interesting that the Cambrian shows high disparity before showing high diversity. That means that there are many different “body plans,” as you can see from the number of phyla that show up in the Cambrian, but not necessarily lots of species within each phylum (the opposite pattern would be, say, lots and lots of species of beetles that all look fairly similar to one another – that would be essentially no disparity and high diversity). One might have imagined that the phyla would show up gradually over the course of the Phanerozoic eon, as the number of species increased and colonized various habitats, but it turns out that life did not evolve in this way.
Furthermore, (1), few or no new phyla show up after the Cambrian and (2), in the Cambrian even within each phylum, we see a large number of shapes, some of which became extinct shortly afterward. Paleontologists have wondered why new body plans arose so quickly and “easily” in the Cambrian, but never again since. This is a good example of a pattern in the evolution of life that would not be able to understand without the fossil record.
The Precambrian-Cambrian transition is considered one of the most important times in this history of life, and is studied intensively. Within the past 10-20 years our knowledge of the fossil record of this interval has grown enormously, as well as other clues such as the chemistry of the ocean at this time. For many this research is interesting not only because of its significance for understanding the history of life, but because it is filled with interpretation of weird and wonderful creatures that became extinct shortly after their origin.
Fossils are usually the “hard parts” – the skeleton or shell – of past organisms, since the “soft parts” – the organic tissues – almost always rot away long before they become permanently buried in sediments. Therefore, much of what we think we know about the history of life comes from the fossil record of organisms with hard parts. We know little about the history of jelly fish, for example, even though they have probably been common since early Cambrian or before.
Sometimes, however, unusual circumstances lead to the preservation of soft parts. We saw this in the Precambrian rocks, in which microscopic cells were preserved in chert. The main way to preserve soft parts of animals is to create chemical conditions that prevent the growth of bacteria or other organisms that would decay soft parts. The main way to do this is to create a place with little water overturn, so that oxygen is used up and not replaced. These exceptionally preserved fossils are called Lagerstaetten.
A famous locality in Canada known as the Burgess Shale has preserved many soft-bodied organisms from the Middle Cambrian. This fauna is incredibly important because it gives us a window in on a critical interval in Earth history, when animals were first evolving into the many groups that we find today. In the early 1990s paleontologists started publishing on a newly described exceptionally preserved fauna, from Chengjiang, China. This fauna is slightly older than the Burgess Shale fauna, and has many of the same kinds of animals, plus some new ones. There are also remarkable lagerstaetten in Sweden from the late Cambrian, a site from the late Proterozoic of China, and a new Cambrian site in Nevada, and others.
[1] Ga = Giga ans (French) = billions of years (and Ma = millions of years)
[2] Trace fossils are made by the movement of organisms through sediment, leaving behind trails, tracks, burrows, and so on. The Precambrian trace fossils are simple traces of worm-like movement through sediment. Many traces are interpreted through traces made in modern environments.
[3] The Precambrian-Cambrian was originally defined to correspond to the record of the first animals, but now we have found records of animals before the boundary.
[4] Phyla are the largest categories in the Linnean hierarchy for animals. This hierarchy includes phyla, classes, orders, families, genera, and species. Evolutionary biologists are moving away from assigning groups of organisms to a particular level in this hierarchy, but the levels are still useful for some applications such as this one.