This era consists of seven eights of the earth’s history. Its fossil record consists of very simple organisms, most organisms with no nuclei (prokaryotes) – like bacteria. Eventually, organisms with nuclei (eukaryotes) appear. Multi-celled organism appear towards the end of the period but they are not rabbits or any thing of the sort. They are all still microscopic.
The Cambrian Era
This period begins with the sudden appearance of complex life forms – sudden in that there is nothing from the previous period would lead you to expect them. From the simple microscopic single celled organisms we had at the end of the Precambrian era, we have the fossils of almost all the major phyla in the animal kingdom. This is called the Cambrian Explosion.
A phylum (plural phyla) as pertains to this discussion is a ‘body plan’. An example is the arthropods whose body plan includes an external skeleton, a segmented body, and jointed appendages. Examples of animals in this class are spiders, scorpions and centipedes.
A popular explanation for Cambrian explosion is that the fossilization of hard bodied animals is more likely and so the soft-bodied precursors to the Cambrian fossils were not preserved. Of course, there are quite a few soft bodied fossils from both the Precambrian and the Cambrian eras, so this argument is not wholly adequate.
The Cambrian era ended with a mass extinction.
The Ordovician Era
This era was dominated by invertebrates – animals without backbones. Many types of snails and clams are found for the first time in this era. While the organisms were new, the body plans had already been set back in the Cambrian era. Some vertebrates are also found in this era.
The Silurian Period
Like the Ordovician, life in this period was mostly marine. There were new animals found, but once again with no new body plans.
__________________________________________________________________A word of caution: Pictorial representations of the fossil record suffer from one problem: they can be manipulated to show what the author wishes to show. If I picked only the microscopic organisms from each period, I can produce a pictorial representation that is mostly unchanged to suggest that evolution did not occur. If I pick organisms with gradually increasing size, I can give the impression of organisms getting more complex with each period – although this says nothing about complexity, but size. I will work on providing an accurate (as far as I know) pictorial representation of the geologic time periods, but it will be slow work. I also want to emphasize that evolution is not about new types of organisms appearing – they do so all the time in the geologic record. It’s about the progression. If every time period had new organisms but all the organisms had no significant new features, this would be uninteresting. If, on the other hand, new organisms appeared and each successive organism was less complex that its predecessors, that would hardly support Darwin’s theory.