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Word Count: 1,094
In Namibia where the rock record is excellent Vendian fossils are followed immediately by Cambrian fossils and they may have overlapped briefly in time In Siberia a whole suite of small shelly fossils appears in the rocks as the Cambrian period begins together with sponges of several different types Most of the small shells are tiny cones and tubes that we dont understand properly but at least some of them are complex animals including the first molluscs Soon archaeocyathid sponges were forming reef patches These are dramatic changes that reflect rapidly evolving life New research pegs the base of the Cambrian at about 543 Ma and the worlds fauna was completely revolutionized in the 20 my that followed The same set of small shelly fossils is now known worldwide and their discovery shows that there were at least two major steps in the evolution of animal skeletons because the next stage of the Cambrian sees the appearance of more abundant and more complex creatures For 10 my or more there were no animals larger than a few millimeters long except for the archaeocyathid sponges Then worldwide in the 2 my between 530 Ma and 528 Ma we see the appearance of a much larger variety of marine life Dominant among these animals were trilobites brachiopods and echinoderms Trilobites are arthropods complex creatures with thick jointed armor covering them from head to tail They had antennae and large eyes they were mobile on the seafloor using long jointed legs and they were something like crustaceans and horseshoe crabs in structure They did not have the complex mouth parts of living crustaceans so their diet may have been restricted to sediment or very small or soft prey They burrowed actively leaving traces of their activities in the sediment and they are by far the most numerous fossils in Cambrian rocks The number of fossils they left behind was increased by the fact that they molted their armor
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