Modern communities contain a great variety of organisms which differ considerably in their preservation potential. Animals with well calcified rigid skeletons such as molluscs will obviously have the best chance of fossilization; others such as echinoderms have well calcified skeletons, but are made up of very small pieces and are thus prone to disarticulation and dispersal. Many Crustacea have articulated and poorly calcified skeletons which cannot be easily preserved.
Some of the most abundant organisms in modern communities such as polychaetes, turbellaria, nematodes, sipunculid worms and many coelenterates have muscular, hydrostatic skeletons and any hard parts are only microscopic, for instance jaws, skin spicules and setae. These soft-bodied organisms can be preserved only under special circumstances and usually no trace is left of these abundant animals. Although they leave no important hard parts the activities of these animals are often preserved as trace-fossils, burrows, grazing trails, and rock and shell borings. Nevertheless there is considerable loss of information during fossilization and the fact that polychaetes and other worms must have been the most abundant and diverse group of animals is rarely evident in fossil communities.
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