Page "Invertebrate paleontology" Paragraph 12

The chitinous jaws of annelids ( such as the marine scolecodonts ) are sometimes preserved as fossils ; while many arthropods and inarticulate brachiopods have easily fossilized hard parts of calcite, chitin, or keratin.

The most common and often-found macrofossils are the very hard calcareous shells of articulate brachiopods ( that is, the everyday " lampshells ") and of mollusks ( such as the omnipresent clams, snails, mussels and oysters ).

On the other hand, non-shelly slugs and non-tubiferous worms ( for instance, earthworms ) have only occasionally been preserved due to their lack of hard parts.