Invertebrate Paleontology - Invertebrate Fossilization

Invertebrate Fossilization

When it comes to the fossil record, soft-bodied and minuscule invertebrates—such as hydras, jellies, flatworms, hairworms, nematodes, ribbon worms, rotifers and roundworms -- are infrequently fossilized. As a result, paleontologists and other fossil hunters must often rely on trace fossils, microfossils, or chemofossil residue when scouting for these prehistoric creatures.

Hard-bodied and large invertebrates are much-more commonly preserved; typically as sizeable macrofossils. These invertebrates are more frequently preserved because their hard parts—for example, shell, armor, plates, tests, exoskeleton, jaws or teeth -- are composed of silica (silicon dioxide), calcite or aragonite (both forms of calcium carbonate), chitin (a protein often infused with tricalcium phosphate), or keratin (an even-more complex protein), rather than the vertebrate bone (hydroxyapatite) or cartilage of fishes and land-dwelling tetrapods.

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.