Invertebrate paleontology

Finally, throughout geologic time, invertebrates have remained non-craniate creatures; that is, they never developed a cranium, nerve-chord brain, skull, or hard protective braincase (unlike many vertebrates).

Nevertheless, most earth science departments continue to employ this term; and paleontologists find it both useful and practical in evaluating fossil invertebrates and—consequently—invertebrate evolution.

These invertebrates are more frequently preserved because their hard parts fossilise more readily—for example, shell, armor, plates, tests, exoskeleton, jaws or teeth.

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, shell-less slugs and non-tubiferous worms (for instance, earthworms) lack hard parts and therefore such organisms are poorly represented in the fossil record.