Page 44 - The Restless Earth Fossils

so Many Fossils, so little time 43

cones. Horseshoe crablike creatures called trilobites also make
good index fossils. In some of the oldest oceans, small trilobites
floated near the surface of the water, larger species swam actively
(probably preying on microscopic organisms), and other species
crawled and dug in the ocean-bottom ooze. Clams like Inoceramus
and shelled creatures called brachiopods that held on to rocks and
other objects with short stalks can be added to the list, along with
various corals and sponges. The United States Geological Survey
(USGS) shows some of the index fossils used to identify points in
geological time on their Web site.
All the creatures mentioned above live in oceans and are mac-
roscopic—visible with the naked eye. Some of the best index fos-
sils are microscopic—only visible with magnification. Scientists
can find (if they know where to look) lots of microscopic forms
that come from both watery and terrestrial environments. Once
plants invaded the land, they produced spores, and later pollen,
in vast quantities. The shapes and surface patterns of spores and
pollen grains are very species-specific. Paleontologists can infer a
lot about the ecology of terrestrial habitats from spores and pol-
len that fell into lakes, bogs, or swamps.
Ocean microfossils that are useful as index fossils include
the following:

Conodonts: Teethlike structures in very old rocks. In 1983,
conodonts were found to be body parts of soft-bodied
animals not unlike modern lancelets (related to primi-
tive fish).
Foraminifera (or “forams” for short): Tiny animals with shells
that existed from 300 million years ago to the present.
Ostracods: Small crustaceans related to water fleas.
Coccoliths: Calcareous plates found on certain algae.
Graptolites: Colonial animals that lived in ancient oceans.

Index fossils serve as great aids to scientists in determining
the order of events and species in the geological record. They thus
help mark what paleontologists refer to as relative time. But

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