of time. When a particular index fossil is found, the relative age of the bed is immediately
known.
Many fossils may qualify as index fossils. Ammonites, trilobites, and graptolites are often
used as index fossils, as are variousmicrofossils, or fossils of microscopic organisms. Fossils
of animals that drifted in the upper layers of the ocean are particularly useful as index fossils,
as they may be distributed all over the world.
In contrast to index fossils, living fossilsare organisms that have existed for a tremen-
dously long period of time without changing very much at all. For example, the Lingulata
brachiopods have existed from the Cambrian period to the present, a time span of over 500
million years! Modern specimens of Lingulata are almost indistinguishable from their fossil
counterparts (Figure11.8).
Figure 11.8: FossilLingula(left) and ModernLingula(right). ( 25 )Clues from Fossils
Fossils are our best form of evidence about the history of life on Earth. In addition, fossils
can give us clues about past climates, the motions of plates, and other major geological
events.
The first clue that fossils can give is whether an environment wasmarine(underwater) or
terrestrial(on land). Along with the rock characteristics, fossils can indicate whether the
water was shallow or deep, and whether the rate of sedimentation was slow or rapid. The