Fossils are useful in a number of different ways, for each specimen
provides some information about when it lived, where it lived, and how
it lived.
Fossils are very important, for example, in tracing the development of
the plants and animals of our earth. This is possible because the
fossils in the older rocks are usually primitive and relatively simple;
but a study of similar specimens that lived in later geologic time shows
that the fossils become progressively more complex and more advanced in
the younger rocks.
Some fossils, for example, the reef-building corals, appear to have
always lived under much the same conditions as they live today. Hence,
it is reasonably certain that the rocks containing fossil reef corals
found in place (that is, where they were originally buried), were
deposited in warm, fairly shallow, salt water. By studying the
occurrence and distribution of such marine fossils, it is possible to
outline the location and extent of prehistoric seas. Moreover, the type
of fossils present will frequently give some indication as to the bottom
conditions, depth, temperature, and salinity of these ancient bodies of
water.
Probably the most important use of fossils is for purposes of
_correlation_—the process of demonstrating that certain rock layers are
closely related to each other. By correlating or “matching” the beds
containing specific fossils, it is possible to determine the
distribution of geologic units of similar age. Some fossils have a very
limited vertical or geologic range and a wide horizontal or geographic
range. In other words, they lived but a relatively short period in
geologic time but were rather widely distributed during their relatively
short life. Such fossils are known as _index fossils_ or _guide fossils_
and are especially useful in correlation because they are normally only
associated with rocks of one certain age.
[Illustration: Fig. 7. Sketches of two types of micropaleontological
slides. (a) Multiple space faunal slide. (b) Single-hole slide.]
Microfossils are often very valuable as guide fossils for the petroleum
geologist. The micropaleontologist washes the well cuttings from the
drill hole and separates the tiny fossils from the surrounding rocks.
The specimens are then mounted on special slides (fig. 7) and studied
under the microscope. Information derived from these fossils often
provides valuable data on the age of the subsurface formation and the
possibilities of oil production. Microfossils are particularly valuable
in the oil fields of the Gulf Coast region of Texas. In fact, some of
the oil-producing zones in this area have even been named for certain
key genera of microfossils. For example, the “het” zone of Oligocene age
(geologic time scale, Pl. 1) is named for the genus _Heterostegina_,
which is a tiny one-celled animal. Other microfossils, such as
fusulinids, ostracodes, spores, and pollens, are also used to identify
subsurface formations in many other parts of the State.
Plant fossils are very useful as climatic indicators but are not too
reliable for purposes of age determination. They do, however, provide
much information about the development of plants throughout geologic
time.