References Cited

Paleontologists generally come much too late to find anything but skeletons. However, they find something denied to the biologist — the time element. The crowning achievement of paleontology has been the demonstration, from the history of life, of the validity of the evolutionary theory (paraphrased from Kurtén, 1953).

In Darwin’s day, the fossil record was poorly known, but this is no longer true. A major focus for geologists is establishing the times of origin of the rock formations in the crust of Earth — the science of geochronology. For paleontologists, it is important to know which rock formations were formed at the same time and thus can be correlated, which rocks were formed at different times, and to put the formations into a time sequence from oldest to youngest in any area under study. Fossils are key to establishing the sequence of the ages of layered sedimentary rocks, and they are the direct proof of the changes that have occurred in living organisms through time on our planet.

In the mid-1600s, about 200 years before Darwin published his theory of evolution, the Danish scientist Nicholas Steno found that it was possible to establish the order in which layered rocks were deposited. He recognized that particles of sand, mud, and gravel settle from a fluid according to their relative weight. Slight changes in particle size, composition, or transporting agent result in the formation of layers in the rocks; these layers are also called beds or strata. Layering, or bedding, is the most obvious feature of sedimentary rocks. The study of layered (sedimentary) rocks is called stratigraphy. 

Sedimentary rocks are formed particle by particle and bed by bed, and the layers  are stacked one on another. Thus, in any sequence of undisturbed layered rocks, a given bed must be older than any bed on top of it. This Principle of Superposition is fundamental to understanding the age of rocks; at any one place it indicates the relative ages of the rock layers and of the fossils they contain. Because rock types such as sandstone, limestone, and shale are formed repeatedly through time, it is usually not possible to use rock types alone to determine the time in which rock formations were formed, or to correlate them to other areas. To determine the age of most sedimentary rocks, scientists study the fossils they contain.

In the late 18th and early 19th centuries, English geologists and French paleontologists discovered that the age of rocks could be determined and correlated by their contained fossils. Rocks of the same age contain the same, or very similar, fossil species, even when the rock units extend over a large area or the exposures are not continuous. They also noted that there was a distinct, observable succession of fossils from older to younger rocks that did not repeat itself. These geoscientists were the first to use fossils to correlate the time of formation of the rocks in which the fossils occur. Three concepts are important in the study and use of fossils: (1) Fossils are the remains of once living organisms; (2) The vast majority of fossils are the remains of the hardparts of extinct organisms; they belong to species no longer living anywhere on Earth; (3) The kinds of fossils found in rocks of different ages differ because life on Earth has changed through time.

If we begin at the present and examine older and older layers of rock, we will arrive at 
a level where no human fossils are found. If we continue backward in time, we successively come to layers where no fossils of birds are present, no mammals, no reptiles, no four-footed animals, no fishes, no shells, and no members of the animal kingdom. These concepts are summarized in the general principle called the Law of Fossil Succession. The kinds of animals and plants found as fossils change through time. When we find the same kinds of fossils in rocks in different places, we know the rocks are of the same age.
 

(Previous Page || Next Page)