The Stratigraphic Record

The layers of rock exposed in the Grand Canyon provide a trip through the geological history of the Earth. (Courtesy: Cornell University)
Strata Smith had shown how to do stratigraphy, using fossils, and this method rapidly spread to other parts of the world. It was a simple extension of earlier concepts regarding the stratified nature of the geologic record already used by naturalists and mining engineers on the continent (Johann Lehmann, Johann Fuchsel, Abraham Werner). The quaint old subdivisions, "Primary" (indurated layered rocks), "Tertiary" (poorly indurated material on top) and "Quaternary" (loose stuff on the very top of everything), quickly became obsolete. The major eras and epochs were defined on the basis of their fossil content, in the 19th century. (Of course, we are still using the words Tertiary and Quaternary, but they have a different meaning, being defined in terms of fossils rather than physical properties.)

Most of this work was completed by the early 20th century, but then received new impetus by an important new development, that of systematic exploration and use of microfossils in stratigraphy. This had been neglected, but became especially important in connection with oil exploration based on drilling. Only a limited amount of material is recovered by drilling, and a search for macrofossils (visible fossils) is hopeless. However, microfossils are commonly quite abundant, and can readily serve to determine stratigraphic placement.

In the 20th century, while macrofossil studies continued and produced some interesting insights on the rules of evolution, most of the important advances in paleontology were centered on the remains of microscopic organisms. This new emphasis provided for an increase in stratigraphic resolution by a factor between ten and a hundred. It allowed a detailed reconstruction of environmental conditions and subtle changes therein (through statistical census of microfossils). And it permitted a tie-in of the fossil record to geochemistry, helping to create a new kind of system approach to Earth history, where the presence and absence of types of fossils holds clues to the evolution of ocean and atmosphere.

An entirely new kind of paleontology was introduced by the American chemist Harold Urey (1893-1981) (discoverer of deuterium, Nobel Prize 1934) who showed that temperature fluctuations could be reconstructed from measuring oxygen isotopes in fossils. This new method was soon applied to microfossils, with striking results concerning climate history.

Also, this new emphasis on microscopy in paleontology led back into the Precambrian era, the new "prehistory" of geologic history (after human history had been relegated to a comparatively short time span at the top of the column). The Precambrian, of course, contains all of Life's evolution before the invention of many-celled organisms, and it spans more than six days, if we assign a week to Earth's history. Yet, a typical Earth history textbook from fifty years ago only spends about 10% of the print on the Precambrian (Archean and Proterozoic) and 90% on the Phanerozoic (the last 540 million years).

In more recent books, the imbalance tends to be less marked. Also, the period immediately preceding the Cambrian, the Ediacaran (590-540 M years ago) gets much more attention. It is an intriguing period because in it we find traces of some of the most primitive many-celled organisms known, partly as prints in soft sediments, partly as tracks. Some are bilaterally symmetric and segmented, like pill bugs and people. (Our feet, arms, eyes and ears come in pairs, our spine comes in segments.) Perhaps, in the Ediacaran, we are not yet far from a common ancestor for arthropods and chordates (the larger groups to which pill bugs and people belong).

On the scale of Earth's history, the most familiar geologic period (thanks to Steven Spielberg's Jurassic Park) is hardly "ancient" at all. The dinosaurs, basically fish that walk and breathe air, are not very different from us (as also emphasized by Spielberg). They do have a somewhat smaller brain and ask "What's for dinner?" more urgently than "Who are we?".

The Jurassic period, which lasted between 208 and 146 million years ago is the central part of the "Mesozoic era". It comprises the "Triassic", "Jurassic", and "Cretaceous" periods, each with its unique (but closely related) fossils. Ammonites played a dominant role in subdividing these periods. Below the Mesozoic is the "Paleozoic era" and above it is the "Cenozoic era". Perhaps the best-known and best-beloved section through the Paleozoic is that exposed in the Grand Canyon of the Colorado. On top, we have the limestones of the Kaibab plateau, of Permian age, the last of the great periods of the Paleozoic era. Going down the Bright Angel Trail, past both marine and terrestrial deposits, we reach the Cambrian in the lowermost part of the section, which is underlain by Precambrian Vishnu schist, exposed in the inner gorge.

More or less, the same groups of organisms are present in all the major eras since the Precambrian, although a few major groups are now extinct (as are the ammonites). Among these forms are, for example, trilobites (things that look like giant pill bugs, more or less), graptolites (floating colonies of microscopic many-celled organisms) and (in the Permian) wheat-grain-sized protists called fusulinids. Lamp shells (brachiopods) and various types of echinoderms are likewise important in subdividing or zoning the Paleozoic; their diversity was much greater then than now. The Cenozoic (Tertiary and Quaternary) has mainly thoroughly modern-looking organisms. It owes its detailed zonation entirely to microfossils. The Quaternary (the last 2 million years) is the time of the ice ages and the evolution of the different species of Homo from ancestral apes, culminating in the rise of Homo sapiens.