Paleontology 137
J Bret Bennington
Paleontology as a tool of the Geologist - History of the Geologic Timescale
For the next several weeks we will be thinking about how paleontological data is used to solve a variety of problems dealing with stratigraphy, paleoclimatology, paleochronology, paleogeography, and depositional environments.
These are problems that do not address the fossils or the history of life directly, rather they are problems where fossils are used as tools to answer questions about stratagraphic relations or the physical conditions that existed in the past.
Stratigraphy
Historically, paleontology has found its greatest usefulness in the science of stratigraphy.
What is stratigraphy? Stratigraphy combines the recognition that the earth has a history with the recognition that layers of rock forming the earth's surface can be ordered and placed within that history. This involves both assigning some age of formation to a rock layer (dating) as well as determining which other layers exposed in other places are equivalent in age (correlation). The end result of stratigraphy is a global geological time scale with the relative age and physical relations between all strata of the world known.
Paleontology is vital to the enterprise of stratigraphy because there is nothing temporally unique about the lithology of rock. A Devonian sandstone and a Jurassic sandstone are not distinguishable on the basis of the rock itself.
However, the history of life is a unique sequence of the evolution and extinction of fossil species. Each species is present on the earth for only a single expanse of time and once extinct never returns. Fossils provide the means of distinguishing the Devonian from the Jurassic sandstone. For rocks that cannot be dated using radioisotopes, fossils provide the only reliable means for assigning strata their proper position in geological time.
So important is paleontology to stratigraphy that has often been called the 'handmaid of stratigraphy' to suggest its true usefulness and importance to the rest of geology (if not outright subservience) in working out the succession and age of strata in the rock record.
In a way this is ironic because the practice of stratigraphy as we know it today and our whole modern sense of the history of the earth, including the Geologic Time Scale, was developed from insights gained through the study of fossils.
Early stratigraphers did not adopt paleontology as a useful tool so much as the study of fossils led geologists to a recognition of the existence of stratigraphy!
Stratigraphy and the growth of the Geologic Time Scale
The story of how paleontology invented stratigraphy can be told through the growth of the geologic time scale.
If you recall from the lecture on the history of paleontology, by the end of the 1700's it was widely recognized that there was a general sequence to the material underlying the surface of the earth. This sequence was named primary, transitional, seconday, tertiary, and diluvial, a terminology left over from previously popular theories that attributed the rocks of the earth to crystalization and settling from a primordial ocean, as well as the recent action of the biblical flood.
However, through the 1700's, excavations around Europe and the increasing discovery and study of fossils were rapidly making older geological theories untenable.
We have already discussed the work of Georges Cuvier, who working with another man named Alexandre Brongniart, demonstrated the succession of extinct vertebrates in the strata of the geological basin surrounding Paris.
It was Cuvier who used fossils to convincingly demonstrate that the earth and the life inhabiting it had a long history – a succession of worlds or series of 'revolutions' in the history of life. However, Cuvier was more concerned with unraveling the general history of life than with using fossils to study strata. Brongniart did eventually realize the value of fossils for correlating rock layers, but not until after he was influenced by his contemporaries in England.
At the same time that Cuvier was arguing that fossils showed a history of life of earth, various distinct formations of strata were being studied and named throughout Europe.
For example, the coal-bearing rocks of France and England were referred to as 'carboniferous soils' and the rocks in the Jura mountains between France and Switzerland were named 'Jurassic' by A. von Humbolt.
Pieces of the stratigraphic puzzle were coming into focus in different places in Europe, but what was lacking was any basis for connecting the different pieces. Physical position in a stratigraphic sequence could be used locally and in places where equivalent strata were correlatable due to obvious lithological similarities (for example the chalks of England and France), but this was not common.
William Smith
Perhaps not surprisingly, the breakthrough did not come from Europe's learned scientific circles, but rather from a 'practical man'.
Born in Oxfordshire, England in 1769, William Smith has little formal education, but trained himself to be a surveyor and civil engineer. He worked planning and digging canals, mines, and stone quarrying operations to great success. As his reputation grew he was enlisted to work all over England.
In his work and his travels Smith collected fossils and took detailed notes on the occurrence and succession of strata in different places, and on the fossils contained within the strata.
Smith noted early in his career that successions of strata were relatively constant from place to place over large geographic areas. His first contribution to geology was the realization that the geographic occurrence of strata on the surface could be mapped as an aid to predicting the rocks in the subsurface that he would encounter when digging. Eventually he would produce the first detailed geologic map ever made.
Smith also noted that there were two potentially confusing aspects to strata. First, sometimes in a familiar sequence of strata a layer or layers normally present would be missing, or other layers would be found in their place. Also, it was easy to be misidentify strata from different sequences that had very similar lithologies.
However, sometime before 1796 Smith realized that a particular sequence of fossils always occured with a particular sequence of rocks, and that rocks of the same lithology from different parts of the sequence could be easily distinguished based on their fossils. This was Smith's great discovery - that the position of strata within a stratigraphic sequence could be determined from fossils alone without paying attention to lithology.
Singlehandedly, without any interaction with the growing geological establishment, Smith invented the science of biostratigraphy.
Smith's big idea has one great hurdle to leap before achieving a wide audience, and this was Smith's social position. He was not a gentleman, and so not a member of the Geological Society of London (nor could he be) and he did not have access to publication in the various scientific journals that had appeared by the 1800's. He did not write well, and even if he had published something many of his contemporaries might not have read it out of principle. Indeed, even after his ideas gained rapid and wide acceptance, many refused to acknowledge their originator.
Fortunately, Smith had two gentlemen friends in the clergy, the Reverends Benjamin Richardson and Joseph Townsend who were actively interested in the geology of the strata around Bath. The story has it that while taking tea at the home of Rev. Townsend, Mr Smith was shown the Reverend's collection of local fossils. Smith proceeded to astonish the two other men by accurately deducing which of the local strata had produced each fossil. Smith then dictated to his friends the local succession of strata, along with their characteristic lithologies and the fossils found within.
This succession was published in a History of Bath, in 1801, but more importantly, the Reverends spread news of Smith's methods by word of mouth. His ideas gained wide acceptance, and by 1831 the Geological Society of London was calling Smith (now a relatively old man of 62) the father of English geology when they awarded him the first Wollason Medal.
Smith's ideas made their mark at a particularly fortuitous time. English geologists were in the process of a drive toward empiracism and a program to collect data in the field on the geology and paleontology of the rocks of England.
Likewise, in the rest of Europe sequences of strata that had previously been recognized on the basis of lithology were now being given formal names and shown to have distinct fossil assemblages.
In 1822, d'Omalius d'Halloy defined the Cretaceous as the chalky strata intermediate between the Tertiary and underlying Jurassic strata. Meanwhile, using fossil ammonites it was shown that the Jurassic strata of France and Switzerland were correlative with the clay and oolite deposits surrounding London.
In 1834 von Alberti recognized a three part sequence of strata in Germany consisting of the Bunter Sandstone, Muschelk Group, and the Keuper Group and called his sequence the Triassic. Later the Upper New Red Sandstone in England is recognized as Triassic also.
Sedgwick and Murchison
Meanwhile, in England geologists were in the field. Two of these geologists, Adam Sedgwick and Roderick Impy Murchison, set for themselves the task of unraveling the geology of the so called 'transitional strata'. In the summer of 1831, both men set out for the field.
The transitional strata of England were best exposed in Wales. However, the extensively folded and faulted nature of the rocks made figuring them out rather difficult. Murchison decided to start from the southern region of Wales where the folding of the rocks was not very intense. Furthermore, he could start his work at the base of the 'Old Red Sandstone' which was known to underlie the coal bearing strata (recently named the Carboniferous System by Conybeare and Phillips).
Murchison worked down the stratagraphic pile and discovered rich deposits of marine fossils unlike those of the overlying rocks. Using Smith's principles he was able to demonstrate an extensive sequence of strata with a unique fossil fauna extending northwestward across Wales.
Murchison named this new system the Silurian, after the tribe that occupied the Welsh borderland where his work began. After publishing his work on the Silurian many other examples from around Europe and the Americas began to be found of rocks with this particular fossil fauna.
Meanwhile, Murchison's friend Sedgwick was having a more difficult time. The strata in the north of Wales were more heavily deformed and not very fossiliferous. However, after several years of work he was able to work out a stratagraphic sequence that he christened the Cambrian, after the latin name for Wales.
At first, neither man knew how the Cambrian and Silurian would join together. This would eventually become a problem....
Meanwhile, working together, Sedgwick and Murchison were solving a controversy over the position of some other 'transitional' strata in the south of England. At first, based on lithology, they were going to assign these rocks to Sedgwick's Cambrian system. However, after working with a paleontologist named Lonsdale, they determined that a sequence of transitional strata in the Devonshire region of England was intermediate in age between the Silurian below and the Carboniferous above, and equivalent in age to the Old Red Sandstone. The named this system after Devonshire, calling it the Devonian.
Although most of their lives had been spent in cooperation and friendship, the question of where the Cambrian and the Silurian met began to intrude. Murchison became convinced in his travels in Sweden and Russia (where he had hobnobbed with much royalty and named the Permian system after strata underlying Triassic rocks adjacent to the Urals in the province of Perm in Russia) that the Silurian system marked the first appearance of life.
It had already been noted that the fossils of Sedgwick's upper Cambrian were very similar to fossils of Murchisons lower Silurian. And, unfortunately for Sedgwick, his lower Cambrian rocks were unfossiliferous. Murchison began to insist that the Cambrian was really just lower Silurian. This led to a long a bitter debate that completely estranged them and lasted until after both of their deaths. Eventually, it was shown that the Cambrian rocks did contain a unique fossil fauna composed mostly of trilobites and that the strata that overlapped both the Cambrian and Silurian as originally defined were distinctive enough in their fossil faunas to warrent being called a separate system, name the Ordovician (after another Welsh tribe) in 1879 by Charles Lapworth.
Let me pause here to point out that the underlying theme to this whole story is the overwhelming importance of the fossils in working out the true stratigraphic relationships of the rocks. As these systems were proposed in England and Europe their equivalents were rapidly located in America and other parts of the world as similar fossils assemblages were described and correlated.
Charles Lyell
We have accounted for the strange names of the Paleozoic and the Mesozoic portions of the geologic timescale, but what about the Tertiary with all of its 'cenes?
The Tertiary was subdivided by Charles Lyell, another and perhaps the most famous English geologist. In 1833 Lyell published his monumental 'Principles of Geology' in which he laid out a comprehensive theory of the earth that stated that all events of the past could be interpreted by studying processes that are occurring today, that all geological change was gradual and occurred over immense spans of time, and that changes in the species that inhabited the earth were also gradual as well as cyclical.
It was to demonstrate the third of these propositions that Lyell subdivided the Tertiary strata. He did this by calculating the percentage of fossil species in different groups of Tertiary strata that are still alive today.
Lyell subdivided the tertiary as follows:
1-5% fauna still extant - Eocene (dawn of the recent)
20-40% fauna still extant - Miocene (less recent)
50-90% fauna still extant - Lower Pliocene (more recent)
90-100% fauna still extant - Upper Pliocene (more recent)
What Lyell wanted to show was that, given his proposition that change in species was gradual, strata could be ordered based on how similar their fossil faunas were to each other. He also wanted to bolster his arguement for gradual change by showing that there must be large spans of unrepresented time between the known formations of the Tertiary shown by the abrupt changes in percent of species between the stages.
Although Lyell's arguements for a strictly gradual earth history did not survive the test of time, his recognition that fossils could be used to make a chronology on which to hang strata agreed well with the work of his colleagues and his Tertiary timescale remains with us today.
In later editions of his book Lyell renamed the Upper Pliocene the Pleistocene. The Paleocene (early dawn of the recent) was named in 1874 by Schimper to include strata at the base of the Eocene that contained a flora that he felt was intermediate between Cretaceous and Eocene. Also, the Oligocene (little recent) was named in 1854 to include strata in the lower Miocene and upper Eocene that were found to overlap.
By 1890 an essentially modern geologic timescale had been established on the basis of the unique paleontology of the different systems of strata. These systems were being widely correlated around the world, particularly in the growing United States were the opening of the West and the growth of state geological surveys created an explosion of geological exploration.