Smith’s law or principle of faunal succession states that sedimentary strata contain different fossils (fauna and flora) that succeed each other vertically in a specific and predictable order that can be identified over a wide horizontal distance.
This law, also known as the law of fossil succession, created by William Smith, used assemblages of fossils to subdivide, characterize and correlate rock strata in a different region. It laid a foundation for modern biostratigraphy.
Discover more about the law or principle of faunal succession, including its definition and significance. We will also give you a historical background and something about past lifeforms and fossils.
Insight into past lifeforms and fossils
We want you to understand and appreciate the law of faunal succession easily. To do so, you need a glimpse of the history of life on Earth and fossils.
Different organisms or lifeforms have existed throughout the history of Earth. However, only certain species or distinctive organisms existed at a certain geologic time. They afterward became extinct and never emerged again. Instead, others appeared. i.e., successional evolution.
Fossils are the remains of these various ancient faunas and flora (plants and animals) or organisms that once lived, 99.9% of which are extinct today. These fossilized past organisms are in sedimentary rocks formed when they existed, not after or before. Therefore, fossils document life’s evolution through time inside sedimentary rocks.
Since only the distinctive organisms that existed at certain geologic times were fossilized in sedimentary rock units that formed then, these organisms’ age relates to the strata age. Thus, you can predict and map where certain organisms exist.
Also, note sedimentary rock layers form successively, the younger on top of the older, i.e., law of superposition. Similarly, the distinctive fossils in these rock strata are in a successive order that they existed. Each rock strata preserves life forms that existed during its formation, and this life form is different from the one above and below it. In other terms, fossils occur at certain strata positions, representing when the individual organisms existed, not randomly or haphazardly.
The change or evolution of organisms was to make them adapt better to the environment. Subsequent organisms had complex organisms. For instance, evolutionists correctly predicted birds were preceded by primitive, flightless creatures, i.e., feathered dinosaurs.
Lastly, igneous rocks don’t contain fossils, as the heat will destroy or melt any biological material. Similarly, high-grade metamorphic rocks don’t have fossils, and the heat and/or pressure would distort fossils, making them unrecognizable.
Historical background and conception
William Smith (1769–1839), an English geologist and canal surveyor, developed the faunal succession principle or law.
He observed that rock layers were arranged following a predictable pattern, and you would find these layers in the same relative position, i.e., the same order of superposition. This observation concerns lithostratigraphy, i.e., rock bodies or strata classification based on observable lithology and their relative position in strata.
Smith also noted that groups of fossils in rock layers appeared in a definite order in successive strata in different regions. Even when the lithology of sedimentary rock changed, the fossil assemblage remained the same. This is what the law of faunal succession is about in the most basic way.
From this second observation, Smith realized you could use fossils in rock units to place them in their correct stratigraphic position even though they are scattered outcrops. This laid foundation for biostratigraphy, which correlates and assigns the relative age of rock strata considering their fossil assemblage.
His work was based on observations in the field. He never understood why specific fossils changed vertically on strata. Also, he did not know about the biological evolution that Charles Darwin postulated 50 years later, i.e., in Darwin’s Origin of Species, and only had a vague time unit, which is not what we know today.
Smith then developed a fossil assemblage order from the oldest at the bottom to the uppermost at the top. Using this, he could predict the stratigraphic position of a certain layer. This knowledge helped him predict rocks they could encounter during canal construction, where to put the bridges’ foundations, and the best route. Also, he could know the fossils present by looking at strata-fossil relationships, even in a new area.
In subsequent years, his work sampling, mapping, and drawing strata from canals, railway and road cuttings, escarpments, and quarries earned him the name ‘Smith Strata.’ Later in 1815, he produced England and Wales’s first comprehensive geologic map.
The other person who concurrently recognized faunal success in France was Alexander Brongniart (1770-1847). However, many people don’t mention this French mineralogist, geologist, chemist, zoologist, and paleontologist. He deserves recognition.
Today, fossil usage has become important in chronostratigraphy. It supersedes lithostratigraphy. However, both are important.
What does the faunal succession principle state?
The law of faunal succession states that sedimentary rock strata contain various fossils, and these fossils vertically succeed each other in a definite, reliable, and predictable order identifiable over wide horizontal distances.
In simple terms, this basic historical geology principle acknowledges three things 1) sedimentary rock strata have a unique and characteristic assemblage of fossils 2) these fossils appear vertically in a specific, predictable, and reliable order 3) and the order in which the fossils appear is identifiable over a wide horizontal area.
During Smith’s time, lithology was the basis for assigning relative rock ages. However, the principle of faunal succession confirmed that rocks of the same lithology could have different ages. For instance, you can have sandstone, shale, or limestone formation of different ages.
Similarly, the depositional or lithology environment will not affect the groups of fossils present. Even sandier or muddier strata can have the same fossils.
To illustrate the principle of faunal succession, let us assume you have a sedimentary rock stratum with these fossils.
- Paradoxides, a genius of a huge trilobite that lived in the middle Cambrian region, i.e., about 509-500 million years ago
- Mucrospirifer mucronatus, a species in the middle Devonian 416-359 million years ago
- Perisphinctes, a genus of extinct ammonite cephalopod of Jurassic age 201.3 to 145 million years ago
- Scaphites, a heteromorph ammonite genus, was widespread in the late Cretaceous period, 140 to 66 million years ago.
Applying the law of faunal succession, you can deduce thatParadoxides will be on the lowest/oldest rock stratum, followed by Mucrospirifer mucronatus, Perisphinctes, and finally, Scaphites in that specific and predictable order. This order will remain the same in sedimentary rock outcrops in different places or over a wide horizontal area.
There will be no instance where rock strata have any of these fossils in the same layer. It will indicate they existed simultaneously, yet they didn’t. Also, you cannot find Paradoxides in the same layer as Scaphites, just as you cannot have dinosaurs with humans.
Understanding zone and index fossils
To help classify rock relative ages using assemblages of fossils, we use index fossils (key, indicator, or type fossils). These are fossils of organisms that evolved quickly and were geographically widespread. Also, they are known when they existed, had easy-to-recognize features, and lived in several environments.
Index fossils are important diagnostic species. Why? Because using a single fossil that is not an index fossil or that existed for a long time cannot give the proper relative age of rock strata. For instance, whales have existed for a long time, i.e., 50 million years, and were not widely distributed. They can exist in enormous vertical strata and only in a few places, making them unsuitable for rock unit correlation.
On the other hand, zone fossils share the same characteristics as index fossils. However, they occur in a limited biographic region. Therefore, you cannot use them to correlate strata.
Significance of law faunal succession
Why is the principle of faunal succession important?
1. In determining the relative age
By studying the fossil assemblage in rock strata and applying the law of superposition, paleontologists and other geologists can more accurately get the relative ages of rock layers. This applies to overturned strata or those whose lithologies vary.
Even where they have the same lithology but different ages, fossil assemblages will help you separate rock units by relative age.
2. Fossil correlation
The principle of fossil correlation states that assemblages of fossils in a rock unit or layer are of similar age, and therefore, strata with these fossils have like ages. This principle is important in matching outcrop rock units or sequences in different locations. Why? Because they have the same geologic depositional time. Otherwise, they will not have assemblages of fossils.
3. It may help identify unconformities
Unconformities refer to buried erosional or non-depositional surfaces separating rock strata of different ages. They represent a hiatus, i.e., a gap in geologic time or rock records. Some unconformities, like paraconformity, are not easy to spot. However, one can identify them by considering fossil succession in strata. If certain fossils are missing, there is an unconformity, i.e., strata eroded or none formed.
4. Understand past changes on Earth, including climatic
Fossils can help us know past events, including climatic change. For instance, fossils from artic indicate warmer weather 55-33 Ma with evidence of turtles, tapir, alligators, or primates, yet the continents were the same place they are today.
Also, we know of continental drift between Africa and America by looking at the same rocks in South America and Africa.
It helped create the geologic time scale (GTS)
Geologists created a composite stratigraphic succession by matching rocks in worldwide outcrops using faunal succession. They then used isotopic dating to assign absolute ages, i.e., eons, eras, periods, epochs, etc., that make the geologic time scale.
References
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- MacLeod, N. (2005). Stratigraphical principles. In Selley, R. C., Morrison, C. L. R., & Plimer, I. R. (Eds.). Encyclopedia of geology (Vols. 1-5). Elsevier Academic.
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