A fauna or fossil assemblage refers to a collection or group of ancient organisms’ remains or traces all found in the same rock layer or sequence of sedimentary rocks. Their occurrence shows that all these organisms existed simultaneously when the rock layer or sequence formed.
We use assemblages of fossils to assign rocks relative ages. Also, they help correlate between strata in different locations.
One advantage of fossil assemblages is that they help narrow the relative age compared to using a single fossil that is not an index fossil.
Learn more about fossil assemblage, including what it is and how it helps geologists and other scientists.
More about fossils and stratigraphy
Fossils are remains or traces of ancient organisms (fauna or flora) preserved in sedimentary rock strata. Most, 99.9% of which are now extinct. They help us know more about the past life history of the Earth, including paleoenvironments and plate tectonic activities.
William Smith, an English geologist and surveyor, was the first to use fossils to identify and correlate strata in different places. He is the one who postulated the law of faunal succession. It states that sedimentary rock layers have fossils. These fossils appear vertically in a reliable and predictable order identifiable over wide horizontal.
Lastly, using index fossils or fossil assemblages to correlate and give relative ages to strata is known as biostratigraphy. A strata interval with a characteristic index fossil or fossil assemblage is known as a fossil zone. Such a zone is not limited to formation boundaries or lithologies.
What is a fossil assemblage?
A fossil assemblage refers to a collection or group of fossils found in the same sedimentary rock layer. Their occurrence implies they were all present when the rock layer containing them formed.
You can have a life assemblage (biocenosis) or death assemblage (thanatocoenosis) fossils. Life assemblage fossils indicate that the organisms interacted and lived in the same habitat. Such organisms biological community before dying in the same place, i.e., not transported. Also, they indicate a sudden event that caused mass death, such as a catastrophe.
A life assemblage will have a wider range of organisms of various ages (juvenile to adult). There will be no evidence of body erosion. Also, there will be no alignment, i.e., they were buried as they lived and will be well preserved, whole, and not much broken.
On the other hand, death assemblage means that the fossils didn’t live together or interact. They came from different habitats and were transported after their death to the place of deposition. An example is water currents bringing marine fossils together. Another example is a predator depositing bones at the same spot.
You can identify a death assemblage as it will have transportation imprints. For instance, the fossil will be eroded, broken, and sorted according to size. These organisms will be aligned parallel to the direction of the current flow. Also, most will be mature, i.e., die after maturity, and species will be limited.
That is not all. You must know of derived fossils. These are fossils removed from existing rocks and transported as clasts before deposition on younger strata. Not knowing them can result in erroneous rock relative age or correlation.
Derived fossils will appear eroded, polished, abraided, or broken. Also, there will be some loss of detail, and they will not fit in the geologic period.
Significance of fossil assemblage
Some of the reasons why fossil assemblages are important include the following:
1. Creation of geologic time scale (GTS)
The appearance and disappearance of the index fossils and fossil assemblages form the basis of defining geologic periods (time intervals). Also, they can form their boundaries. These two help create the relative geologic time scale.
2. Relative rock dating
Fossil assemblages are important in the relative dating of rocks, including when you don’t have an index fossil. We also know that a certain sedimentary rock with a certain fossil assemblage formed when all respective organisms were present.
To know the relative age of the strata in which you found a fossil assemblage. Then, consider each fossil’s geologic period or lifespan, i.e., when they emerged and disappeared.
Lastly, using the lifespan of each organism in the assemblage, you will narrow the strata’s relative age to when they all lived simultaneously. This relative age will be the overlapping period/strata just before the oldest disappears and after the youngest appears. Of course, you will also apply the law of superposition.
3. Rock correlation
The principle of fossil correlation states that an assemblage of fossils in a rock layer are of similar ages. Therefore, any strata with these fossils have like ages.
We use this principle to correlate or match rock strata with the same fossil assemblage in different locations globally, even when the rock types or lithologies are different.
This biocorrelation helps give more glimpses into the history of the Earth, including plate tectonics, environment, climate, etc.
Fossil assemblage vs. index fossils
Both index fossils and fossil assemblage follow the law of faunal succession and are vital in correlating rocks in different places and assigning relative ages. The former is more convenient than the latter.
However, some scientists and geologists consider the use of fossil assemblage to be more reliable than index fossils. They say that although not as convenient, it eliminates errors or uncertainties of reworked, undiscovered, or missing individual species. Thus, correlations based on fossil assemblage tend to be more useful.
References
- Levin, H. L., & King, D. T. (2016). The Earth through time, 11th edition (11th ed.). Wiley.
- Plummer, C. C., Carlson, D. H., & Hammersley, L. (2016). Physical Geology (15th ed.). McGraw-Hill Education.
- Boggs, S. (2014). Principles of Sedimentology and stratigraphy (5th ed.). Pearson Education.
- Murphy, M. A., & Salvador, A. (Eds.). (2000). International Subcommission on stratigraphic classification of IUGS International Commission on stratigraphy. GeoArabia, 5(2), 231–266. https://doi.org/10.2113/geoarabia0502231
- Prothero, D. R., & Schwab, F. (2014). Sedimentary geology: An introduction to sedimentary rocks and Stratigraphy (3rd ed.). W.H. Freeman and Company.
- Grotzinger, J. P., & Jordan, T. H. (2014). Understanding earth (7th ed.). W.H. Freeman and Company.
- Kusky, T. M., & Cullen, K. E. (2005). Encyclopedia of earth and space science. Facts on File.
- Wicander, R., & Monroe, J. S. (2010). Historical geology: Evolution of the Earth and life through time (6th ed.). Books-Cole.