Relative Fossil Dating and Conodonts
- 演化之聲
- Mar 9
- 5 min read
Updated: Mar 26
In the article "Stratigraphy — Introduction to Paleontology (Part III)," several approaches used in identifying rock strata were introduced, including lithostratigraphy, biostratigraphy, and chronostratigraphy. That article also mentioned radiometric dating as the primary method for determining the age of geological strata.
Because radiometric dating directly determines a numerical age or time interval, it is referred to as absolute dating.
However, radiometric methods are subject to certain limitations. In many sedimentary layers, the conditions required for radiometric dating are absent, making the method difficult or impossible to apply. For this reason, another method mentioned in the previous article—dating based on fossils—becomes an important complementary approach.
In this article, Bai Leng will explain how fossil dating works and introduce one of the most widely used fossil groups for this purpose: conodonts.
Why Fossil Dating Is Necessary
Radiometric dating determines the age of rocks by measuring the decay products of radioactive isotopes present within them. By analyzing the ratio between parent isotopes and their decay products, scientists can estimate the age of the rock layer within a relatively precise time range.
Different isotopes produce different dating methods. In stratigraphic studies, commonly used techniques include:
Uranium–lead dating
Rubidium–strontium dating
Potassium–argon dating
Despite their usefulness, these methods share an important limitation: they work best on igneous rocks.
As a result, unless volcanic activity occurred in a region and produced igneous layers within the stratigraphic sequence, determining the exact age of sedimentary strata becomes difficult. In such situations, fossil dating becomes extremely valuable.
Index Fossils
Fossils used for dating geological layers are known as index fossils. When selecting fossils suitable for this purpose, researchers apply several criteria. These criteria arise from the way fossil dating functions.
Earlier it was mentioned that fossil dating is a complementary method. The reason for this is that fossils rarely provide precise chronological ages on their own. Instead, fossil dating primarily establishes the relative order of strata.
Because this approach cannot directly determine absolute ages, it is called relative dating, in contrast to absolute dating.
The principle is straightforward. Suppose two rock layers, A and B, both contain fossils of the same species. In that case, the two layers likely formed during roughly the same time interval. By examining many such occurrences, scientists can establish the chronological sequence of rock layers in Earth's history.
In certain cases, a rock layer containing index fossils may also include rocks suitable for radiometric dating. When this occurs, researchers can first determine the absolute age of that layer through radiometric methods. They can then infer the age range of the index fossil found within it. Once this age range is established, all other strata containing the same index fossil can be correlated to that time interval. The geological boundary known as the "Golden Spike" (Global Boundary Stratotype Section and Point) is determined using this type of integrated method.
After understanding how fossil dating operates, we can now examine the criteria used to identify good index fossils.
Wide geographic distribution: The broader the fossil's horizontal distribution, the more geological strata it can help correlate.
Large population size: A species that existed in great numbers is more likely to leave abundant fossil remains, making it easier to find specimens.
Rapid evolutionary rate: Species that evolve quickly tend to exist for shorter time intervals. This shorter duration allows for more precise stratigraphic dating.
Distinctive features: Clearly recognizable characteristics help ensure accurate identification. If a fossil species cannot be reliably identified, it cannot serve as an index fossil.
These four criteria define the primary requirements for index fossils. After applying these criteria, several fossil groups have proven especially useful for stratigraphic dating. Examples include coccolithophores (Coccosphaerales), foraminifera (Foraminifera), and the group discussed in this article: conodonts.
Conodonts
Conodonts are an extinct group of animals that first appeared in the Late Cambrian of the Paleozoic Era and persisted until the Late Triassic of the Mesozoic Era. Their fossils are typically extremely small, ranging from about 0.1 to 4 millimeters in length.
The name "conodont" literally means "cone tooth," referring to the tooth-like appearance of their fossils. For a long time, only these tiny tooth-shaped structures were known. Because these elements clearly did not represent complete organisms, scientists remained uncertain about the true nature of conodonts for more than a century after their first formal description in 1856.
This uncertainty led to many competing hypotheses regarding their identity. Some researchers proposed that conodonts belonged to chaetognaths, while others suggested relationships with mollusks, annelids, arthropods, or vertebrates. A few reconstructions were particularly imaginative. One proposal from 1974 depicted conodonts as barrel-shaped organisms covered externally with numerous tooth-like hard structures that supposedly protected them from predators.
The mystery was finally solved in 1983 when fossils preserving soft tissues were discovered in South Africa. These fossils revealed the complete organism and showed that conodonts were actually primitive vertebrates. The tooth-like structures were located in the mouth and functioned similarly to teeth. Phylogenetic studies indicate that conodonts were relatively close to living jawless vertebrates such as hagfish and lampreys.
This discovery also explains why conodont fossils usually consist only of tooth-like elements. Early vertebrates originally possessed skeletons composed largely of soft cartilage rather than mineralized bone. As a result, most parts of their bodies decayed easily and rarely fossilized. Only the hardened feeding elements in the mouth were preserved.
Even today, only three complete conodont body fossils have been discovered.
Sharks present a similar case. Much of what we know about ancient sharks also comes primarily from their teeth, because the rest of their skeleton is largely cartilaginous.
Conodonts as Index Fossils
As members of the index fossil assemblage, conodonts meet all the criteria described earlier.
They were distributed across nearly all marine environments worldwide. Their evolutionary rate was extremely rapid, and more than 1,500 species have already been identified. Conodont elements are also abundant in marine strata of the Paleozoic, where they often occur in large quantities. Their distinctive shapes make them relatively easy to identify.
Because of these properties, conodonts are considered one of the most effective index fossil groups for the Paleozoic Era. Biostratigraphic zones defined using conodonts number 72 in the Paleozoic alone, with an additional 22 zones established for the Triassic Period of the Mesozoic Era, creating an exceptionally detailed stratigraphic framework.
Unfortunately, conodonts became extinct during the Late Triassic. After their disappearance, other microfossil groups—such as coccolithophores and foraminifera—continued to serve as major index fossils up to the present day.
Author: Bai Leng
Reference:
Eichenberg, W. (1930). Conodonten aus dem Culm des Harzes. Paläontologische Zeitschrift
Fåhraeus, Lars E. (1983). Phylum Conodonta Pander, 1856 and Nomenclatural Priority. Systematic Zoology
Terrill, David F., Jarochowska, Emilia, Henderson, Charles M., Shirley, Bryan, Bremer, Oskar (2022). Sr/Ca and Ba/Ca ratios support trophic partitioning within a Silurian conodont community from Gotland, Sweden. Paleobiology
Zhen, Y. Y. (2020). What are conodonts?. Australian Museum
Comments