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What are Index Fossils?
Fossils are the remnants of ancient living things like animals and plants well-preserved from the prehistoric period by the vision of natural conditions. It can be usually a rock in which organisms are preserved. These are found in sedimentary rocks, coal, asphalt deposits, amber and other materials. The scientific discipline that transacts with the study of fossils is called paleontology. The shells, bones, leaves, and feathers become fossils. It can be small or large accordingly.
Index fossils are the fossils that are generally found as well as extensively distributed and are the fossils limited in the period. These helps resolve the age of organic rocks as well as other fossil accumulations. Not only this, but it also helps in establishing the associations between the rock units and is also called indicator fossils or guide fossils.
Index fossils can be defined as the fossils of animals and plants preserved in the rocks present in the earth that can be studied to know about the geological period that has happened over time. It is useful in being different and easily recognizable. It is also present plentifully and has a wide range of distribution on the planet and a short range.
They are relatively useful in defining boundaries in the geological period and also for the strata’s relationship. In the layers of the marine environment, the index fossil fuels used are the protist family that has a hard body covering and large forms called ammonoids. Similarly, in the terrestrial environment, it is of the Cenozoic Era which started approximately 70 million years ago and mammals have been used extensively as date deposits. They also contain hard body parts such as bones, shells, and teeth, thus, evolved quickly.
Meaning of Index Fossil
Index fossils are those fossils that are valuable for dating as well as comparing the different strata where they are found. Some species live in a particular environment and space and animal/plant life is well-preserved in rocks of the Earth that are characteristic of a particular reach of geologic time or environment.
Geological scientists use them. They are important to help in determining the relationship as well as the age of rock patterns. Geologists use microscopic fossils to large fossils to identify geologic time.
They use both large or macrofossils as well as microscopic or microfossils as catalogs to describe as well as identify geologic periods.
Talking about macrofossils, have the advantage over microfossils because they are easy to see and find in the field but actually, they are erratic and rare whereas microfossils are used by mineral resources businesses after extraction of required data from these fossils.
Examples of Index Fossils
There are several types of index fossils which include Brachiopods, Ammonites, Graptolites, Trilobites, and Nannofossils.
Brachiopods: They arose 550 million years ago. Brachiopods were first founded in the Cambrian period. These are mollusks-like sea animals that appeared during the Cambrian (540 to 500 million years ago) some of them still alive. They are supposed to be the most common fossils.
Ammonites: The ancient marine animal fossils are said to be known as ammonite index fossils. They were common during the Mesozoic era (65 to 245 million years ago). They were not found after the Cretaceous period, as they went extinct during the K-T extinction period (a global huge extinction event accountable for eliminating up to 80 percent of all species of animals and very close to the borderline between the Paleogene and Cretaceous periods, about 65 million years ago).
Graptolites: They are shiny and thin. The patterns on rock surfaces look like pencil marks. The name Graptolites arises from the Greek for “writing in the rocks”. These are extensive colonial aquatic hemichordates. They lived during the period of Cambrian unevenly from 540 to 505 mya to the early to mid-Carboniferous about 360 to 320 mya). These are found in mud rocks and shales.
Trilobites: During the Paleozoic Era from 540 to 245 mya, they were common. Approximately half of the Paleozoic fossils are trilobites. At the start of the Paleozoic Era, they evolved and went extinct during the late Permian period about 248 million years ago.
Nannofossils: These are the remains of calcareous nannoplankton, and coccolithophores and are microscopic fossils from various eras. These are time-specific because of their evolutionary rates. Nanofossils are very abundant and it is distributed broadly in the areas. There are several numbers of beneficial nannofossils which include foraminifera and radiolarians.
Which Organisms Make Good Index Fossils?
Most index fossils are from animals and plants that were successful in being plentiful. However, they were susceptible to extinction or changed fast. They have the boom-and-bust characteristic because their time on Earth was short.
Most of these organisms were pelagic which means marine organisms living in the open sea or ocean. However, a few were terrestrial, benthos that live at ocean bottoms, or littoral live near shores. These organisms are the most common index fossils because most of the sedimentary rocks form in the sea. Also, water fluxes moved these pelagic or floating marine organisms, including their larvae and eggs, across the world. Additionally, the bottom of sea environments didn’t affect them much.
The only thing that affected pelagic organisms was food distribution and environmental differences in water bodies. But such a variation was short-term, unlike extinction, they would return when conditions were favorable. Similarly, plants containing spores or seeds moved by wind could be extensive. However, not many plant index fossils happen. Perhaps, they are not easy to preserve in sedimentary rocks.
Finally, some benthic groups are good fossil guides. However, the ocean environment at the bottom affects them. Also, in the Cenozoic Era, some mammals have acted as key fossils but for specific regions. A good example of it is the quick-evolving rodents.
Characteristics of Index Fossils
An index fossil includes the following important characteristics:
Be Abundant
The very important characteristic of index fossils is abundance. These organisms must be in huge numbers. Otherwise, the chance of finding these fossils in a rock formation is next to impossible.
Be Geographically Extensive
An important fossil must happen over a wide geographical area, such as, in different environments and continents. Such organisms must bear various ecological regions. Also, they should be independent of facies (all of the features of a rock including its physical, chemical, and biological characteristics that differentiate it from adjacent rock). Fossils widespread in certain facies specify a paleoenvironment and cannot give comparative age.
However, it is noted that younger plate tectonic events may give a false impression that an organism was found extensively, yet it was confined to a region in which it existed. Also, waves may move hollow shells, giving a false impression they are found widespread, yet they were in one zone.
Have a Short Geologic Time
Perfect index fossils should exist for a short geological time to give precise rock ages. They should appear, extinct, and not ever reappear or evolve into a characteristic organism within a short geological period. Such organisms have a short geological range or time on the Earth and will be in narrower vertical strata.
Organisms with a high rate of evolution will develop unique biological changes in a short period, resulting in more index fossils. Finally, one cannot use organisms that existed for an Era, such as the Paleozoic, to find a period like Silurian, which ended 25 million years ago. It will not give accurate relative ages.
Easy to Recognize or Identify
A guide fossil must have easy-to-recognize syllable structure taxonomic characters like a skeleton. For example, ammonites contain a coiled-body appearance with joints and jumbles. Thus, one can easily notice them. Organisms that are hard to recognize require additional study, making them inappropriate.
Distinctive
A guide fossil can be easy to distinguish but not distinctive from others. Such will confuse. i.e., it is possible to mix it to be a different organism that looks like it. A good fossil should instead be distinctive.
Preservable
Guides fossils must be easy to solidify or fossilize. For example, organisms with bones or hard shells are easier to preserve than soft-bodied ones, for instance, slugs, jellyfish, or worms that decay quickly.
However, this is not the case always. Birds have an intercontinental variety but poorly fossilizes. Their bones degenerate fast and fall apart, leaving wreckages. Thus, one cannot use them to study, making them inappropriate index fossils.
Advantages of Index Fossils
Index Fossils have various advantages discussed below:
- These fossils are used by paleontologists as well as geologists as an important support to determine the association as well as the age of rock categorizations or patterns.
- Microfossils, when required as well as precise knowledge from the fossils are collected, after that, these are usually used by oil prospectors along with other industries that are interested in mineral resources.
- By using some biological or physical methods, stratigraphic connections can also be made.
- Besides the advantages above, lithologic resemblance may also be established between the various rock units by using one or two marker beds.
- It helped and played a significant role in establishing the Continental Drift Theory.
List of Index Fossils
Some of the common index fossils are given below along with their name, and the time they represent.
- Viviparus Glacialis representing early Pleistocene (0.5 million years ago)
- Pecten Gibbus represent Quaternary Period (1.8 mya)
- Calyptraphorus Velatus (Tertiary Period)
- Scaphites Hippocrepis (Cretaceous Period 145 to 66 million years ago)
- Perisphinctes Tiziani (Jurassic Period)
Evidence of the index fossils helped, Alfred Lothar Wegener to establish as well as present the ” Continental Drift hypothesis” in 1912. According to this, all the continents of the world that we can see today, have been created from a single supercontinent called Pangea, and breaking the Pangea into smaller pieces led to the drifting away these pieces from each other around 200 million years ago that was the reason the present continents formed.
He found the fossil remnants, as well as the structure of the rocks around the shorelines of Africa, South America, and India, were similar to each other despite being separated by oceans such as the Indian and the Atlantic Oceans. Even fossils of tropical plants such as cycads and ferns are being found in the Arctic Island. Thus, the existence of index fossils leads to the authentication of the theory of Continental Drift.
Conclusion
In conclusion, we can say that index fossils are commonly found as well as extensively spread and distributed fossils which are inadequate in the period. The architecture of geological time consists of ages, periods, epochs, and eras, which index fossils helped to determine.
They consist of leftovers of animals and plants along with other species in the layers of sedimentary rocks. If we talk about a good index fossil, two basic and required required features, are they should be vulnerable as well as abundant. This boom-and-bust characteristic of fossils makes them a good fossil for paleontologists and geologists.
you can access to an article on ” How to use fossils to determine the age of rocks” from the following link https://www.northcentralpa.com/life/dcnr-how-to-use-fossils-to-determine-the-age-of-rocks
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