Indexed fossils

  • Indexed fossils are the ancient remains of the living things formed.
  • These are formed in a long period in natural conditions and also in a limited period to observe age are called index fossils. Fossils are preserved remains of living things, which study is called Paleontology. It has been found in sedimentary rocks, deposits, coral, amber, and other materials.
  • Index fossils are preserved for a limited period, which is then used for determining the age of organic rocks and act as indicator fossils to form a relationship between different rock units. These are distinct and easily identified, with a wide range of distribution.[Indexed fossils] They are extremely valuable for determining geological time scale boundaries as well as strata correlation

Indexed Fossils

A example for calculate Indexed fossils  

  • For example, Protists with hard body coverings and enormous forms known as ammonoids are the key Indexed fossils fuels employed in the strata of the marine environment. Similarly, mammals have been frequently employed as date deposits in the terrestrial stratum environment, which dates back to the Cenozoic Era, which began 65.5 million years ago.
  • They also have hard body parts like bones, teeth, and shells, and as a result, they have evolved quickly. These vary in range from large to microscopic fossils which are studied by geological scientists. 


  • Distinctive 
  • Found abundantly
  • Restricted to particular geological times
  • Resilient and long-lasting,
  • Vulnerable to environmental change and extinction.

Types of index fossils

  • Ammonites: An ammonite indexed fossils is said to be a fossil of prehistoric sea creatures. They were common during the Mesozoic era (245 to 65 million years ago ). They were not found beyond the Cretaceous period because they went extinct during the K-T extinction ( 65 million years ago ).
  • Brachiopods: They first appeared about 550 million years ago. It was first established during the Cambrian period. These are mollusk-like marine organisms that first appeared during the Cambrian Period (540 to 500 million years ago), and some of them are still alive today. these are the most common fossils.

Argopecten gibbus

                                                                      Orange white-colored striped shells as Indexed fossils

  • Graptolites: They’re slender and gleaming. On the surface of rocks, the patterns resemble pencil marks. Graptolite is a Greek word meaning “writing in the rocks.” These hemichordates are colonial marine hemichordates that can be found all over the world.
  • They lived from the Cambrian Period (approximately 540 to 505 million years ago) until the early to mid-Carboniferous Period (360 to 320 million years ago). these are found in mudrocks and shale. 
  • Nanofossils: The Indexed fossils have microscopic sizes (calcareous nannoplankton, coccolithophores) from different periods. These are time-limited due to their evolutionary rates and are abundant. Radiolarians and foraminifera are two examples of helpful nanofossils. Until now, the principal method has been marine sediments. 
  • Trilobites: They were prevalent during the Paleozoic Era (540 to 245 million years ago). Trilobites make up around half of all Paleozoic fossils. They developed during the start of the Paleozoic Era, but died extinct during the late Permian period, 248 million years ago.  


  1. These fossils help determine the correlation and age of rock.
  2. Microfossils have been used for industrial purposes.
  3. It helped to support the continental drift theory. 
  4. Besides these points, the lithologic similarity between the various rock units can also be defined by using one or two marker beds.

Pecten gibbus

White-colored shells as index fossils

Orange white-colored striped shells as index fossils

Examples of Index fossils 

  • Viviparus glacialis: Early Pleistocene  
  • Pecten gibbus: Quaternary period 
  • Calyptraphorus velatus: Tertiary period 
  • Scaphites hippocrepis: Cretaceous period
  • Thus, these fossils have distinct useful properties for studying the preserved remains of living things and relate to the oldest features of geological times, which proved to be a boom and bust period for geologists and paleontol.

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