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Glite Dictionary
index fossil

index fossil

8.2
A fossil of an animal or plant that lived in one time period and helps identify and date rock layers
  • noun
  • /ˈɪndɛks ˈfɑsəl/
  • Specialized
translation icon : fósil guía
  • The discovery of an index fossil helped the scientists quickly determine the age of the sedimentary rocks.

Examples

  • Trilobites are a classic example of an index fossil used by geologists to date Paleozoic rock layers.

  • Because ammonites have a wide geographic range, they serve as important index fossils for the Mesozoic Era.

  • The index fossil helped scientists date the rock layer.

  • Many index fossils are used in geology.

  • Ammonites are considered excellent index fossils because they lived during specific geological periods and can help date rock layers.

  • Geologists often rely on trilobites as key index fossils to determine the age of sedimentary rocks.

  • The presence of specific index fossils in a rock layer indicates that it was formed during the Ordovician period.

Synonyms

guide fossil
vsindex fossil
  • UK
8.2

A fossil that lived in many places for a short time and shows the age of rock layers

is tied to a specific geological period and may not be widespread

Surface Forms

index fossil singular
index fossils plural

Morphology

index + fossil

The compound clearly combines 'index' (an indicator) and 'fossil' (remains of ancient organisms), so a learner who knows both words can infer it denotes a fossil used as an indicator. However, the precise technical function — fossils used to identify and date specific geological layers — is specialized and not fully predictable from the constituents alone, making the term partially transparent to B1 learners.

Etymology

Index fossil comes from the idea of an index in a book: a fossil that points to a specific time. Scientists use these fossils as markers to 'identify' and 'date' the rock layer where they are found.