They are used as an index fossil in the Ordovician.

This is revealed by the presence of the index fossil Halysites.

It is used in biostratigraphy as an index fossil.

However, misleading results are produced if the index fossils turn out to have longer fossil ranges than first thought.

It has a cosmopolitan distribution and is an important index fossil in biostratigraphy.

They have been recorded from every continent and many are used as zonal or index fossils.

Other index fossils from Coon Creek date a little older than 70.6 million years.

The family includes several important index fossils.

Trilobites are used as index fossils to subdivide the Cambrian period.

Misleading results are produced if the index fossils are incorrectly dated.

They lived during the Jurassic Period and serve as an index fossil for that time period.

In Jurassic rock ammonites are the index fossils.

Many ecphora species are important index fossils.

Nevertheless, in the late 1960s, mini-skirts were distributed all over the world and could be (if preserved) the index fossil for that epoch.

This makes some corals useful index fossils that enabled geologists to date the rocks in which they are found.

They are valuable as index fossils.

Oppel's zones are named after a particular distinctive fossil species, called an index fossil.

To be useful in stratigraphic correlation index fossils should be:

Planktonic Graptolites, an important index fossil, appear during the Tremadocian.

They became the planet's very first reef-building animals and are an index fossil for the Lower Cambrian worldwide.

The Bashkirian contains six biozones based on conodont index fossils:

Index fossils are one of the species from the assemblage of species that characterise the zone.

It has become especially useful to metamorphic and igneous geologists where index fossils are seldom available.

Such index fossils must be distinctive, be globally distributed and have a short time range to be useful.

They are important index fossils for dating Palaeozoic rocks as they evolved rapidly with time and formed many different species.