| Carboniferous and Permian strata often contain useful index fossils belonging to this group. |
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| These rocks are interbedded in Emsian marine sedimentary successions that are well documented by index fossils. |
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| They are very abundant in some Paleozoic formations and include useful index fossils. |
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| Thus, the Silurian-Devonian boundary is anchored to the first occurrence of specific index fossils. |
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| Their distinctive scales are used as index fossils for the relative dating of rock layers. |
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| Unfortunately, geochronologists assume that index fossils represent an absolute pattern. |
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| These fossils are indexes of relative geologic age and may be termed index fossils. |
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| Ostracods—small crustaceans radiated during the Jurassic and are used today as index fossils. |
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| Anisian marine strata are correlated worldwide by seven biozones containing ammonoid cephalopod index fossils. |
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| The spire-bearing spiriferoids were perhaps the most common and have been used as index fossils. |
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| Induan marine strata are correlated worldwide by six biozones containing ammonoid cephalopod index fossils. |
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| They make excellent index fossils because they underwent rapid evolution and attained a broad distribution. |
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| These fossils which are used to identify particular layers of the geological column are known as index fossils. |
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| Locally, some index fossils can be correlated with absolute age dates that are slightly older or slightly younger than the upper and lower age limit of the time slice. |
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| Sometimes they are found in multiple layers, but a statistical study of their distribution shows that even index fossils are seldom found layered on top of one another. |
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| This is a long-term research programme initiated by IGCP Project 381 comprising several research workinggroups on the various types of index fossils on both sides of the South Atlantic. |
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| This feature and their occurrence in marine sedimentary rocks, commonly in great abundance, makes them ideal biostratigraphic index fossils, and they have been used extensively for this purpose in petroleum exploration. |
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| Fusulinid foraminiferans, useful as late Paleozoic index fossils, did not survive the crisis, nor did the cryptostomate and fenestrate bryozoans, which inhabited many Carboniferous and Permian reefs. |
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| Such index fossils must be distinctive, be globally distributed and occupy a short time range to be useful. |
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| The fossil bones belong to ancestors of our modern baleen whales and their age was determined using so-called co-occurring index fossils. |
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| Some bivalves also are used as index fossils. |
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| Such index fossils must be distinctive, be globally distributed and have a short time range to be useful. |
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| However, misleading results are produced if the index fossils turn out to have longer fossil ranges than first thought. |
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| Some echinoids, such as Micraster, which is found in the Cretaceous period Chalk Formation of England and France, serve as zone or index fossils. |
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| It has become especially useful to metamorphic and igneous geologists where index fossils are seldom available. |
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| Ammonites are excellent index fossils, meaning it is often possible to link the rock layer in which a particular species is found to specific geological time periods. |
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