Faunal stage

Faunal stage

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Encyclopedia
In chronostratigraphy
Chronostratigraphy
Chronostratigraphy is the branch of stratigraphy that studies the age of rock strata in relation to time.The ultimate aim of chronostratigraphy is to arrange the sequence of deposition and the time of deposition of all rocks within a geological region, and eventually, the entire geologic record of...

, a stage is a succession of rock strata
Geologic record
The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata — deposits laid down in volcanism or by sediment deposition of weathering detritus including all its fossil content and the information it yields about the history...

 laid down in a single age on the geologic timescale, which usually represents millions of years of deposition. A given stage of rock and the corresponding age of time will by convention have the same name, and the same boundaries.

Rock series are divided into stages, just as geological epochs are divided into ages. Stages can be divided into smaller stratigraphic units called chronozone
Chronozone
A chronozone or chron is a slice of time that begins at a given identifiable event and ends at another. In the fossil record such tracer events are usually keyed to disappearance of a widely distributed and rapidly changing species or the appearance of such a species in the geological record...

s. (See chart at right for full terminology hierarchy.)

The term faunal stage is sometimes used, referring to the fact that the same fauna
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...

 (animals) are found throughout the layer (by definition).

Defining


Stages are primarily defined by a consistent set of fossils (biostratigraphy
Biostratigraphy
Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them. Usually the aim is correlation, demonstrating that a particular horizon in one geological section represents the same period...

) or a consistent magnetic polarity (see paleomagnetism
Paleomagnetism
Paleomagnetism is the study of the record of the Earth's magnetic field in rocks. Certain minerals in rocks lock-in a record of the direction and intensity of the magnetic field when they form. This record provides information on the past behavior of Earth's magnetic field and the past location of...

) in the rock. Usually one or more index fossil
Index fossil
Index fossils are fossils used to define and identify geologic periods . They work on the premise that, although different sediments may look different depending on the conditions under which they were laid down, they may include the remains of the same species of fossil...

s that are common, found worldwide, easily recognized, and limited to a single, or at most a few, stages are used to define the stage's bottom.

Thus, for example, in the (still used) local North American subdivision paleontologist finding fragments of the trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...

 Olenellus would identify the beds as being from the Waucoban Stage whereas fragments of a later trilobite such as Elrathia would identify the stage as Albertan.

Stages were very important in the 19th and early 20th century as they were the major tool available for dating rock beds until the development of seismology
Seismology
Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. The field also includes studies of earthquake effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic,...

 and radioactive dating in the second half of the 20th Century. Microscopic analysis of the rock (petrology
Petrology
Petrology is the branch of geology that studies rocks, and the conditions in which rocks form....

) is also sometimes useful in confirming that a given segment of rock is from a particular age.

Originally, faunal stages were only defined regionally; however as additional stratigraphic tools, and especially geochonological ones, were developed, stages were defined over broader and broader areas. More recently, the adjective "faunal" has been dropped as regional and global correlations of rock sequences have become relatively certain and there is less need for faunal labels to define the age of formations. A tendency developed to use European and, to a lesser extent, Asian, stage names for the same time period world wide, even though the faunas in other regions often had little in common with the stage as originally defined.

International standardization


Boundaries and names are established by the International Commission on Stratigraphy
International Commission on Stratigraphy
The International Commission on Stratigraphy , sometimes referred to by the unofficial "International Stratigraphic Commission" is a daughter or major subcommittee grade scientific daughter organization that concerns itself with stratigraphy, geological, and geochronological matters on a global...

 (ICS) of the International Union of Geological Sciences
International Union of Geological Sciences
The International Union of Geological Sciences is an international non-governmental organization devoted to international cooperation in the field of geology.-About:...

. As of 2008, the ICS is nearly finished a task begun in 1974, subdividing the Phanerozoic
Phanerozoic
The Phanerozoic Eon is the current eon in the geologic timescale, and the one during which abundant animal life has existed. It covers roughly 542 million years and goes back to the time when diverse hard-shelled animals first appeared...

 eonothem
Eonothem
In stratigraphy and geology, an eonothem is the totality of rock strata laid down in the stratigraphic record deposited during a certain eon of the continuous geologic timescale...

 into internationally accepted stages using two types of benchmark. For younger stages, a Global Boundary Stratotype Section and Point
Global Boundary Stratotype Section and Point
A Global Boundary Stratotype Section and Point, abbreviated GSSP, is an internationally agreed upon stratigraphic section which serves as the reference section for a particular boundary on the geologic time scale. The effort to define GSSPs is conducted by the International Commission on...

 (GSSP), a physical outcrop
Outcrop
An outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. -Features:Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial deposits are covered by a mantle of soil and vegetation and cannot be...

 clearly demonstrates the boundary. For older stages, a Global Standard Stratigraphic Age
Global Standard Stratigraphic Age
In the stratigraphy sub-discipline of geology, a Global Standard Stratigraphic Age, officially abbreviated GSSA, is a chronological reference point and criterion in the world's rock records used to define the boundaries between different geological periods, epochs or ages on the overall geologic...

 (GSSA) is an absolute date. The benchmarks will give a much greater certainty that results can be compared with confidence in the date determinations, and such results will have farther scope than any evaluation based solely on local knowledge and conditions.

In many regions around the world local subdivisions and classification criteria are still used along with the newer internationally coordinated uniform system, but once the research establishes a more complete international system, it is expected that local systems will be abandoned.

Stages and lithostratigraphy


Stages can include many lithostratigraphic
Lithostratigraphy
Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology...

 units (for example formations, beds
Bed (geology)
In geology a bed is the smallest division of a geologic formation or stratigraphic rock series marked by well-defined divisional planes separating it from layers above and below. A bed is the smallest lithostratigraphic unit, usually ranging in thickness from a centimeter to several meters and...

, members, etc.) of differing rock types that were being laid down in different environments at the same time. In the same way, a lithostratigraphic unit can include a number of stages or parts of them.

See also

  • Body form
  • European Mammal Neogene
  • Geologic record
    Geologic record
    The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata — deposits laid down in volcanism or by sediment deposition of weathering detritus including all its fossil content and the information it yields about the history...

  • Geologic time scale
    Geologic time scale
    The geologic time scale provides a system of chronologic measurement relating stratigraphy to time that is used by geologists, paleontologists and other earth scientists to describe the timing and relationships between events that have occurred during the history of the Earth...

  • North American Land Mammal Ages
    North American Land Mammal Ages
    The North American Mammal Ages establishes a geologic timescale for prehistoric North American fauna beginning 66.5 Ma during the Paleocene and continuing through to the Late Pleistocene...

  • Fauna (animals)
  • Type locality (geology)
    Type locality (geology)
    Type locality , also called type area or type locale, is the where a particular rock type, stratigraphic unit, fossil or mineral species is first identified....

  • list of geochronologic names
  • List of GSSAs
  • List of GSSPs

External links