Biochronology

Source: Wikipedia, the free encyclopedia.
This article is about chronology based on fossil correlation. For correlation tied to stratigraphic units, see Biostratigraphy.

In

climate variations and mass extinctions
.

Comparison with biostratigraphy

Ediacaran Period, an example of an internationally agreed upon reference point
for this boundary.

In sedimentary rocks, fossils are the only widely applicable tool for time correlation.[1]: 229  Evolution leaves a record of progressive change, sequential and nonrepeating.[1]: 230  A rock unit has a characteristic assemblage of fossils, independent of its lithology.[1]: 229  Thus, the fossils can be used to compare the ages of different rock units.

The basic unit of biochronology is the biostratigraphic zone, or

benthic foraminifera, which readily spread throughout the world's oceans.[1]
: 230 

Another challenge for stratigraphy is that there are often large gaps in the fossil record at a given location. To counter this, biostratigraphers search for a particularly well-preserved section that can be used as the

graptolite Mongraptus uniformus uniformus in a section in Klonk, Czech Republic.[1]
: 237 

In terrestrial deposits, fossils of land mammals and other vertebrates are used as stratigraphic tools, but they have some disadvantages relative to marine fossils. They are seldom evenly distributed through a section, and they tend to occur in isolated pockets with few overlaps between biozones. Thus, correlations between biozones is often indirect, inferred using a knowledge of their sequence of evolution.[1]: 240  This practice was first proposed by H. S. Williams in 1941.

In the United States, biochronology is widely used as a synonym for biostratigraphy, but in Canada and Europe the term is reserved for biochronology that is not tied to a particular stratigraphic section.[2] This form of biochronology is not recognized by the International Stratigraphic Guide, but it is "really what a great many paleontologists and stratigraphers are after ... an optimum network of fossil correlations, thought to embody a reliable and high-resolution isochronous time (lines) framework."[3]

Land mammal ages

Skull of Bison antiquus from the La Brea Tar Pits.

A Cenozoic chronology based on mammal taxa has been defined on all the continents except Antarctica.[4]: 939  Because the continents have been separated through most of the Cenozoic, each continent has its own system.[5] Most of the units are based on assemblage zones, layers of strata that contain three or more distinctive fossils.[6]: 4, 15 

North America

Main article: North American land mammal age

In 1941, a committee chaired by Horace E. Wood II compiled a list of 19 "provincial ages" for North America, later called North American Land Mammal Ages (NMLAs). An example of an NMLA is the

chronostratigraphic stages, so some authors place quotes around "Ages".[1]: 240 [4]: 943 [8]

South America

Main article: South American land mammal age

The development of South American land mammal ages is largely due to two brothers, Florentino Ameghino and Carlos Ameghino. As of 1983, there were 19 ages, all but one of which were based on sections in Argentina.[9] Since then three more ages have been added for the Paleocene.[10]: 16 

Europe

Main article: European land mammal age

The first European European land mammal age (ELMA), the

Mammal Paleogene zones have also been defined.[10]
: 15 

Asia and Africa

See also: Asian land mammal age

Asian land mammal ages are more recently named and more tentative than the ages for the above continents, with poor geochronological constraints. There is no consensus for the names of some of the ages.[10]: 17  However, the picture is rapidly improving, since Central Asia has some of the world's best records of Neogene mammals.[5]: 11  In Africa, sequences of fossils (including those of primates) have been determined and some land mammal ages designated, but not yet formally defined.[10]: 11 

Other tetrapod-based biochronologies

Land-mammal ages mostly represent intervals in the Cenozoic; they have not been proposed for the Mesozoic. However, related systems have been proposed for other periods of prehistory. Land-vertebrate "ages" (LVAs) based primarily on dinosaur faunas have been proposed for the late Cretaceous in western North America.[12]

Land vertebrate faunachrons

Main article: Triassic land vertebrate faunachrons

The most widely utilized pre-Cenozoic tetrapod biochronology system involves Land vertebrate faunachrons (LVFs). The LVF system was originally designed by Spencer G. Lucas to correlate terrestrial faunal assemblages of the Triassic period.[13] LVFs have also been used in Permian biochronology.[14] Although LVFs are a common method used to date Triassic terrestrial sediments, their reliability is more heavily debated than that of Land Mammal Ages.[15][16]

Geochronology

The order of evolutionary events that have been used to sequence land mammal records have been verified using geochronological methods.[1]: 241  Although first and last occurrences of taxa can vary with location, assemblages show little variation.[1]: 240  Fossils of mammals also have the advantage that the mammals have evolved rapidly.

The resolution of terrestrial fossil records have improved as the methods have improved. Although

reversals of the Earth's magnetic field.[17]: 10  This has made it possible to correlate terrestrial sediments with the time scale from marine sediments, and compare them directly with global climate change and mass extinctions.[17]
: 10 

Paleontologists have moved towards finer zonation of terrestrial fossils, with the potential to divide the Cenozoic into time intervals of 300,000 years or less.[17]: 10  They have also attempted to convert some of the intervals, including the Wasatchian age/stage and Clarkforkian age/stage, into biostratigraphic units.[6]: 16  However, the fossil record remains discontinuous even in North America, and Woodburne speculates that "mammal age correlations provide results that are satisfactory to their users."[6]: 18 

References

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  3. ISBN 9780444594488
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  4. ^
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  8. doi:10.1130/GSAB-52-1
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  9. ^ Larry G., Marshall; Hoffster, Robert; Pascual, Rosendo (1983). "Mammals and stratigraphy: geochronology of the continental mammal-bearing Tertiary of South America". Palaeovertebrata. Retrieved 15 April 2024.
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  11. ^
    doi:10.1016/S0031-0182(97)00083-7
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  13. ISSN 0031-0182
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  14. ^ Lucas, Spencer G. (2005). "Permian Tetrapod Faunachrons". New Mexico Museum of Natural History and Science Bulletin. 30: 197–201.
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  16. ^ Irmis, Randall B.; Martz, Jeffrey W.; Parker, William G.; Nesbitt, Sterling J. (March 2010). "Re-evaluating the correlation between Late Triassic terrestrial vertebrate biostratigraphy and the GSSP-defined marine stages" (PDF). Albertiana. 38: 40–53.
  17. ^
    ISBN 9780521355193
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