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This article is about chronology based on fossil correlation. For correlation tied to stratigraphic units, see Biostratigraphy.

Biochronology is the correlation in time of biological events using fossils.

Comparison with biostratigraphy[edit]

A golden spike marking the bottom of the 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 biozone, a collection of fossils found together in a rock unit. This is used as the basis of a biochron, "a unit of time in which an association of taxa is interpreted to have lived."[1]:229 However, a biozone may vary in age from one location or another. For example, a given taxon may migrate, so its first appearance varies from place to place. In particular, facies-controlled organisms (organisms that lived in a particular sedimentary environment) are not well suited for biochronology because they move with their environment and may change little over long periods of time.[1]:230–231 Thus, biostratigraphers search for species that are particularly widespread, abundant, and not tied to particular sedimentary environments. This is particularly true of free-swimming animals such as 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 type section for a particular biostratographic unit. As an example, the boundary between the Silurian and Devonian periods is marked by the first appearance of 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.[3]

Dating systems[edit]

Skull of Bison antiquus from the La Brea Tar Pits.

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). Quotes are often placed around "Ages" to indicate that they are not true chronostratigraphic stages.[1]:240[4]:943[5]

The first European European land mammal age (ELMA), the Villafranchian, was defined in 1865. It was based on sedimentary units near Villafranca d'Asti in Italy.[6] Several more were proposed between 1950 and 1975; and in 1975 Mein introduced a finer division called Mammal Neogene (MN) zones.[6] A total of 30 Mammal Paleogene zones have also been defined.[7]:15

Land-mammal "ages" have since been introduced for Asia, Europe and South America.

Land-mammal "ages" mostly represent intervals in the Cenozoic; they have not been proposed for the Mesozoic with the exception of four from the late Cretaceous in western North America.[8] However, land-vertebrate "ages" (LVAs) or faunachrons have been proposed that use other vertebrates such as dinosaurs.[8]

An example of an NMLA is the Rancholabrean, named after the Rancho La Brea fossil site. One of its characteristic fossils is the bison, which first appears in the Rancholabrean.[9]


  1. ^ a b c d e f g h i Prothero, Donald R. (1989). Interpreting the stratigraphic record. New York: W.H. Freeman. ISBN 0716718545. 
  2. ^ Lucas, Spencer G. (1997). "Biostratigraphy". In Padian, Kevin; Currie, Philip J. Encyclopedia of Dinosaurs. Burlington: Elsevier. pp. 65–68. ISBN 9780080494746. 
  3. ^ Gradstein, Felix M (2012). "Biochronology". In Gradstein, Felix M; Ogg, J G; Schmitz, Mark et al. The geologic time scale 2012. Amsterdam: Elsevier. pp. 43–62. ISBN 9780444594488. 
  4. ^ Hilgen, F J; Jourens, L J; Van Dam, J A (2012). "The Neogene Period". In Gradstein, Felix M; Ogg, J G; Schmitz, Mark et al. The geologic time scale 2012. Amsterdam: Elsevier. pp. 923–978. ISBN 9780444594488. 
  5. ^ Wood, H. E.; Chaney, R. W.; Clark, J.; Colbert, E. H.; Jepsen, G. L.; Reeside, J. B.; Stock, C. (1 January 1941). "Nomenclature and correlation of the North American continental Tertiary". Geological Society of America Bulletin 52 (1): 1–48. doi:10.1130/GSAB-52-1. 
  6. ^ a b Lindsay, Everett (October 1997). "Eurasian mammal biochronology: an overview". Palaeogeography, Palaeoclimatology, Palaeoecology 133 (3-4): 117–128. doi:10.1016/S0031-0182(97)00083-7. 
  7. ^ Rose, Kenneth D. (2006). The beginning of the age of mammals. Baltimore, Md.: Johns Hopkins Univ. Press. ISBN 9780801884726. 
  8. ^ a b Lucas, Spencer G. (1997). "Land-mammal ages". In Padian, Kevin; Currie, Philip J. Encyclopedia of Dinosaurs. Burlington: Elsevier. pp. 395–397. ISBN 9780080494746. 
  9. ^ Bell, Christopher J; Lundelius, Jr, Ernest L; Barnosky, Anthony D; Graham, Russell W; Lindsay, Everett H; Ruez, Dennis R; Semken Jr, Holmes A; Webb, S. David; Zakrewski, Richard J (2004). "The Blancan, Irvingtonian, and Rancholabrean Mammal Ages". In Woodburne, Michael O. Late Cretaceous and Cenozoic Mammals of North America : biostratigraphy and geochronology. New York: Columbia university press. ISBN 978-0231130400. 
  • Lindsay, Everett H; Tedford, Richard H (1989). "Development and Application of Land Mammal Ages in North America and Europe, a Comparison". In Lindsay, Everett H.; Fahlbusch, Volker; Mein, Pierre. European Neogene mammal chronology. NATO ASI Series 180. New York: Plenum Press. pp. 601–624. doi:10.1007/978-1-4899-2513-8_36. ISBN 978-1-4899-2513-8. 
  • Larry G., Marshall; Hoffster, Robert; Pascual, Rosendo (1983). "Mammals and stratigraphy: geochronology of the continental mammal-bearing Tertiary of South America". Palaeovertebrata. Retrieved 29 August 2015. 
  • Prothero, Donald R (1998). "The chronological, climatic, and paleogeographic background to North American mammalian evolution". In Janis, Christine M.; Scott, Kathleen M; Jacobs, Louise L. Evolution of Tertiary mammals of North America. Cambridge: Cambridge Univ. Pr. ISBN 9780521355193. 
  • Woodburne, Michael O., ed. (2004). Late Cretaceous and Cenozoic Mammals of North America : biostratigraphy and geochronology. New York: Columbia university press. ISBN 978-0231130400. 

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