Paleogene

The Paleogene (alternatively Palaeogene) period is an interval of about 42 million years defined on the geologic timescale as spanning roughly from 65 to 23 million years ago (mya) and being most notable as the time in which mammals became very diversified and dominant after the great Cretaceous-Tertiary extinction event killed an estimated 76% percent of the species on Earth, including most of the dinosaurs. The Paleogene period also encompassed the dramatic global warming event known as the Paleocene-Eocene Thermal Maximum.

The Paleogene period, the first of the Cenozoic era's two periods, begins at the end of both the Cretaceous period and the Mesozoic era and is followed by the Neogene period, the current and final period of the Cenozoic era. The Paleogene period comprises three epochs—the Paleocene, Eocene, and Oligocene.

The Paleogene period and its sister Neogene period are a relatively recently introduced set of geological names that have displaced the earlier set: Tertiary and Quaternary. The names "Tertiary" and "Quaternary" remain as a legacy recorded in such traditional names as the Cretaceous-Tertiary extinction.

The Paleogene climate, geography, geology, and biota came on the foundation of previous stages and was itself the foundation for modern life.

Paleogene or Tertiary?

While the two sets of names for periods within the Cenozoic era—Paleogene and Neogene or Tertiary and Quaternary—both encompass all seven of the Cenozoic era's epochs (Paleocene, Eocene, and Oligocene, Miocene, Pliocene, Pleistocene, and Holocene), the epochs are apportioned differently by the two sets. The Paleogene and Neogene periods comprise three and four epochs respectively, but the Tertiary and Quaternary periods formerly comprised five and two epochs respectively. In the now-dominant nomenclature, Tertiary is no longer a "period" but rather simply a sub-era spanning the Paleogene period plus part of the Neogene period, while the Quaternary is simply a sub-era within the Neogene period. While the use of "Tertiary" has been widespread and continues, the International Commission on Stratigraphy no longer endorses this term as part of the formal stratigraphic nomenclature. Scientists in the fields such as geology, zoology paleontology, and botany, whose published articles have long spoken of "Tertiary" and "Quaterary," have been and continue to be opposed to these changes (Hinton 2006).

Lower boundary: The Cretaceous-Tertiary extinction event

Badlands near Drumheller, Alberta where erosion has exposed the KT boundary.

The Paleogene period opens immediately after the mass extinction event at the end of the Cretaceous period, known as the Cretaceous-Tertiary extinction event or K-T extinction event. Many forms of life perished, encompassing approximately 50 percent of all plant and animal families and 76 percent of species (dos Reis et al. 2014; Barnosky et al. 2011), with the most conspicuous loss being of the non-avian dinosaurs. The last of the pterosaurs also vanished, as well as the great sea reptiles of the Cretaceous, the mosasaurs and plesiosaurs. Among mollusks, the ammonites, a diverse group of coiled cephalopods, were exterminated. Although mammals suffered, not all of them were exterminated. As much as 57 percent of the plant species in North America may have become extinct as well.

The K-T boundary that marks the separation between the Cretaceous and Paleogene periods is marked in the geological record of much of the Earth by a discontinuity in the fossil fauna, with high iridium levels. There is also fossil evidence of abrupt changes in plants and animals. There is some evidence that a substantial but very short-lived climatic change may have occurred in the very early decades of the Paleocene. There are a number of theories about the cause of the K-T extinction event, with most evidence supporting the impact of a ten km (6 mile) diameter asteroid near Yucatan, Mexico.

Paleocene-Eocene Thermal Maximum

Climate change during the last 65 million years. The Paleocene-Eocene Thermal Maximum is labeled PETM and is likely to be understated by a factor of two or more due to coarse sampling and averaging in this data set.

The end of the Paleocene epoch (55.5-54.8 mya) was marked by one of the most rapid and extreme global warming events recorded in geologic history. This event is variously labeled the Paleocene-Eocene Thermal Maximum (PETM) or the Initial Eocene Thermal Maximum (IETM). During this time, sea surface temperatures rose between 5 and 8°C (between 9 and 14°F) over a period of a few thousand years, and in the high Arctic, sea surface temperatures rose to a sub-tropical temperature of about 23°C (73°F). These changes upset oceanic and atmospheric circulation and led to the extinction of numerous deep-sea benthic foraminifera and on land, a major turnover in mammals.

Paleogene climate and paleogeology

Some continental motion took place during the Paleogene period. Climates cooled somewhat over the duration of the Paleogene period and inland seas retreated from North America early in the period.

There appears to have been a land bridge in the early Oligocene epoch (about 33 mya) between North America and Europe as the faunas of the two regions are very similar (Mayr 2001).

Paleogene biota

The Paleogene period is most notable as being the time in which mammals evolved from small, simple forms into diverse animals in the wake of the mass extinction that ended the preceding Cretaceous period. Some of these mammals would evolve into large forms that would dominate the land, while others would become capable of living in marine, specialized terrestrial, and even airborne environments. Within a geologically brief period of time in the early Eocene (about 53 mya), most of the modern mammal orders appeared.

Birds also evolved considerably during this period changing into roughly modern forms.

Most other branches of life on earth remained relatively unchanged in comparison to birds and mammals during this time period.

See also

• Geologic time scale
• Paleobotany
• Paleoclimatology
• Tertiary

Notes

References

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• Barnosky, A. D., N. Matzke, S. Tomiya, et al. 2011. Has the Earth’s sixth mass extinction already arrived? Nature 471:51-57.
• dos Reis, M., P. C. Donoghue, and Z. Yang. 2014. Neither phylogenomic nor palaeontological data support a Palaeogene origin of placental mammals. Biology Letters April 27, 2014. Retrieved May 2, 2014.
• Hooker, J. J. 2005. Tertiary to present: Paleocene. In R. C. Selley, L. R. McCocks, and I. R. Plimer, Encyclopedia of Geology, Vol. 5. Oxford: Elsevier Limited. ISBN 0-12-636380-3.
• Mayr, E. 2001. What Evolution Is. New York: Basic Books. ISBN 0-465-04425-5.