Paleocene

The Paleocene is a geologic epoch that lasted from 65.5 ± 0.3 million years ago (mya) to 55.8 ± 0.2 mya. It is the first epoch of the Paleogene perid in the modern Cenozoic era. As with most other older periods of the geologic time scale, the strata that define the epoch's beginning and end are well identified, but the exact dates are uncertain.

The Paleocene epoch immediately followed the mass extinction event at the end of the Cretaceous, 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, the most conspicuous being the non-avian dinosaurs. Some recognize this mass extinction now as occurring at the Cretaceous-Paleogene boundary rather than the K-T boundary, because of the (Hinton 2006).

The die-off of the dinosaurs left unfilled ecological niches worldwide, and the name "Paleocene" comes from Greek and refers to the "old(er) (paleo) – new (ceno)" fauna that arose during the epoch, prior to the emergence of modern mammalian orders in the Eocene.

Paleogene period
Paleocene epoch	Eocene epoch	Oligocene epoch
Danian | Selandian Thanetian	Ypresian | Lutetian Bartonian | Priabonian	Rupelian | Chattian

Paleocene boundaries and subdivisions

The K-T boundary that marks the separation between Cretaceous and Paleocene is visible 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 flora and fauna. 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 10 km diameter asteroid near Yucatan, Mexico.

The end of the Paleocene (55.5/54.8 Ma) was marked by one of the most significant periods of global change during the Cenozoic, a sudden global change, the Paleocene-Eocene Thermal Maximum, which upset oceanic and atmospheric circulation and led to the extinction of numerous deep-sea benthic foraminifera and on land, a major turnover in mammals.

The Paleocene is usually broken into early, middle, and late sub-epochs which correspond to the following faunal stages, from youngest to oldest:

Thanetian	(58.7 ± 0.2 – 55.8 ± 0.2 Ma)
Selandian	(61.7 ± 0.2 – 58.7 ± 0.2 Ma)
Danian	(65.5 ± 0.3 – 61.7 ± 0.2 Ma)

Paleocene climate

The early Paleocene was slightly cooler than the preceding Cretaceous, though temperatures rose again late in the epoch. The climate was warm and humid world-wide, with subtropical vegetation growing in Greenland and Patagonia. The poles were cool and temperate, North America, Europe, Australia and southern South America were warm and temperate; tropical climates characterized equatorial areas, and North and South of the Equator climates were hot and arid.

Paleocene paleogeography

In many ways, the Paleocene continued processes that had begun during the late Cretaceous Period. During the Paleocene, the continents continued to drift toward their present positions. North America and Asia were still intermittently joined by a land bridge, while Greenland and North American were beginning to separate. The Laramide orogeny of the late Cretaceous continued to uplift the Rocky Mountains in the American west, only ending in the succeeding epoch.

South and North America remained separated by equatorial seas, only joining during the Neogene; the components of the former southern supercontinent Gondwanaland continued to split apart, with Africa, South America, Antarctica and Australia pulling away from each other. Africa was heading north towards Europe, slowly closing the Tethys Ocean, and India began its migration to Asia that would lead to the huge tectonic collision and formation of the Himalayas.

The inland seas in North America (Western Interior Seaway) and Europe had receded by the beginning of the Paleocene, making way for new land-based flora and fauna.

Paleocene flora

Terrestrial Paleocene strata immediately overlying the K-T boundary is in places marked by a "fern spike": a bed especially rich in fern fossils. Ferns are often the first species to colonize areas damaged by forest fires; thus the fern spike may indicate post-Chicxulub devastation.

In general, the Paleocene is marked by the development of modern plant species. Cacti and palm trees appeared. Paleocene and later plant fossils are generally attributed to modern genera or to closely related taxa.

The warm temperatures world-wide gave rise to thick tropical, sub-tropical and deciduous forest cover around the globe (the first recognizably modern rain forests) with ice-free polar regions covered with coniferous and deciduous trees. With no large grazing dinosaurs to thin them, Paleocene forests were probably denser than those of the Cretaceous.

Flowering plants (angiosperms), first seen in the Cretaceous, continued to develop and proliferate, and along with them coevolved the insects that fed on these plants and pollinated them.

Paleocene fauna

Mammals

Mammals had first appeared in the Triassic, and developed alongside the dinosaurs, exploiting ecological niches untouched by the larger and more famous Mesozoic animals: in the insect-rich forest underbrush, and high up in the trees. These smaller mammals (as well as birds, reptiles, amphibians, and insects) survived the mass extinction at the end of the Cretaceous, which wiped out the dinosaurs, and mammals diversified and spread throughout the world.

While early mammals were small nocturnal animals with herbivorous and insectivorous diets, the demise of the dinosaurs and the beginning of the Paleocene saw mammals growing bigger, more ferocious, and finally becoming the dominant predators and spreading throughout the world. Ten million years after the death of the dinosaurs, the world was filled with rodent-like mammals, medium sized mammals scavenging in forests, and large herbivorous and carnivorous mammals hunting other mammals, birds, and reptiles.

Paleocene mammals did not yet have specialized teeth or limbs, and their brain to body mass ratios were quite low; compared to later forms, they are considered primitive, or archaic. It was not until the Eocene, 55 Ma, that true modern mammals developed.

Fossil evidence from the Paleocene is scarce, and there is relatively little known about mammals of the time. Because of their small size—a constant until late in the epoch—early mammal bones are not well-preserved in the fossil record, and most of what we know comes from fossil teeth (a much tougher substance), and only a few skeletons.

Mammals of the Paleocene include:

• Monotremes: three species of monotremes have survived to modern times: the duck-billed platypus, and two species of Echidnas. Monotrematum sudamericanum lived during the Paleocene.
• Marsupials: modern kangaroos are marsupials, characterized by giving birth to embryonic babies, who crawl into the mother's pouch and suckle until they are developed. The Bolivian Pucadelphys andinus is a Paleocene example.
• Multituberculates: the only major branch of mammals to go extinct, this rodent-like grouping includes the Paleocene Ptilodus.
• Placentals: this grouping of mammals became the most diverse and the most successful. Members include hoofed ungulates, primates and carnivores, such as the Paleocene mesonychid.

Reptiles

Due to the climatic conditions of the Paleocene, reptiles were more widely distributed over the globe than at present. Among the sub-tropical reptiles found in North America during this epoch are champsosaurs (aquatic reptiles that resemble modern gharials), crocodilians, soft-shelled turtles, palaeophid snakes, varanid lizards, and Protochelydra zangerli (similar to modern snapping turtles).

Examples of champsosaurs of the Paleocene include Champsosaurus gigas, the largest champsosaur ever discovered. This creature was unusual among Paleocene reptiles in that C. gigas became larger than its known Mesozoic ancestors: C. gigas is more than twice the length of the largest Cretaceous specimens (3 meters versus 1.5 meters). Reptiles as a whole decreased in size after the K-T event. Champsosaurs declined towards the end of the Paleocene and became extinct at the end of the Eocene.

Examples of Paleocene crocodylians are the euschian crocodylid Leidyosuchus formidabilis, the apex predator and the largest animal of the Wannagan Creek fauna, and the alligatorid Wannaganosuchus.

Dinosaurs may have survived to some extent into the early Danian stage of the Paleocene Epoch circa 64.5 Mya. The controversial evidence for such is a hadrosaur leg bone found from Paleocene strata from 64.5 Mya in Australia.

Birds

Birds began to diversify during the epoch, occupying new niches. Most modern bird types had appeared by mid-Cenozoic, including perching birds, cranes, hawks, pelicans, herons, owls, ducks, pigeons, loons, and woodpeckers.

Large carnivorous flightless birds (also called Terror Birds) have been found in late Paleocene fossils, including the fearsome Gastornis in Europe.

Early owl types such as Ogygoptynx and Berruornis appear in the late Paleocene in the U.S. and France respectively.

Paleocene oceans

Warm seas circulated throughout the world, including the poles. The earliest Paleocene featured a low diversity and abundance of marine life, but this trend reversed later in the epoch. Tropical conditions gave rise to abundant marine life, including coral reefs. With the demise of marine reptiles at the end of the Cretaceous, sharks became the top predators. The end of the Cretaceous also saw extinctions of the ammonites, and many species of foraminifera.

Marine faunas also came to resemble modern faunas, with only the marine mammals and the Charcharinid sharks missing.

References

ISBN links support NWE through referral fees

• Ogg, Jim; June, 2004, Overview of Global Boundary Stratotype Sections and Points (GSSP's) http://www.stratigraphy.org/gssp.htm Accessed April 30, 2006.

Further reading

• Paleocene Mammals, an excellent site with lots of info
• BBC Changing Worlds: Paleocene, general information
• Maryland Paleocene Fossils
• Paleos: Paleocene
• Paleocene Evolutionary Radiation
• PaleoMap Project
• John Alroy, "Evidence of a Paleocene Evolutionary Radiation"

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