The Rocky Mountains were being uplifted, the Americas had not yet joined, the Indian Plate had begun its collision with Asia, and the North Atlantic Igneous Province was forming in the third-largest magmatic event of the last 150 million years.

In the Paleocene, with a global average temperature of about 24–25 °C (75–77 °F), compared to 14 °C (57 °F) in more recent times, the Earth had a greenhouse climate without permanent ice sheets at the poles, like the preceding Mesozoic.

As such, there were forests worldwide—including at the poles—but they had low species richness in regards to plant life, and were populated by mainly small creatures that were rapidly evolving to take advantage of the recently emptied Earth.

The word "Paleocene" was first used by French paleobotanist and geologist Wilhelm Philipp Schimper in 1874 while describing deposits near Paris (spelled "Paléocène" in his treatise).

[8][n 1] British geologist John Phillips had proposed the Cenozoic in 1840 in place of the Tertiary,[9] and Austrian paleontologist Moritz Hörnes had introduced the Paleogene for the Eocene and Neogene for the Miocene and Pliocene in 1853.

It is generally thought that a 10 to 15 km (6 to 9 mi) wide asteroid impact, forming the Chicxulub Crater in the Yucatán Peninsula in the Gulf of Mexico, and Deccan Trap volcanism caused a cataclysmic event at the boundary resulting in the extinction of 75% of all species.

[24] The Danian was first defined in 1847 by German-Swiss geologist Pierre Jean Édouard Desor based on the Danish chalks at Stevns Klint and Faxse, and was part of the Cretaceous, succeeded by the Tertiary Montian Stage.

[24] The Selandian was first proposed by Danish geologist Alfred Rosenkrantz in 1924 based on a section of fossil-rich glauconitic marls overlain by gray clay which unconformably overlies Danian chalk and limestone.

The beginning of this stage was defined by the end of carbonate rock deposition from an open ocean environment in the North Sea region (which had been going on for the previous 40 million years).

[53] Because of this and a drop in sea levels resulting from tectonic activity, the Western Interior Seaway, which had divided the continent of North America for much of the Cretaceous, had receded.

Based on this and estimated plant-gas exchange rates and global surface temperatures, the climate sensitivity was calculated to be +3 °C when CO2 levels doubled, compared to 7 °C following the formation of ice at the poles.

[100] Around 62.2 mya in the late Danian, there was a warming event and evidence of ocean acidification associated with an increase in carbon;[101] at this time, there was major seafloor spreading in the Atlantic and volcanic activity along the southeast margin of Greenland.

[28] From 59.7 to 58.1 Ma, during the late Selandian and early Thanetian, organic carbon burial resulted in a period of climatic cooling, sea level fall and transient ice growth.

[108] This was due to an ejection of 2,500–4,500 gigatons of carbon into the atmosphere, most commonly explained as the perturbation and release of methane clathrate deposits in the North Atlantic from tectonic activity and resultant increase in bottom water temperatures.

[71][129] For example, the floral diversity of what is now the Holarctic region (comprising most of the Northern Hemisphere) was mainly early members of Ginkgo, Metasequoia, Glyptostrobus, Macginitiea, Platanus, Carya, Ampelopsis, and Cercidiphyllum.

For example, what is now Castle Rock, Colorado featured a rich rainforest only 1.4 million years after the event, probably due to a rain shadow effect causing regular monsoon seasons.

[129] Conversely, low plant diversity and a lack of specialization in insects in the Colombian Cerrejón Formation, dated to 58 mya, indicates the ecosystem was still recovering from the K–Pg extinction event 7 million years later.

[120] Flowering plants (angiosperms), which had become dominant among forest taxa by the middle Cretaceous 110–90 mya,[130] continued to develop and proliferate, more so to take advantage of the recently emptied niches and an increase in rainfall.

[126] There is some evidence that, in the Gulf Coast, there was an extinction event in the late Paleocene preceding the PETM, which may have been due to the aforementioned vulnerability of complex rainforests, and the ecosystem may have been disrupted by only a small change in climate.

The Iceberg Bay Formation on Ellesmere Island, Nunavut (latitude 75–80° N) shows remains of a late Paleocene dawn redwood forest, the canopy reaching around 32 m (105 ft), and a climate similar to the Pacific Northwest.

One possibility is that the interior of the continent favored deciduous trees, though prevailing continental climates may have produced winters warm enough to support evergreen forests.

However, the fossil record of birds in the Paleocene is rather poor compared to other groups, limited globally to mainly waterbirds such as the early penguin Waimanu.

[180] The final record of albanerpetontids from North America and outside of Europe and Anatolia, an unnamed species of Albanerpeton, is known from the Paleocene aged Paskapoo Formation in Canada.

[183] Almost immediately following the K–Pg extinction event, ray-finned fish — today, representing nearly half of all vertebrate taxa – became much more numerous and increased in size, and rose to dominate the open-oceans.

Acanthomorphs—a group of ray-finned fish which, today, represent a third of all vertebrate life—experienced a massive diversification following the K–Pg extinction event, dominating marine ecosystems by the end of the Paleocene, refilling vacant, open-ocean predatory niches as well as spreading out into recovering reef systems.

[193] Several Paleocene freshwater fish are recorded from North America, including bowfins, gars, arowanas, Gonorynchidae, common catfish, smelts, and pike.

[197] There is a gap in the ant fossil record from 78 to 55 mya, except for the aneuretine Napakimyrma paskapooensis from the 62–56 million year old Canadian Paskapoo Formation.

[198] Given high abundance in the Eocene, two of the modern dominant ant subfamilies—Ponerinae and Myrmicinae—likely originated and greatly diversified in the Paleocene, acting as major hunters of arthropods, and probably competed with each other for food and nesting grounds in the dense angiosperm leaf litter.

Myrmicines expanded their diets to seeds and formed trophobiotic symbiotic relationships with aphids, mealybugs, treehoppers, and other honeydew secreting insects which were also successful in angiosperm forests, allowing them to invade other biomes, such as the canopy or temperate environments, and achieve a worldwide distribution by the middle Eocene.

[182] Marine invertebrate diversity may have taken about 7 million years to recover, though this may be a preservation artifact as anything smaller than 5 mm (0.20 in) is unlikely to be fossilized, and body size may have simply decreased across the boundary.