Paleocene

The Paleocene IPA: /ˈpæli.əsiːn, -i.oʊ-, ˈpeɪli-/ PAL-ee-ə-seen, -⁠ee-oh-, PAY-lee-), or Palaeocene, is a geological epoch that lasted from about 66 to 56 million years ago. It is the first epoch of the Paleogene Period in the modern Cenozoic Era. The name comes from Ancient Greek words meaning "old" and "new", translating to "the old part of the Eocene".

This time period began after the K–Pg extinction event, caused by an asteroid impact (Chicxulub impact) and possibly volcanism (Deccan Traps), which killed off many species, including the non-avian dinosaurs. The Paleocene was a time of recovery and change on Earth. The climate was warm, with no ice sheets at the poles, and forests grew even at high latitudes.

During the Paleocene, the continents were still shifting. The Rocky Mountains were beginning to form, and the Indian Plate was moving toward Asia. In the oceans, life was also changing. Mammals began to diversify, and the first placental and marsupial mammals appeared. The end of the Paleocene was marked by a major climate event called the Paleocene–Eocene Thermal Maximum (PETM), which released large amounts of carbon into the atmosphere and caused a spike in global temperatures.

Etymology

The word "Paleocene" was first used in 1874 by French scientist Wilhelm Philipp Schimper. It comes from Ancient Greek words meaning "old" and "new", specifically referring to "the old part of the Eocene". The Eocene epoch marks the dawn of modern life, and the Paleocene comes right before it.

Over time, scientists have standardized how we divide Earth's history. In 1978, the Paleocene was officially defined as the first epoch of the Paleogene Period, along with the Eocene and Oligocene. Today, we use the spelling "Paleocene" in North America and mainland Europe, while "Palaeocene" is used in the UK.

Geology

The Paleocene Epoch was the time period right after the K–Pg extinction event, which marked the end of the Cretaceous Period and the Mesozoic Era. It lasted for about 10 million years, from 66 to 56 million years ago, and was the first part of the Cenozoic Era and the Paleogene Period. During this time, the Earth went through many changes, including the formation of important coal and natural gas deposits.

Geologists divide the Paleocene into three main ages: the Danian, Selandian, and Thanetian. These ages help scientists understand and study the rocks and fossils from this time. The end of the Paleocene was marked by a big warming event called the Paleocene–Eocene thermal maximum, which caused changes in the oceans and affected many sea creatures.

Paleogeography

During the Paleocene, the continents continued to move toward their current positions. In the Northern Hemisphere, land bridges connected North America and Eurasia. These included Beringia between North America and East Asia, and routes through Greenland and Scandinavia.

The Rocky Mountains in North America were still rising due to tectonic activity. Volcanism in the North Atlantic created a large igneous province, which may have helped open the North Atlantic Ocean and affected the climate. In the Southern Hemisphere, the continents of Gondwanaland continued to separate, with Antarctica still connected to South America and Australia. Africa moved toward Europe, and the Indian subcontinent headed toward Asia.

Climate

The Paleocene climate was warm and tropical, much like the time before it. The poles were temperate, and there were no ice caps. The average global temperature was around 24–25 °C (75–77 °F), much warmer than today’s average of 14 °C (57 °F).

Different parts of the world had varied climates. Some areas were tropical, others arid, and some had humid, monsoonal weather. The oceans were also warm, with deep water temperatures likely between 8–12 °C (46–54 °F). There were changes in climate during this time, including warming events and periods of cooling, influenced by factors like volcanic activity and changes in carbon levels.

Flora

The warm, wet climate of the Paleocene supported tropical and subtropical forests filled with conifers and broad-leafed trees. In places like Patagonia, there were tropical rainforests, cloud rainforests, mangrove forests, swamp forests, savannas, and sclerophyllous forests. Fossils from Colombia and North Dakota show that many modern plant families, like palm trees, legumes, and aroids, existed then, just as they do today.

With large herbivorous dinosaurs gone, forests grew denser. Plants developed new ways to survive, such as growing taller to reach sunlight or producing larger seeds to give seedlings extra food. Wildfires became less common because there was less oxygen in the air than before. After the mass extinction event that ended the age of dinosaurs, many plant species died out, but ferns were often the first to grow back, helping to restore the forests. Over time, flowering plants became more common, evolving alongside the insects that pollinated them and the animals that ate their fruits.

Fauna

In the K–Pg extinction event, every land animal over 25 kg was wiped out, leaving open several niches at the beginning of the epoch.

Mammals

Mammals had first appeared in the Late Triassic, and remained small and nocturnal throughout the Mesozoic to avoid competition with dinosaurs, though, by the Middle Jurassic, they had branched out into several habitats. In general, Paleocene mammals retained this small size until near the end of the epoch. Following the K–Pg extinction event, mammals very quickly diversified and filled the empty niches. Modern mammals are subdivided into therians (modern members are placentals and marsupials) and monotremes. These three groups all originated in the Cretaceous. Paleocene marsupials include Peradectes, and monotremes Monotrematum. The epoch featured the rise of many crown placental groups—such as the earliest afrotherian Ocepeia, xenarthran Utaetus, rodent Tribosphenomys and Paramys, the forerunners of primates the Plesiadapiformes, earliest carnivorans Ravenictis and Pristinictis, possible pangolins Palaeanodonta, possible forerunners of odd-toed ungulates Phenacodontidae, and eulipotyphlans Nyctitheriidae. Though therian mammals had probably already begun to diversify around 10 to 20 million years before the K–Pg extinction event, average mammal size increased greatly after the boundary, and a radiation into frugivory and omnivory began.

Birds

According to DNA studies, modern birds rapidly diversified following the extinction of the other dinosaurs in the Paleocene, and nearly all modern bird lineages can trace their origins to this epoch with the exception of fowl and palaeognaths. This was one of the fastest diversifications of any group, probably fueled by the diversification of fruit-bearing trees and associated insects, and the modern bird groups had likely already diverged within four million years of the K–Pg extinction event. 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. The earliest arboreal crown group bird known is Tsidiiyazhi, a mousebird dating to around 62 Ma. The fossil record also includes early owls such as the large Berruornis from France, and the smaller Ogygoptynx from the United States.

Reptiles

It is generally believed all non-avian dinosaurs went extinct at the K–Pg extinction event 66 Ma. In the wake of the K–Pg extinction event, 83% of lizard and snake (squamate) species went extinct, and the diversity did not fully recover until the end of the Paleocene. However, since the only major squamate lineages to disappear in the event were the mosasaurs and polyglyphanodontians, and most major squamate groups had evolved by the Cretaceous, the event probably did not greatly affect squamate evolution. Nonetheless, there was a faunal turnover of squamates, and groups that were dominant by the Eocene were not as abundant in the Cretaceous. Only small squamates are known from the early Paleocene—the largest snake Helagras was 95 cm in length—but the late Paleocene snake Titanoboa grew to over 13 m long, the longest snake ever recorded. Kawasphenodon peligrensis from the early Paleocene of South America represents the youngest record of Rhynchocephalia outside of New Zealand.

Freshwater crocodilians and choristoderans were among the aquatic reptiles to have survived the K–Pg extinction event, probably because freshwater environments were not as impacted as marine ones. One example of a Paleocene crocodile is Borealosuchus, which averaged 3.7 m in length. Turtles experienced a decline in the Campanian during a cooling event, and recovered during the PETM at the end of the Paleocene. Turtles were not greatly affected by the K–Pg extinction event, and around 80% of species survived.

Amphibians

There is little evidence amphibians were affected very much by the K–Pg extinction event, probably because the freshwater habitats they inhabited were not as greatly impacted as marine environments. The true toads evolved during the Paleocene.

Fish

The small pelagic fish population recovered rather quickly, and there was a low extinction rate for sharks and rays. Overall, only 12% of fish species went extinct. During the Cretaceous, fishes were not very abundant, probably due to heightened predation by or competition with ammonites and squid, although large predatory fish did exist. Almost immediately following the K–Pg extinction event, ray-finned fish 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. In specific, percomorphs diversified faster than any other vertebrate group at the time.

Insects and arachnids

Insect recovery varied from place to place. The middle-to-late Paleocene French Menat Formation shows an abundance of beetles (making up 77.5% of the insect diversity)—especially weevils, jewel beetles, leaf beetles, and reticulated beetles—as well as other true bugs—such as pond skaters—and cockroaches. To a lesser degree, there are also orthopterans, hymenopterans, butterflies, and flies.

Marine invertebrates

Among marine invertebrates, plankton and those with a planktonic stage in their development were most impacted by the K–Pg extinction event, and plankton populations crashed. Nonetheless, the rapid diversification of large fish species indicates a healthy plankton population through the Paleocene. Marine invertebrate diversity may have taken about 7 million years to recover. Sand dollars first evolved in the late Paleocene.