6.1 Paleogene period

The Paleogene period marks the start of the Cenozoic era, following the mass extinction that wiped out non-avian dinosaurs. Spanning 66 to 23 million years ago, it saw rapid evolution of mammals and major climate shifts.

This period is divided into three epochs: Paleocene, Eocene, and Oligocene. Each epoch had distinct animal and plant life, reflecting changing environments as Earth transitioned from the warm Paleocene to the cooler Oligocene.

Paleogene period overview

• The Paleogene period is the first part of the Cenozoic era, following the mass extinction event at the end of the Cretaceous period that wiped out the non-avian dinosaurs
• It spans from approximately 66 million years ago to 23 million years ago and is divided into three epochs: Paleocene, Eocene, and Oligocene
• The Paleogene is characterized by the rapid evolution and diversification of mammals, as well as significant changes in climate and geography

Transition from Mesozoic

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• The Paleogene period begins immediately after the Cretaceous-Paleogene (K-Pg) extinction event, which marked the end of the Mesozoic era
• The K-Pg extinction resulted in the disappearance of non-avian dinosaurs, marine reptiles, and many other groups, leaving ecological niches open for new species to fill
• Surviving lineages, such as mammals, birds, and certain reptiles, underwent rapid adaptive radiation and diversification in the early Paleogene

Duration of Paleogene

• The Paleogene period lasted approximately 43 million years, from 66 million years ago to 23 million years ago
• It is subdivided into three epochs:
  • Paleocene (66-56 million years ago)
  • Eocene (56-33.9 million years ago)
  • Oligocene (33.9-23 million years ago)
• Each epoch is characterized by distinct faunal and floral assemblages, as well as climatic and geographic conditions

Paleocene epoch

• The Paleocene is the oldest epoch of the Paleogene period, spanning from 66 to 56 million years ago
• It represents the initial recovery and diversification of life following the K-Pg extinction event
• The Paleocene is characterized by the rise of mammals and the decline of archaic groups that survived the extinction

Mammalian radiation

• Mammals underwent a rapid adaptive radiation during the Paleocene, filling ecological niches left vacant by the extinction of dinosaurs
• Paleocene mammals were generally small, but they diversified into various forms, including insectivores, herbivores, and carnivores
• Examples of Paleocene mammals include early primates (Plesiadapiformes), primitive ungulates (Condylarthra), and early carnivores (Creodonta)

Archaic ungulates

• Archaic ungulates, such as condylarths and uintatheres, were among the most diverse and abundant mammals of the Paleocene
• Condylarths were small to medium-sized, generalized herbivores that gave rise to later hoofed mammals, such as horses and rhinos
• Uintatheres were large, herbivorous mammals with unique skull morphologies, including multiple horn-like protrusions

Multituberculates decline

• Multituberculates, a group of small, rodent-like mammals that thrived during the Mesozoic, began to decline in the Paleocene
• The decline of multituberculates is thought to be due to competition with the rapidly diversifying placental mammals
• By the end of the Paleocene, multituberculates were largely extinct, with only a few lineages persisting into the Eocene

Eocene epoch

• The Eocene epoch spans from 56 to 33.9 million years ago and is known for its warm global climate and the continued diversification of mammals
• It is subdivided into three stages: Early Eocene (56-47.8 Ma), Middle Eocene (47.8-38 Ma), and Late Eocene (38-33.9 Ma)
• The Eocene is characterized by the appearance of modern mammal groups and the development of complex ecosystems

Climatic optimum

• The Early Eocene Climatic Optimum (EECO) was a period of exceptionally warm global temperatures, with little to no polar ice
• Tropical and subtropical forests extended into high latitudes, and warm oceans allowed for the dispersal of species between continents
• The EECO promoted high biodiversity and the evolution of specialized adaptations in plants and animals

Diversification of mammals

• Mammals continued to diversify and evolve throughout the Eocene, giving rise to many modern groups
• Perissodactyls (odd-toed ungulates) and artiodactyls (even-toed ungulates) first appeared in the Early Eocene and underwent rapid diversification
• Other notable Eocene mammals include early proboscideans (elephants), sirenians (sea cows), and bats

Primitive primates

• The Eocene saw the diversification of early primates, including the first anthropoids (monkeys, apes, and humans)
• Primitive primates, such as adapiforms and omomyids, were abundant in Eocene forests, occupying various ecological niches
• The earliest known simian primates, such as Eosimias and Bahinia, appeared in the Middle Eocene of Asia and Africa

Perissodactyls vs Artiodactyls

• Perissodactyls and artiodactyls are two major groups of hoofed mammals that first appeared in the Early Eocene
• Perissodactyls, which include horses, rhinos, and tapirs, have an odd number of toes on their hind feet and are generally larger than artiodactyls
• Artiodactyls, which include cattle, deer, and pigs, have an even number of toes on their hind feet and are more diverse than perissodactyls

Appearance of whales

• The earliest whales, known as archaeocetes, first appeared in the Early Eocene, evolving from land-dwelling, hoofed ancestors
• Primitive whales, such as Pakicetus and Ambulocetus, were semi-aquatic and retained limb morphologies adapted for walking on land
• Throughout the Eocene, whales underwent a series of adaptations for fully aquatic life, including the loss of hind limbs and the development of a streamlined body shape

Oligocene epoch

• The Oligocene epoch spans from 33.9 to 23 million years ago and is characterized by a global cooling trend and the expansion of grasslands
• It is subdivided into two stages: Early Oligocene (33.9-28.1 Ma) and Late Oligocene (28.1-23 Ma)
• The Oligocene marked a significant faunal turnover, with the decline of many archaic mammal groups and the rise of more modern forms

Cooling climate

• The Oligocene climate was cooler and drier compared to the preceding Eocene, with the growth of the Antarctic ice sheet and a drop in global sea levels
• The cooling trend led to the decline of tropical and subtropical forests and the expansion of open habitats, such as grasslands and savannas
• Many plant and animal species adapted to these new environments, developing specialized feeding and locomotor strategies

Expansion of grasslands

• Grasslands began to expand during the Oligocene, particularly in the mid-latitudes of North America and Eurasia
• The spread of grasses was facilitated by the cooling and drying climate, as well as the co-evolution of grazing mammals
• Grassland expansion had a significant impact on mammalian evolution, leading to the diversification of grazing specialists, such as horses and ruminants

Rise of ruminants

• Ruminants, a group of artiodactyls that includes cattle, deer, and giraffes, underwent a major diversification during the Oligocene
• The evolution of complex stomachs and efficient digestive systems allowed ruminants to thrive on the tough, fibrous vegetation of grasslands
• Early ruminants, such as Leptomeryx and Archaeomeryx, gave rise to the diverse array of modern ruminant families

Decline of primitive mammals

• Many archaic mammal groups that had thrived during the Eocene, such as creodonts and condylarths, declined or went extinct during the Oligocene
• The changing climate and the rise of more specialized mammal groups likely contributed to the demise of these primitive forms
• Some lineages, such as the multituberculates and plesiadapiform primates, had already gone extinct by the beginning of the Oligocene

Paleogene flora

• The Paleogene flora was characterized by the dominance of angiosperms (flowering plants) and the diversification of modern plant groups
• Floral assemblages varied depending on latitude and climate, with tropical forests in the equatorial regions and temperate forests at higher latitudes
• Paleogene plant fossils provide valuable insights into the evolution and ecology of terrestrial ecosystems

Angiosperm dominance

• Angiosperms, which had first appeared in the Cretaceous, became the dominant plant group during the Paleogene
• The rapid diversification of angiosperms led to the evolution of a wide variety of growth forms, including trees, shrubs, and herbaceous plants
• Angiosperm-dominated forests were prevalent throughout the Paleogene, with diverse assemblages of families such as Fagaceae (oaks and beeches), Juglandaceae (walnuts), and Leguminosae (legumes)

Tropical vs temperate forests

• Paleogene forests can be broadly categorized into tropical and temperate types, depending on their latitudinal distribution and climatic conditions
• Tropical forests, found in equatorial regions, were characterized by high diversity, a closed canopy, and the presence of species adapted to warm, humid conditions (Dipterocarpaceae, Moraceae)
• Temperate forests, found at higher latitudes, had lower diversity and were dominated by deciduous and coniferous trees adapted to seasonal changes in temperature and precipitation (Pinaceae, Betulaceae)

Paleogene plant fossils

• Paleogene plant fossils are preserved in various forms, including compressions, impressions, and permineralizations
• Leaf fossils are particularly abundant and informative, as they can be used to reconstruct past climates and environments through the analysis of their morphology and stomatal density
• Other important Paleogene plant fossils include wood, seeds, fruits, and pollen, which provide insights into the reproductive biology and dispersal strategies of ancient plants

Paleogene fauna

• The Paleogene fauna was dominated by mammals, which underwent a rapid diversification following the K-Pg extinction
• Birds and reptiles also played important roles in Paleogene ecosystems, with the evolution of modern groups and the persistence of some archaic forms
• Insect diversity was high during the Paleogene, with many groups adapting to the new angiosperm-dominated landscapes

Mammals vs reptiles

• Mammals were the most diverse and abundant terrestrial vertebrates during the Paleogene, having radiated into a wide variety of niches following the extinction of the dinosaurs
• Reptiles, including turtles, crocodilians, and squamates (lizards and snakes), were also present but played a less dominant role compared to mammals
• Some archaic reptile groups, such as champsosaurs and choristoderes, persisted into the early Paleogene before going extinct

Avian evolution

• Birds underwent significant diversification during the Paleogene, with the evolution of many modern orders and the appearance of specialized forms
• The Eocene saw the evolution of large, flightless birds, such as Gastornis and Diatryma, which likely filled predatory niches in the absence of large mammalian carnivores
• Other notable Paleogene birds include early representatives of Passeriformes (perching birds), Psittaciformes (parrots), and Piciformes (woodpeckers and allies)

Insect diversity

• Insect diversity was high during the Paleogene, with many groups adapting to the new plant communities dominated by angiosperms
• The diversification of pollinating insects, such as bees, butterflies, and moths, was closely tied to the evolution of flowering plants
• Other important Paleogene insect groups include ants, termites, and beetles, which played key roles in nutrient cycling and plant-insect interactions

Paleogene extinctions

• The Paleogene was marked by several extinction events, although none were as severe as the K-Pg extinction that preceded it
• The most significant Paleogene extinction event was the Grande Coupure at the Eocene-Oligocene boundary, which resulted in a major faunal turnover
• Other minor extinctions occurred throughout the Paleogene, often associated with climatic changes or competition between evolving groups

Grande Coupure event

• The Grande Coupure ("Great Break") was a significant extinction event that occurred around 33.9 million years ago, coinciding with the Eocene-Oligocene boundary
• It was characterized by the extinction of many European mammal species and the immigration of Asian forms, resulting in a major faunal turnover
• The Grande Coupure affected various mammal groups, including primates, perissodactyls, and rodents, and led to the disappearance of many endemic European lineages

Causes of extinctions

• The causes of Paleogene extinctions were likely multifaceted and varied depending on the specific event and groups affected
• Climate change, particularly cooling trends and the onset of glaciation, was a major driver of extinctions, as many species were adapted to the warmer conditions of the early Paleogene
• Competition between evolving groups, such as the replacement of archaic mammals by more derived forms, also contributed to extinctions throughout the Paleogene

Paleogene stratigraphy

• Paleogene stratigraphy involves the study of rock layers and their associated fossils to reconstruct the geological and biological history of the period
• Key Paleogene formations and fossil sites are distributed globally, providing insights into the diversity and evolution of life during this time
• Paleogene index fossils, such as certain mammal and mollusk species, are used to correlate and date strata from different regions

Key Paleogene formations

• The Willwood Formation in Wyoming, USA, is a rich source of Early Eocene mammal fossils, including primates, perissodactyls, and condylarths
• The Messel Pit in Germany is a UNESCO World Heritage site that preserves exceptionally detailed Middle Eocene fossils, including birds, bats, and insects
• The White River Formation in the central United States spans the Eocene-Oligocene boundary and contains abundant mammal fossils, such as oreodonts, entelodonts, and early horses

Paleogene index fossils

• Perissodactyl genera, such as Hyracotherium (early horse) and Brontops (brontothere), are useful index fossils for the Eocene of North America and Europe
• Planktonic foraminifera, such as Hantkenina and Turborotalia, are important index fossils for the Eocene in marine sediments worldwide
• The mollusk genus Turritella is a common index fossil for the Oligocene of Europe and North America, with different species used to define regional stages

Paleogene paleogeography

• Paleogene paleogeography was characterized by the gradual breakup of the supercontinent Gondwana and the opening of the Atlantic and Indian Oceans
• The configuration of continents and oceans had a significant impact on climate, ocean circulation, and the distribution of flora and fauna
• Paleogeographic reconstructions are based on various lines of evidence, including plate tectonics, paleomagnetic data, and the distribution of fossil organisms

Continental configurations

• During the early Paleogene, the continents were still relatively close together, with North America and Europe connected by land bridges
• The separation of South America and Australia from Antarctica led to the development of the Antarctic Circumpolar Current, which contributed to the cooling trend of the late Paleogene
• The collision of the Indian subcontinent with Asia in the Early Eocene resulted in the formation of the Himalayas and the Tibetan Plateau, affecting regional and global climate patterns

Oceanic circulation patterns

• Paleogene ocean circulation was influenced by the opening and closing of seaways, as well as changes in global temperature gradients
• The early Paleogene was characterized by warm, sluggish oceans with weak thermal gradients, allowing for the widespread distribution of tropical and subtropical species
• The cooling trend of the late Paleogene led to the strengthening of ocean currents and the development of more pronounced latitudinal temperature gradients, contributing to the differentiation of marine faunal provinces

Transition to Neogene

• The transition from the Paleogene to the Neogene period, which began 23 million years ago, was marked by significant changes in climate, fauna, and flora
• The Oligocene-Miocene boundary, at 23 Ma, is defined by the appearance of certain mammal and microfossil species in the stratigraphic record
• The Neogene saw the continued evolution and diversification of modern plant and animal groups, as well as the development of new ecosystems in response to changing environmental conditions

Faunal turnover

• The Paleogene-Neogene transition witnessed a significant faunal turnover, with the extinction of many archaic mammal groups and the rise of more modern forms
• In Europe and Asia, the "Proboscidean Datum Event" marked the dispersal of African proboscideans (elephants and their relatives) into the northern continents, coinciding with the extinction of many endemic mammal species
• In North America, the "Miocene Mam