🦕Paleontology Unit 6 – Cenozoic Era

Key Events and Timeline

• Cenozoic Era began ~66 million years ago (mya) following the Cretaceous-Paleogene (K-Pg) extinction event
• Divided into three periods: Paleogene (66-23 mya), Neogene (23-2.6 mya), and Quaternary (2.6 mya to present)
  • Paleogene includes Paleocene, Eocene, and Oligocene epochs
  • Neogene includes Miocene and Pliocene epochs
  • Quaternary includes Pleistocene and Holocene epochs
• Paleocene epoch (66-56 mya) marked by recovery and diversification of mammals and birds
• Eocene epoch (56-33.9 mya) characterized by warm global temperatures and the emergence of modern mammal orders
• Oligocene epoch (33.9-23 mya) saw cooling global temperatures and the expansion of grasslands
• Miocene epoch (23-5.3 mya) featured the evolution of grazing mammals and the spread of hominids
• Pliocene epoch (5.3-2.6 mya) marked by further global cooling and the evolution of early human ancestors
• Pleistocene epoch (2.6 mya-11,700 years ago) characterized by repeated glacial-interglacial cycles and the evolution of modern humans
• Holocene epoch (11,700 years ago to present) began with the end of the last glacial period and continues to the present day

Climate and Environmental Changes

• Cenozoic Era began with a warm, greenhouse climate following the K-Pg extinction event
• Early Eocene Climatic Optimum (EECO) ~52-50 mya marked the warmest period of the Cenozoic
  • Global temperatures were 12-14°C higher than present
  • Reduced temperature gradient between the equator and poles
• Mid-Eocene saw a gradual cooling trend, with the development of the Antarctic Circumpolar Current
• Eocene-Oligocene boundary ~34 mya marked by a rapid drop in global temperatures and the onset of Antarctic glaciation
  • Caused by a combination of factors, including the opening of the Drake Passage and the drop in atmospheric CO2 levels
• Miocene climate was characterized by a general cooling trend and the expansion of grasslands and savannas
• Mid-Pliocene Warm Period ~3.3-3 mya saw global temperatures 2-3°C higher than present
• Quaternary Period marked by repeated glacial-interglacial cycles driven by variations in Earth's orbit (Milankovitch cycles)
  • Ice sheets expanded and contracted, causing sea level fluctuations of up to 120 meters
  • Interglacial periods, such as the current Holocene, characterized by warmer temperatures and higher sea levels

Major Fauna and Flora

• Paleocene epoch saw the recovery and diversification of mammals, including the appearance of early primates (Plesiadapiformes)
• Eocene epoch marked the emergence of modern mammal orders, such as Artiodactyla (even-toed ungulates), Perissodactyla (odd-toed ungulates), and Primates
  • Early horses (Hyracotherium) and whales (Pakicetus) appeared during the Eocene
• Oligocene epoch featured the diversification of grassland-adapted mammals, such as early camels (Poebrotherium) and horses (Mesohippus)
• Miocene epoch saw the evolution of grazing mammals, including the first true horses (Merychippus), and the spread of hominids (Proconsul)
• Pliocene epoch marked the evolution of early human ancestors, such as Australopithecus afarensis, and the appearance of modern elephant genera (Loxodonta and Elephas)
• Pleistocene megafauna included large mammals such as woolly mammoths (Mammuthus primigenius), saber-toothed cats (Smilodon), and giant ground sloths (Megatherium)
  • Many of these species went extinct during the Late Pleistocene, likely due to a combination of climate change and human hunting
• Cenozoic flora was characterized by the expansion of angiosperms (flowering plants) and the diversification of grasses
  • Grasslands and savannas became widespread during the Miocene, supporting the evolution of grazing mammals

Evolutionary Trends

• Cenozoic Era saw the rise of mammals as the dominant terrestrial vertebrates following the K-Pg extinction event
• Mammalian evolution was characterized by a trend towards increased body size (Cope's Rule) and specialization for particular niches
  • Example: The evolution of horses from small, multi-toed browsers (Hyracotherium) to large, single-toed grazers (Equus)
• Primates evolved and diversified throughout the Cenozoic, with key adaptations such as opposable thumbs, stereoscopic vision, and enlarged brains
  • Hominins, the group that includes modern humans and our extinct ancestors, diverged from other apes during the late Miocene (~6-7 mya)
• Birds diversified and adapted to various niches, with the evolution of features such as long beaks in hummingbirds and the loss of flight in ratites (ostriches, emus)
• Marine mammals, such as whales and seals, evolved from land-dwelling ancestors and adapted to an aquatic lifestyle
  • Example: The evolution of whales from early semi-aquatic forms (Pakicetus) to fully aquatic, baleen-bearing species (Balaenoptera)
• Insects, particularly ants and termites, became ecologically dominant and played important roles in nutrient cycling and soil formation
• Flowering plants (angiosperms) diversified and coevolved with pollinating insects, leading to the development of complex plant-insect relationships

Extinction Events

• Cenozoic Era began with the Cretaceous-Paleogene (K-Pg) extinction event ~66 mya, which wiped out non-avian dinosaurs and many other species
  • Caused by the impact of a large asteroid or comet, as evidenced by the Chicxulub crater in Mexico and the global iridium anomaly
  • Led to the extinction of ~75% of all species and opened up ecological niches for the survivors, particularly mammals
• Late Eocene extinctions ~37-33 mya affected many marine species, including foraminifera, mollusks, and early cetaceans
  • Likely caused by global cooling and the onset of Antarctic glaciation
• Mid-Miocene Climatic Optimum ~15-17 mya was followed by a gradual cooling and drying trend, leading to the extinction of many forest-adapted species
• Late Pleistocene extinctions ~50,000-10,000 years ago affected primarily large mammals, including mammoths, mastodons, and giant ground sloths
  • Caused by a combination of factors, including climate change, human hunting, and habitat loss
  • Resulted in the loss of ~70% of North American megafauna species and ~80% of South American megafauna species
• Holocene extinctions have accelerated due to human activities such as habitat destruction, overhunting, and the introduction of invasive species
  • Example: The extinction of the dodo (Raphus cucullatus) in the 17th century due to human hunting and the introduction of non-native predators

Human Evolution

• Hominins, the group that includes modern humans and our extinct ancestors, diverged from other apes during the late Miocene (~6-7 mya)
• Early hominins, such as Ardipithecus and Australopithecus, were characterized by bipedalism and relatively small brain sizes
  • Example: Australopithecus afarensis, known from the famous "Lucy" fossil, lived ~3.9-2.9 mya in East Africa
• The genus Homo evolved ~2.8 mya, marked by an increase in brain size and the development of stone tool technology
  • Early species include Homo habilis and Homo erectus, the first hominin to leave Africa and spread across Eurasia
• Homo sapiens, anatomically modern humans, evolved in Africa ~300,000-200,000 years ago and later spread to all continents
  • Neanderthals (Homo neanderthalensis) and Denisovans (Denisova hominins) were contemporaries of early modern humans and interbred with them before going extinct ~40,000-30,000 years ago
• The development of agriculture and the rise of civilizations during the Holocene epoch marked a significant shift in human societies and their impact on the environment

Fossil Record and Key Discoveries

• Cenozoic fossil record is extensive and well-studied, providing insights into the evolution and diversification of mammals, birds, and other groups
• Eocene lagerstätten (sites with exceptionally preserved fossils) such as the Green River Formation in North America and the Messel Pit in Germany have yielded detailed fossils of early mammals, birds, and insects
  • Example: The Messel Pit has preserved fossils of early horses (Eurohippus), primates (Darwinius), and the oldest known bat (Onychonycteris)
• Oligocene and Miocene fossil sites, such as the White River Badlands in South Dakota and the Ashfall Fossil Beds in Nebraska, have provided insights into the evolution of grassland-adapted mammals
• Pliocene and Pleistocene fossil sites in Africa, such as Laetoli in Tanzania and Olduvai Gorge in Kenya, have yielded important hominin fossils and evidence of early human behavior
  • Example: The Laetoli footprints, dated to ~3.6 mya, provide evidence of bipedalism in early hominins
• Pleistocene cave sites, such as Chauvet Cave in France and Lascaux Cave in Spain, contain well-preserved cave art and fossils of Ice Age mammals
• Advances in dating techniques, such as radiocarbon dating and uranium-series dating, have allowed for more precise age determinations of Cenozoic fossils
• The development of molecular phylogenetics has revolutionized our understanding of evolutionary relationships among Cenozoic species

Impact on Modern Ecosystems

• Cenozoic Era saw the development of many modern ecosystems, such as grasslands, savannas, and tropical rainforests
• The expansion of grasslands during the Miocene and Pliocene led to the evolution of grazing mammals and the coevolution of grasses and herbivores
  • Example: The spread of C4 grasses during the late Miocene allowed for the diversification of horses, antelope, and other grazing mammals
• The Pleistocene glacial-interglacial cycles had a profound impact on the distribution and composition of modern ecosystems
  • Example: The repeated expansion and contraction of ice sheets led to the development of distinct biomes, such as tundra and boreal forests
• The Late Pleistocene extinctions resulted in the loss of many large mammal species, which likely had cascading effects on ecosystem structure and function
  • Example: The extinction of mammoths and other megaherbivores may have led to changes in plant communities and nutrient cycling
• Human activities during the Holocene, such as agriculture, deforestation, and urbanization, have significantly altered modern ecosystems and biodiversity
  • Example: The conversion of grasslands and forests to agricultural land has led to habitat loss and fragmentation for many species
• Conservation efforts aim to protect and restore modern ecosystems, often using insights from the Cenozoic fossil record to inform management decisions
  • Example: The reintroduction of wolves to Yellowstone National Park in the 1990s was based in part on the understanding of their ecological role during the Pleistocene