5.3 Cretaceous period

The Cretaceous period, spanning 79 million years, marked the end of the Mesozoic Era. It saw dinosaurs reach their peak diversity, the rise of flowering plants, and ended with a mass extinction event that reshaped life on Earth.

This period was characterized by warm climates, high sea levels, and the breakup of Pangaea. The Cretaceous left behind rich fossil records and valuable resources, providing key insights into Earth's geological and biological history.

Cretaceous period overview

• The Cretaceous period is the longest period of the Mesozoic Era, spanning approximately 79 million years
• It is known for the diversification and dominance of dinosaurs, the evolution of flowering plants, and the mass extinction event at the end of the period
• The Cretaceous period provides important insights into the evolution and extinction of various plant and animal groups, as well as significant changes in Earth's climate and geography

Beginning and end of Cretaceous

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• The Cretaceous period began approximately 145 million years ago, following the Jurassic period
• It ended around 66 million years ago with the Cretaceous-Paleogene (K-Pg) extinction event, which marked the end of the Mesozoic Era and the beginning of the Cenozoic Era
• The K-Pg extinction event was caused by a massive asteroid impact and resulted in the extinction of non-avian dinosaurs and many other species

Duration of Cretaceous period

• The Cretaceous period lasted for about 79 million years, making it the longest period of the Mesozoic Era
• It is divided into two epochs: the Early Cretaceous (145-100.5 million years ago) and the Late Cretaceous (100.5-66 million years ago)
• The long duration of the Cretaceous period allowed for significant evolutionary changes and the diversification of various plant and animal groups

Cretaceous paleogeography

• During the Cretaceous period, Earth's continents continued to drift apart, resulting in the gradual breakup of the supercontinent Pangaea
• The opening of the Atlantic Ocean and the formation of the Western Interior Seaway in North America had significant impacts on global climate and ocean circulation patterns
• The Tethys Ocean, which separated the northern continents from the southern continents, reached its maximum extent during the Cretaceous period

Continental drift during Cretaceous

• The separation of Gondwana (the southern supercontinent) continued, with Africa and South America splitting apart and India beginning to move northward towards Asia
• North America and Eurasia remained connected for much of the Cretaceous, but began to separate towards the end of the period
• The movement of continents affected global climate patterns and influenced the distribution and evolution of flora and fauna

Oceanic anoxic events

• The Cretaceous period witnessed several oceanic anoxic events (OAEs), which were periods of widespread oxygen depletion in the oceans
• OAEs were caused by a combination of factors, including increased volcanic activity, changes in ocean circulation, and high levels of organic matter burial
• These events had significant impacts on marine life, leading to extinctions and changes in the composition of marine ecosystems (e.g., the Cenomanian-Turonian OAE)

Cretaceous sea level

• Sea levels during the Cretaceous period were generally higher than present-day levels, with the highest sea levels occurring during the Late Cretaceous
• High sea levels resulted in the flooding of continental margins and the creation of extensive shallow marine environments (e.g., the Western Interior Seaway in North America)
• Changes in sea level throughout the Cretaceous period influenced the distribution and evolution of marine organisms and affected the preservation of fossil deposits

Cretaceous climate

• The Cretaceous period experienced a generally warm climate, with no evidence of permanent polar ice caps
• However, the climate was not uniform throughout the period, with fluctuations between warmer and cooler intervals
• The warm climate of the Cretaceous had significant impacts on the distribution and diversity of both terrestrial and marine life

Climate trends in Cretaceous

• The Early Cretaceous was characterized by a relatively cool climate, with evidence of seasonal ice at high latitudes
• The Late Cretaceous saw a significant warming trend, with global temperatures reaching their maximum during the Turonian stage (93.9-89.8 million years ago)
• Towards the end of the Cretaceous, the climate began to cool, possibly due to a combination of factors such as decreased volcanic activity and changes in ocean circulation patterns

Causes of Cretaceous climate change

• Volcanic activity, particularly the formation of large igneous provinces (LIPs), likely contributed to the warm climate of the Cretaceous by releasing greenhouse gases into the atmosphere
• Changes in ocean circulation patterns, influenced by the opening and closing of oceanic gateways (e.g., the Drake Passage), also played a role in regulating global climate
• Variations in Earth's orbit and the amount of solar radiation received may have contributed to shorter-term climate fluctuations within the Cretaceous period

Impact of climate on Cretaceous life

• The warm climate of the Cretaceous allowed for the expansion of tropical and subtropical environments, which supported a diverse array of plant and animal life
• Higher global temperatures and increased humidity likely contributed to the evolution and diversification of flowering plants (angiosperms) during the Cretaceous
• Marine life, including reef-building organisms, thrived in the warm, shallow seas created by high sea levels during the Cretaceous

Cretaceous flora

• The Cretaceous period witnessed significant changes in Earth's flora, with the rise and diversification of flowering plants (angiosperms)
• Gymnosperms, such as conifers and cycads, remained important components of Cretaceous ecosystems, but they were gradually overshadowed by the expanding angiosperm groups
• Cretaceous plant fossils provide valuable insights into the evolution and ecology of terrestrial ecosystems during this time

Dominant plant groups

• Angiosperms underwent a rapid evolutionary radiation during the Cretaceous, becoming the dominant plant group by the end of the period
• Conifers, such as pines and redwoods, were widespread and diverse, adapting to a range of environments from coastal areas to upland forests
• Cycads and bennettitaleans, which had been important components of Mesozoic floras, began to decline in diversity and abundance during the Cretaceous

Angiosperm evolution and diversification

• The oldest unequivocal angiosperm fossils date back to the Early Cretaceous, around 130 million years ago
• By the mid-Cretaceous, angiosperms had diversified into a wide range of morphologies and ecological roles, including trees, shrubs, and herbaceous plants
• The rapid diversification of angiosperms is attributed to a combination of factors, including coevolution with insect pollinators, adaptations for efficient water and nutrient transport, and the ability to exploit a wide range of habitats

Cretaceous plant fossils

• Cretaceous plant fossils are preserved in various forms, including compressions, impressions, and permineralizations
• Important Cretaceous plant fossil localities include the Crato Formation in Brazil, the Yixian Formation in China, and the Raritan Formation in the eastern United States
• Amber deposits from the Cretaceous, such as those found in Myanmar and Lebanon, provide exceptional preservation of delicate plant structures and associated organisms (e.g., insects and fungi)

Cretaceous fauna

• The Cretaceous period is known for its diverse and iconic fauna, including dinosaurs, marine reptiles, and early birds and mammals
• The warm climate and high sea levels of the Cretaceous supported a wide range of terrestrial and marine ecosystems, each with its own unique assemblage of animals
• Cretaceous fossils provide important insights into the evolution, ecology, and extinction of various animal groups during the Mesozoic Era

Dinosaur diversity in Cretaceous

• Dinosaurs reached their peak diversity during the Cretaceous period, with a wide range of herbivorous and carnivorous species inhabiting terrestrial ecosystems worldwide
• Notable Cretaceous dinosaurs include Tyrannosaurus rex, Triceratops, Velociraptor, and Spinosaurus
• The evolution of new anatomical features and adaptations, such as horns, frills, and feathers, contributed to the diversification of dinosaurs during the Cretaceous

Marine reptiles of Cretaceous

• The Cretaceous seas were home to a diverse array of marine reptiles, including plesiosaurs, pliosaurs, ichthyosaurs, and mosasaurs
• Plesiosaurs and pliosaurs were long-necked and short-necked predators, respectively, that hunted fish and other marine creatures
• Ichthyosaurs, which had dominated Mesozoic marine ecosystems, declined in diversity and went extinct by the mid-Cretaceous
• Mosasaurs, a group of large, predatory marine lizards, evolved and diversified during the Late Cretaceous, filling the ecological niches left by ichthyosaurs

Cretaceous bird evolution

• The Cretaceous period witnessed the early evolution and diversification of birds, which evolved from small, feathered theropod dinosaurs
• The oldest known bird, Archaeopteryx, lived during the Late Jurassic, but the Cretaceous saw the emergence of more derived bird lineages, including the ancestors of modern birds (Neornithes)
• Cretaceous birds exhibited a wide range of adaptations for different lifestyles, including diving (Hesperornithiformes), aerial insect-catching (Enantiornithes), and seed-eating (Jeholornithiformes)

Early mammal diversification

• Mammals underwent significant diversification during the Cretaceous period, although they remained small and inconspicuous compared to the dominant dinosaurs
• Early Cretaceous mammals included both monotremes (egg-laying mammals) and therians (marsupials and placentals), with a wide range of dental and skeletal adaptations for different diets and lifestyles
• The Late Cretaceous saw the emergence of more derived mammal lineages, including early representatives of modern groups such as primates, rodents, and ungulates

Invertebrate life in Cretaceous

• Invertebrates, including mollusks, arthropods, and echinoderms, were abundant and diverse in Cretaceous marine and terrestrial ecosystems
• Rudist bivalves, a group of reef-building mollusks, were particularly important in the warm, shallow seas of the Cretaceous
• Ammonites, a group of cephalopods, reached their peak diversity during the Cretaceous before going extinct at the end of the period
• Insects, including early bees, ants, and butterflies, diversified in association with the rise of flowering plants

Cretaceous mass extinctions

• The Cretaceous period witnessed several significant extinction events, including the Cenomanian-Turonian extinction event and the end-Cretaceous mass extinction
• These extinction events had profound impacts on the diversity and composition of Earth's flora and fauna, paving the way for the rise of new groups in the aftermath
• The causes of Cretaceous extinctions are complex and multifaceted, involving a combination of environmental and ecological factors

Cenomanian-Turonian extinction event

• The Cenomanian-Turonian extinction event occurred approximately 94 million years ago, during the mid-Cretaceous
• This event primarily affected marine organisms, with evidence of elevated extinction rates among mollusks, foraminifera, and calcareous nannoplankton
• The cause of the Cenomanian-Turonian extinction is linked to the oceanic anoxic event (OAE 2), which resulted in widespread oxygen depletion in the oceans

End-Cretaceous mass extinction

• The end-Cretaceous mass extinction, also known as the Cretaceous-Paleogene (K-Pg) extinction, occurred approximately 66 million years ago
• This event resulted in the extinction of non-avian dinosaurs, pterosaurs, marine reptiles, ammonites, and many other groups, marking the end of the Mesozoic Era
• The K-Pg extinction had a significant impact on both terrestrial and marine ecosystems, with an estimated 75% of all species going extinct

Causes of Cretaceous extinctions

• The end-Cretaceous mass extinction is primarily attributed to the impact of a large asteroid or comet, which struck the Earth in what is now the Yucatan Peninsula of Mexico
• The impact released a vast amount of energy and debris into the atmosphere, leading to global cooling, acid rain, and disruption of photosynthesis
• Other factors, such as massive volcanic eruptions in the Deccan Traps of India and long-term changes in climate and sea level, may have contributed to the overall environmental stress during this time

Survivors of Cretaceous extinctions

• Some groups of organisms, including crocodilians, turtles, and certain bird and mammal lineages, survived the end-Cretaceous mass extinction
• The survivors of the K-Pg extinction often exhibited adaptations that allowed them to cope with the changing environmental conditions, such as the ability to burrow, a generalist diet, or small body size
• The extinction of dominant groups, such as non-avian dinosaurs, created ecological opportunities for the survivors, leading to the diversification of mammals and birds in the early Cenozoic

Cretaceous geology

• The Cretaceous period is characterized by a diverse array of sedimentary and igneous rocks, reflecting the complex interplay of tectonic, climatic, and biological processes during this time
• Cretaceous strata are widely distributed across the globe, with important fossil sites and economic resources found on every continent
• The study of Cretaceous geology provides valuable insights into the Earth's history, including changes in sea level, climate, and the evolution of life

Cretaceous stratigraphy

• The Cretaceous period is divided into two epochs: the Early Cretaceous (145-100.5 million years ago) and the Late Cretaceous (100.5-66 million years ago)
• Each epoch is further subdivided into stages, which are defined by the appearance or disappearance of specific fossil species or changes in sedimentary patterns
• Important Cretaceous stratigraphic units include the Wealden Group (Europe), the Dakota Formation (North America), and the Bahariya Formation (Africa)

Important Cretaceous fossil sites

• The Yixian Formation in Liaoning Province, China, is famous for its exceptionally preserved feathered dinosaurs, early birds, and mammals
• The Hell Creek Formation in the western United States is known for its diverse assemblage of Late Cretaceous dinosaurs, including Tyrannosaurus rex and Triceratops
• The Araripe Basin in northeastern Brazil contains the Crato and Santana Formations, which yield exquisitely preserved insects, fish, and pterosaurs

Cretaceous fossil preservation

• Cretaceous fossils are preserved in a variety of ways, depending on the depositional environment and the nature of the organism
• Rapid burial in fine-grained sediments, such as those found in marine or lacustrine settings, can result in exceptional preservation of soft tissues and delicate structures
• Permineralization, the replacement of organic material by minerals, is common in Cretaceous wood and bone fossils
• Amber, fossilized tree resin, provides a unique window into Cretaceous forest ecosystems, preserving insects, spiders, and plant material in exquisite detail

Cretaceous resources

• The Cretaceous period is an important source of economic resources, particularly fossil fuels and mineral deposits
• The formation of these resources is closely tied to the tectonic, climatic, and biological conditions that prevailed during the Cretaceous
• The exploration and extraction of Cretaceous resources have significant implications for modern society and the global economy

Cretaceous oil and gas deposits

• Many of the world's major oil and gas reserves are found in Cretaceous sedimentary basins, such as the Western Interior Basin of North America and the South Atlantic marginal basins of Africa and South America
• The source rocks for these hydrocarbons are often organic-rich marine shales, deposited during times of high sea level and anoxic conditions
• The migration and accumulation of oil and gas in Cretaceous reservoirs are controlled by the interplay of tectonic structures, such as faults and folds, and the properties of the reservoir rocks (e.g., porosity and permeability)

Other economic resources from Cretaceous

• Cretaceous sedimentary rocks are important sources of coal, particularly in Australia, where the Eromanga Basin contains significant deposits of sub-bituminous and bituminous coal
• Phosphate deposits, formed from the accumulation of phosphatic organisms and sediments, are found in Cretaceous strata in various parts of the world, including North Africa and the Middle East
• Cretaceous limestones and chalks are widely used as building materials and in the production of cement, with notable examples including the White Cliffs of Dover in England and the Austin Chalk of Texas