5.2 Jurassic period

The Jurassic period, spanning 56 million years, was a crucial chapter in Earth's history. It saw the breakup of Pangaea, the dominance of dinosaurs, and the evolution of new marine reptiles. This period shaped the planet's geography and biodiversity.

During the Jurassic, warm and humid conditions prevailed. Shallow seas covered large areas, creating diverse habitats. Gymnosperms dominated plant life, while dinosaurs ruled the land. Marine reptiles and pterosaurs thrived, and mammals began to diversify.

Jurassic period overview

• The Jurassic period is the second segment of the Mesozoic Era, following the Triassic and preceding the Cretaceous
• Characterized by the breakup of the supercontinent Pangaea, the dominance of dinosaurs on land, and the evolution of new marine reptile groups
• Witnessed the early diversification of mammals and the first appearance of birds

Position in geologic timescale

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• The Jurassic is the middle period of the Mesozoic Era, which is part of the Phanerozoic Eon
• Follows the Triassic Period and is succeeded by the Cretaceous Period
• Spans from approximately 201.3 million years ago (Ma) to 145 Ma

Duration of Jurassic period

• The Jurassic period lasted for about 56 million years
• Subdivided into three epochs: Early Jurassic (201.3 to 174.1 Ma), Middle Jurassic (174.1 to 163.5 Ma), and Late Jurassic (163.5 to 145 Ma)
• Each epoch is further divided into ages and stages based on biostratigraphy and other stratigraphic markers

Jurassic paleogeography

• The Jurassic saw significant changes in the configuration of landmasses due to the ongoing breakup of Pangaea
• The Tethys Ocean expanded, creating new marine habitats and influencing global climate patterns
• Shallow epicontinental seas covered large areas of the continents, providing habitats for diverse marine life

Pangaea breakup

• During the Jurassic, the supercontinent Pangaea began to split into northern Laurasia and southern Gondwana
• The rifting of Pangaea led to the formation of the Atlantic Ocean basin and the widening of the Tethys Ocean
• The breakup of Pangaea had profound effects on climate, ocean circulation, and biogeography

Tethys Ocean expansion

• The Tethys Ocean, which separated Laurasia from Gondwana, significantly expanded during the Jurassic
• The widening of the Tethys created new marine habitats and facilitated the dispersal of marine organisms
• The Tethys Ocean played a crucial role in heat transport and global climate regulation

Shallow epicontinental seas

• Extensive shallow seas covered large areas of the continents during the Jurassic, particularly in North America, Europe, and Asia
• These epicontinental seas provided habitats for diverse marine life, including ammonites, belemnites, and marine reptiles
• The presence of shallow seas influenced sedimentation patterns and the formation of important fossil-bearing strata (Solnhofen Limestone)

Jurassic climate

• The Jurassic climate was generally warm and humid, with reduced temperature gradients between the equator and poles
• The absence of permanent polar ice caps contributed to higher sea levels and the expansion of shallow marine environments
• Milankovitch cycles, driven by variations in Earth's orbital parameters, influenced long-term climate patterns during the Jurassic

Warm, humid conditions

• Jurassic global temperatures were generally higher than present-day, with evidence of widespread humid conditions
• Elevated atmospheric CO2 levels, possibly due to volcanic activity, contributed to the greenhouse climate
• Warm, humid conditions supported the growth of lush vegetation and diverse ecosystems

Lack of polar ice caps

• The Jurassic climate was characterized by the absence of permanent polar ice caps
• Reduced ice volume contributed to higher global sea levels and the flooding of continental margins
• The lack of polar ice caps influenced ocean circulation patterns and heat distribution

Milankovitch cycles

• Milankovitch cycles, which are periodic changes in Earth's orbital parameters, influenced long-term climate patterns during the Jurassic
• Variations in eccentricity, obliquity, and precession affected the distribution of solar radiation and seasonal contrasts
• Milankovitch cycles are recorded in Jurassic sedimentary sequences and are used for high-resolution stratigraphic correlation

Jurassic flora

• Jurassic flora was dominated by gymnosperms, including conifers, cycads, and ginkgoes
• Bennettitales, an extinct group of seed plants, were diverse and widespread during the Jurassic
• The Jurassic witnessed the early evolution and diversification of angiosperms (flowering plants)

Gymnosperm dominance

• Gymnosperms, which are seed-bearing plants without enclosed ovaries, were the dominant plant group during the Jurassic
• Conifers (Araucariaceae, Pinaceae) were diverse and adapted to a wide range of environments
• Ginkgoes and cycads were also common components of Jurassic floras

Bennettitales and cycads

• Bennettitales, an extinct group of seed plants with superficial similarities to cycads, were diverse and widespread during the Jurassic
• Cycads, which are palm-like gymnosperms, were also common in Jurassic floras
• Both Bennettitales and cycads were important food sources for herbivorous dinosaurs

Early angiosperm evolution

• The Jurassic witnessed the early evolution and diversification of angiosperms (flowering plants)
• The oldest unequivocal angiosperm fossils date back to the Early Cretaceous, but molecular evidence suggests an earlier origin in the Jurassic
• Early angiosperms were likely small, herbaceous plants with simple flowers, co-evolving with insect pollinators

Jurassic fauna

• Jurassic fauna was characterized by the dominance of dinosaurs on land, the diversification of marine reptiles, and the evolution of pterosaurs
• Mammals, which originated in the Late Triassic, underwent early diversification during the Jurassic
• Jurassic ecosystems supported a wide range of invertebrates, including ammonites, belemnites, and bivalves

Dinosaur diversity and dominance

• Dinosaurs reached their peak diversity and ecological dominance during the Jurassic
• Sauropods (Brachiosaurus, Diplodocus) were the largest land animals, while theropods (Allosaurus, Ceratosaurus) were the apex predators
• Ornithischian dinosaurs (Stegosaurus, Camptosaurus) diversified and adapted to various herbivorous niches

Marine reptile adaptations

• Jurassic oceans were home to a diverse array of marine reptiles, including ichthyosaurs, plesiosaurs, and pliosaurs
• Marine reptiles evolved various adaptations for aquatic life, such as streamlined bodies, paddle-like limbs, and tail flukes
• Some marine reptiles (pliosaurs) reached gigantic sizes and were apex predators in Jurassic marine ecosystems

Pterosaur evolution and diversity

• Pterosaurs, the first vertebrates capable of powered flight, underwent significant diversification during the Jurassic
• Jurassic pterosaurs included both primitive (rhamphorhynchoids) and advanced (pterodactyloids) forms
• Pterosaurs occupied various ecological niches, including aerial predators, filter feeders, and possible seed dispersers

Mammal origins and early diversification

• Mammals, which originated in the Late Triassic, underwent early diversification during the Jurassic
• Jurassic mammals were small, mostly insectivorous or omnivorous, and adapted to various niches (arboreal, fossorial)
• The Jurassic saw the emergence of key mammalian traits, such as hair, lactation, and specialized dentition

Iconic Jurassic dinosaurs

• The Jurassic period is known for its diverse and iconic dinosaur fauna, including sauropods, theropods, and ornithischians
• Sauropods, such as Brachiosaurus, Diplodocus, and Apatosaurus, were the largest land animals of the Jurassic
• Theropods, including Allosaurus, Ceratosaurus, and Megalosaurus, were the dominant predators

Sauropods: Brachiosaurus, Diplodocus, Apatosaurus

• Brachiosaurus was a large sauropod with a long neck and front legs longer than its hind legs, adapted for browsing high vegetation
• Diplodocus was a long-necked, whip-tailed sauropod with a horizontal posture, possibly adapted for ground-level browsing
• Apatosaurus (formerly known as Brontosaurus) was a large, robust sauropod with a long neck and tail, known for its massive size

Theropods: Allosaurus, Ceratosaurus, Megalosaurus

• Allosaurus was a large, carnivorous theropod with a powerful skull and serrated teeth, likely an apex predator in Jurassic ecosystems
• Ceratosaurus was a medium-sized theropod with a distinctive nasal horn and blade-like teeth, possibly a specialized predator
• Megalosaurus was one of the earliest named theropods, a large carnivore from the Middle Jurassic of Europe

Ornithischians: Stegosaurus, Camptosaurus

• Stegosaurus was a large, herbivorous ornithischian with distinctive plates and spikes along its back and tail, possibly for display or defense
• Camptosaurus was a medium-sized, bipedal ornithischian with a beak-like snout, adapted for browsing vegetation
• Ornithischians diversified during the Jurassic, giving rise to various herbivorous lineages (thyreophorans, ornithopods, marginocephalians)

Jurassic mass extinctions

• The Jurassic period witnessed several episodes of extinctions and faunal turnovers, although not as severe as the end-Triassic or end-Cretaceous mass extinctions
• The Jurassic began with the recovery from the end-Triassic extinction, which affected both marine and terrestrial ecosystems
• The Toarcian Oceanic Anoxic Event (T-OAE) in the Early Jurassic led to a significant turnover in marine fauna

End-Triassic extinction recovery

• The Jurassic period began with the recovery from the end-Triassic mass extinction, which saw the disappearance of many Triassic lineages
• The recovery was gradual, with the diversification of new groups (dinosaurs, crocodylomorphs, mammals) filling ecological niches
• Marine ecosystems recovered with the radiation of ammonites, bivalves, and marine reptiles

Toarcian turnover

• The Toarcian Oceanic Anoxic Event (T-OAE) in the Early Jurassic led to widespread anoxia in marine environments
• The T-OAE caused a significant turnover in marine fauna, particularly affecting ammonites, bivalves, and brachiopods
• The event may have been triggered by volcanic activity and the release of greenhouse gases, leading to global warming and ocean acidification

Minor extinctions and faunal turnovers

• The Jurassic witnessed several minor extinctions and faunal turnovers, often associated with changes in sea level, climate, or ocean chemistry
• The end-Callovian extinction affected ammonites and other marine invertebrates, possibly due to a global cooling event
• The Jurassic-Cretaceous boundary saw a minor extinction event, particularly affecting ammonites and bivalves, but its causes remain uncertain

Jurassic fossil formations

• The Jurassic period is represented by numerous fossil-bearing formations worldwide, providing insights into the diversity and evolution of Jurassic life
• Notable Jurassic fossil formations include the Morrison Formation of North America, the Solnhofen Limestone of Germany, and the Tendaguru Formation of Tanzania
• These formations preserve a wide range of fossils, including dinosaurs, marine reptiles, pterosaurs, and invertebrates

Morrison Formation of North America

• The Morrison Formation is a Late Jurassic sedimentary sequence exposed in the western United States, known for its rich dinosaur fossils
• The formation has yielded numerous iconic dinosaurs, such as Brachiosaurus, Diplodocus, Allosaurus, and Stegosaurus
• The Morrison Formation represents a diverse range of paleoenvironments, including floodplains, wetlands, and semi-arid regions

Solnhofen Limestone of Germany

• The Solnhofen Limestone is a Late Jurassic Lagerstätte (fossil deposit with exceptional preservation) in southern Germany
• The formation is famous for its exquisitely preserved fossils, including the early bird Archaeopteryx, pterosaurs, and marine invertebrates
• The Solnhofen Limestone represents a shallow marine environment with occasional influxes of freshwater and volcanic ash

Tendaguru Formation of Tanzania

• The Tendaguru Formation is a Late Jurassic sedimentary sequence in Tanzania, known for its diverse dinosaur fauna
• The formation has yielded sauropods (Giraffatitan), theropods (Elaphrosaurus), and ornithischians (Kentrosaurus)
• The Tendaguru Formation represents a range of paleoenvironments, including coastal plains, tidal flats, and shallow marine settings

Jurassic paleoenvironments

• Jurassic paleoenvironments were diverse, encompassing coastal and shallow marine settings, terrestrial floodplains and wetlands, and arid interior regions
• The breakup of Pangaea and the expansion of the Tethys Ocean influenced the distribution and evolution of Jurassic ecosystems
• Jurassic paleoenvironments supported a wide range of organisms, from marine invertebrates to terrestrial vertebrates

Coastal and shallow marine settings

• Coastal and shallow marine environments were widespread during the Jurassic, due to high sea levels and the presence of epicontinental seas
• These settings were characterized by carbonate platforms, coral reefs, and lagoons, which supported diverse marine invertebrates and vertebrates
• Coastal environments also served as important nesting sites for marine reptiles, such as ichthyosaurs and plesiosaurs

Terrestrial floodplains and wetlands

• Terrestrial floodplains and wetlands were common in the Jurassic, particularly in the expansive continental interiors
• These environments were characterized by meandering rivers, lakes, and swamps, which supported lush vegetation and diverse terrestrial fauna
• Floodplains and wetlands were important habitats for dinosaurs, crocodylomorphs, and early mammals

Arid interior regions

• Arid interior regions developed in some parts of Pangaea during the Jurassic, particularly in the rain shadows of mountain ranges
• These environments were characterized by sand dunes, salt flats, and ephemeral streams, supporting adapted flora and fauna
• Arid regions were inhabited by specialized dinosaurs (e.g., small theropods, early ceratopsians) and other drought-tolerant organisms

Economic resources from Jurassic

• Jurassic sedimentary rocks are important sources of economic resources, including petroleum, coal, and building stones
• The presence of organic-rich sediments and suitable reservoir rocks in Jurassic sequences has led to the formation of significant petroleum reserves
• Jurassic coal deposits, formed from the accumulation of plant material in swamps and wetlands, are mined in several regions worldwide

Petroleum reserves

• Jurassic source rocks, such as marine shales and limestones, are important sources of petroleum in many sedimentary basins worldwide
• The North Sea, Gulf of Mexico, and Middle East host significant Jurassic petroleum reserves
• Jurassic reservoir rocks, including sandstones and carbonates, provide suitable porosity and permeability for oil and gas accumulation

Coal deposits

• Jurassic coal deposits are found in several regions, particularly in the northern hemisphere (Europe, Asia, North America)
• These coal deposits formed from the accumulation of plant material in extensive swamps and wetlands under warm, humid conditions
• Jurassic coals are exploited for electricity generation and industrial processes

Jurassic building stones

• Jurassic sedimentary rocks, particularly limestones and sandstones, are used as building stones in many regions
• The Portland Stone, a Late Jurassic limestone from southern England, has been widely used in construction (St. Paul's Cathedral, United Nations Headquarters)
• Other notable Jurassic building stones include the Cotswold Stone (limestone) from central England and the Bavarian Jurassic limestones