Chapter 13: The Mesozoic Era

Chapter 13 – Mesozoic Era
13.1 Introduction to the Mesozoic Era
 Second tectonic cycle of the Phanerozoic begins
o Pangaea rifted apart
o Atlantic Ocean opens
o Tethys becomes a major circumequatorial seaway
 Transition from ancient Paleozoic world to modern Cenozoic world
o Rediversification of biosphere following Late Permian extinctions
13.2 Tectonic Cycle: Impacts on the Hydrosphere, Atmosphere, and Rock Cycle
 Rifting of Pangea
o Tethys gradually widened and deepened between Laurasia and Gondwana
o Central Atlantic began to open as North America rifted from South
o Gondwana began to break apart
 Sea Level, CO2, and Sedimentary Facies
o Late Paleozoic-to-Early Mesozoic climatic extremes gave way to a world
of widespread seas and overall warmth
o Sea level remained high through much of the Mesozoic as seafloor
spreading centers displaced water from the ocean basins, producing epeiric
o Increased rifting, volcanism, and seafloor spreading increased atmospheric
CO2 levels
o Limestone (especially calcareous oozes) were widespread in the epeiric
o Evaporites were also prevalent due to overall climatic warmth
o Tropical marine faunas and terrestrial floras (coals) extended to relatively
high latitudes in both hemispheres
o Little (if any) evidence exists for glacial tillites; Earth may have been too
warm to sustain permanent ice caps
 Ocean Circulation and Chemistry
o Warm conditions were also promoted by ocean circulation due to
changing ocean-continent configurations
o Oxygen isotope rations suggest that the oceans were warm and their
circulation relatively sluggish
o Ocean circulation probably depended primarily on the production of
hypersaline waters in restricted basins and shallow shelves, because polar
ice caps were absent
o Jurassic and Cretaceous oceans tended toward anoxia, producing carbonrich black shales in short (< 1 million years) “oceanic anoxic events”
o Rifting also decreased the magnesium concentration in seawater,
promoting calcite seas
13.3 Tectonic Cycle and Orogeny
 Eastern North America
o The eastern margin of North America became a passive margin and was
tectonically quiet
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o The initial rifting of Pangaea is indicated by rift basins filled with lake
o Sediment eroded from the Appalachians was shed east, building out the
continental shelf as the Atlantic opened
 Cordilleran Orogenic Belt
o The Cordillera is one of he longest mountain chains in the world,
extending from Alaska down through the Rockies, Central America, and
the Andes in South America
o The Cordillera is still active today as evidenced by volcanism and
earthquakes associated with the Pacific Ring of Fire
o After the Early-to-Middle Paleozoic, the western margin of North America
had become an ocean-continent convergent margin, with ocean crust being
subducted eastward beneath the North American plate
o A number of island arcs and microcontinents accreted as terranes to the
western margin of North America, moving northward along faults for
thousands of kilometers
 Orogenic Episodes
o The first Mesozoic orogenic episode in western North America was a
continuation of the Sonoma Orogeny, in which an island arc and
microcontinent (along with other terranes) accreted to the western margin
o The next phase of mountain building, referred to as the Cordilleran
Orogeny, involved phases of uplift in different regions: Nevadan, Sevier
and Orogeny (Jurassic to Eocene)
o During the Nevadan Orogeny, enormous granitic batholiths were
emplaced as ocean crust was subducted to sufficient depth to produce
o The following Sevier Orogeny may have been initiated by the collision of
a microcontinent with North America in the Cretaceous; igneous activity
shifted eastward, perhaps due to a decrease in the angle of subduction
o The Laramide Orogeny began during the Late Cretaceous and continued
into the Cenozoic; it was responsible for many features of the Rocky
Mountains seen today
13.4 Orogeny, Sea Level, and Sedimentation
 During the Early Triassic, dry arid conditions persisted in many continental
interiors, resulting in the deposition of redbeds
 These deposits gave way to Early Jurassic desert sandstones, followed by Middle
Jurassic epicontinental sea deposits
o The Sundance Sea twice invaded the foreland basin east of the main
orogenic belt of the Nevadan Orogeny
 The eastward shift in subduction associated with the Sevier Orogeny led to the
formation of another foreland basin; the immense Cretaceous Interior Seaway ultimately
developed in this basin
o This seaway stretched a thousand miles, from the Rocky Mountains to the
eastern margin of the Great Plains, and connected the Gulf of Mexico to
waters of the far north
13.5 Diversification of the Marine Biosphere
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Plankton and Microfossils
o Conodonts and radiolarians persisted from the Paleozoic into the
Mesozoic, but only the radiolarians flourished; they were joined in the
plankton by newly evolved groups: the coccolithophorids, planktonic
foraminifera, diatoms, and dinoflagellates
 Benthic Ecosystems
o Members of the Modern Fauna radiated into niches left vacant by the Late
Permian extinctions
o These included bivalves with longer siphons, carnivorous gastropods, sea
urchins, crustaceans and fish
o Survivors of the Paleozoic Fauna rediversified, including the ammonoids,
which were joined by another group of cephalopods, the belemnoids
o Reef faunas rediversified, with a new order of reef-forming corals, the
scleractinians; these remained the dominant reef builders of the Mesozoic,
except for rudist bivalves that temporarily displaced them in the midCretaceous
 Marine Vertebrates
o Early Mesozoic placodonts and nothosaurs gave way to ichthyosaurs,
plesiosaurs, and mosasaurs
13.6 Diversification of the Terrestrial Biosphere
 Plants and Insects
o Early Triassic land floras continued to be dominated by gymnosperms
such as seed ferns; these communities also contained seedless vascular
plants (spore plants such as ferns, horsetails and club mosses)
o These began to be replaced by new taxa of gymnosperms: cycads, ginkgos
and conifers
o By mid-to-late Cretaceous, these gave way to angiosperms, perhaps as a
result of coevolution with insects
 Vertebrates
o Among the vertebrates, the dinosaurs dominated, which arose from the
archosaur group of reptiles
o Dinosaurs were composed to two lineages: the Saurischia (lizard-hipped)
and the Ornithischia (bird-hipped)
o The saurischians included both large herbivorous sauropods and
carnivorous theropods
o The dominant ornithischians of the jurassic were the stegosaurs, which
eventually were replaced by the ankylosaurs, ornithopods, and
ceratopsians in the Cretaceous
 Evolution of Flight
o Pterosaurs – these reptiles were probably gliders rather than active fliers;
they lacked feathers
o Birds – The first bird, Archaeopteryx, which appeared in the Jurassic is a
classic missing link between reptiles and birds, exhibiting traits of both
groups (feathers, claws on winds, teeth, long tail); it probably evolved
from small bipedal theropods
 Mammals
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o True mammals appeared by the end of the Traissic, evolving from the
o Therapsids may have possessed several mammilian traits: endothermy and
hair, legs positioned underneath the body, and possibly a four-chambered
o During the Mesozoic, mammals evolved their distinctive single lower jaw
bone, increasingly specialized teeth, inner ear, and secondary palate
(allowing them to eat and breathe at the same time)
o Late Cretaceous mammals were dominated by insectivores; it is from
these taxa that modern mammals diversified after the demise of the
dinosaurs in the Late Cretaceous
13.7 Extinction
 Late Triassic Extinctions
o The Late Triassic mass extinction nearly wiped out the therapsids,
bivalves, gastropods, ammonoid and nautiloid cephalopods, plesiosaurs,
and icthyosaurs; the conodonts and placodonts died out
o The rapid rise of the dinosaurs may have been due to the opening up of
habitats and niches as a result of this event
o The mechanisms of this extinction event are poorly understood; an
extraterrestrial impact may have occurred, volcanism may have been
involved, as well as changes in ocean circulation or productivity
 Late Cretaceous Extinctions
o There is general consensus that one or more meteors hit the Earth at the
end of the Cretaceous, and that impact was the primary cause of the
demise of the dinosaurs and other taxa on land and in the sea
o Falling sea levels and climatic cooling may have also played a role, as
well as large scale volcanism associated with the Deccan Traps of India
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