Geology Reference
In-Depth Information
The breakup of Pangaea infl uenced global climatic
and atmospheric circulation patterns. Although the
temperature gradient from the tropics to the poles gradu-
ally increased during the Mesozoic, overall global tem-
peratures remained equable.
An increased rate of seafl oor spreading during the Cre-
taceous Period caused sea level to rise and transgressions
to occur.
Except for incursions along the continental margin and
two major transgressions (the Sundance Sea and the
Cretaceous Interior Seaway), the North American craton
was above sea level during the Mesozoic Era.
The Eastern Coastal region was the initial site of the sepa-
ration of North America from Africa that began during
the Late Triassic. During the Cretaceous Period, it was
inundated by marine transgressions.
The Gulf Coastal region was the site of major evaporite
accumulation during the Jurassic as North America rifted
from South America. During the Cretaceous, it was inun-
dated by a transgressing sea, which, at its maximum, con-
nected with a sea transgressing from the north to create
the Cretaceous Interior Seaway.
Mesozoic rocks of the western region of North America
were deposited in a variety of continental and marine
environments. One of the major controls of sediment
distribution patterns was tectonism.
Western North America was affected by four interrelated
orogenies: the Sonoma, Nevadan, Sevier, and Laramide.
Each involved igneous intrusions, as well as eastward
thrust faulting and folding.
The cause of the Nevadan, Sevier, and Laramide orog-
enies was the changing angle of subduction of the oceanic
Farallon plate under the continental North American
plate. The timing, rate, and, to some degree, the direction
of plate movement were related to seafl oor spreading and
the opening of the Atlantic Ocean.
Orogenic activity associated with the oceanic-continental
convergent plate boundary in the Cordilleran mobile belt
explains the structural features of the western margin of
North America. It is thought, however, that more than
25% of the North American western margin originated
from the accretion of terranes.
Mesozoic rocks contain a variety of mineral resources,
including coal, petroleum, uranium, gold, and copper.
The marine invertebrate survivors of the Permian mass
extinction diversifi ed and gave rise to increasingly diverse
Mesozoic marine invertebrate communities.
Land plant communities of the Triassic and Jurassic
consisted of seedless vascular plants and gymnosperms.
The angiosperms, or fl owering plants, evolved during the
Early Cretaceous, diversifi ed rapidly, and were soon the
most abundant land plants.
Dinosaurs evolved during the Late Triassic, but were most
abundant and diverse during the Jurassic and Cretaceous.
The two distinct orders of dinosaurs, based on pelvic
structure, are Saurischia (lizard-hipped) and Ornithischia
(bird-hipped).
Bone structure, predator-prey relationships, and other
features have been cited as evidence of dinosaur endo-
thermy. Although there is still no solid consensus, many
paleontologists think that some dinosaurs were indeed
endotherms.
That some theropods had feathers indicates that they
were warm-blooded and provides further evidence of
their relationship to birds.
Small pterosaurs were probably active, wingfl apping
fl iers, whereas large ones may have depended on
thermal updrafts and soaring to stay aloft. At least one
pterosaur species had hair or feathers, so it was probably
endothermic.
The fi sh-eating, porpoise-like ichthyosaurs were thor-
oughly adapted to an aquatic life, whereas the plesiosaurs
with their paddle-like limbs could most likely come out
of the water to lay their eggs. The marine reptiles known
as mosasaurs were most closely related to lizards.
Birds probably evolved from small carnivorous dinosaurs.
The oldest known bird,
Archaeopteryx,
appeared dur-
ing the Jurassic; however, few other Mesozoic birds are
known.
The earliest mammals evolved during the Late Triassic,
but they are diffi cult to distinguish from advanced cyn-
odonts. Details of the teeth, middle ear, and lower jaw are
used to distinguish the two.
Several types of Mesozoic mammals existed, but most
were small and their diversity was low. Both marsupi-
als and placentals evolved during the Cretaceous from a
group known as eupantotheres.
Because the continents were close together and climates
were mild during much of the Mesozoic, animals and
plants occupied much larger geographic ranges than they
do now.
Among the victims of the Mesozoic mass extinctions
were dinosaurs, fl ying reptiles, marine reptiles, and
several groups of marine invertebrates. A huge mete-
orite impact may have caused these extinctions, but
some paleontologists think that other factors were also
important.
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