The
Mesozoic Era is one of three
geologic eras of the
Phanerozoic eon.
The division of time into eras dates back to
Giovanni Arduino, in the 18th century,
although his original name for the era now called the "Mesozoic"
was "Secondary" (making everything after, including the modern era,
the "Tertiary"; the current term
Quaternary was later proposed for the modern era,
following the same numbering principle). Lying between the
Paleozoic and the
Cenozoic, "Mesozoic" means "middle animals",
deriving from the
Greek prefix
meso-/
μεσο- for "between" and
zoon/
ζωον meaning "
animal"
or "living being". It is often called the "Age of the
Reptiles", after the dominant
fauna of the era.
The Mesozoic was a time of
tectonic,
climatic and
evolutionary activity. The
continents gradually shifted from a state of
connectedness into their present configuration; the drifting
provided for
speciation and other
important evolutionary developments. The climate was exceptionally
warm throughout the period, also playing an important role in the
evolution and diversification of new
animal
species. By the end of the era, the basis of modern life was in
place.
Geologic periods
Following the Paleozoic, the Mesozoic extended roughly 180 million
years: from 251 million years ago (
Ma) to when
the Cenozoic era began 65 Ma. This time frame is separated into
three geologic
periods. From oldest
to youngest:
The lower (Triassic) boundary is set by the
Permian-Triassic extinction
event, during which approximately 90% to 96% of marine species
and 70% of terrestrial vertebrates became
extinct. It is also known as the "Great Dying"
because it is considered the largest mass extinction in the Earth's
history.
The upper (Cretaceous) boundary is set at the
Cretaceous-Tertiary
extinction event (now more accurately called the
Cretaceous–Paleogene (or K–Pg) extinction event), which may have
been caused by the impactor that created Chicxulub
Crater
on the Yucatán Peninsula
. Approximately 50% of all genera became
extinct, including all of the non-
avian
dinosaurs.
Paleogeography and tectonics
Compared to the vigorous convergent plate
mountain-building of the late Paleozoic, Mesozoic
tectonic deformation was comparatively mild. Nevertheless, the era
featured the dramatic rifting of the
supercontinent Pangaea. Pangaea gradually split into a northern
continent,
Laurasia, and a southern
continent,
Gondwana.
This created the
passive continental
margin that characterizes most of the Atlantic
coastline
(such as along the U.S. East Coast) today.
By the end of the era, the continents had rifted into nearly their
present form.
Laurasia became North America and Eurasia, while Gondwana split into South America, Africa,
Australia, Antarctica
and the Indian
subcontinent, which collided with the Asian
plate during the Cenozoic, the impact giving rise to the Himalayas
.
Africa
At the beginning of the Mesozoic Era, Africa was joined with
Earth's other continents in Pangaea. Africa shared the
supercontinent's relatively uniform fauna which was dominated by
theropods, prosauropods and primitive ornithischians by the close
of the Triassic period. Late Triassic fossils are found throughout
Africa, but are more common in the south than north. The boundary
separating the Triassic and Jurassic marks the advent of an
extinction event with global impact, although African strata from
this time period have not been thoroughly studied.
Early Jurassic strata are distributed in a similar fashion to Late
Triassic beds, with more common outcrops in the south and less
common fossil beds which are predominated by tracks to the north.
As the Jurassic proceeded, larger and more iconic groups of
dinosaurs like sauropods and ornithopods proliferated in Africa.
Middle Jurassic strata are neither well represented nor well
studied in Africa. Late Jurassic strata are also poorly represented
apart from the spectacular Tendeguru fauna in Tanzania. The Late
Jurassic life of Tendeguru is very similar to
that found in western
North America's
Morrison
Formation.
Midway through the Mesozoic, about 150-160 million years ago,
Madagascar separated from Africa, although it remained connected to
India and the rest of the Gondwanan landmasses. Fossils from
Madagascar include
abelisaurs and
titanosaurs.
Later into the Early Cretaceous epoch, the India-Madagascar
landmass separated from the rest of Gondwana. By the Late
Cretaceous, Madagascar and India had permanently split ways and
continued until later reaching their modern configurations.
By contrast to Madagascar, mainland Africa was relatively stable in
position through-out the Mesozoic. Despite the stable position,
major changes occurred to its relation to other landmasses as the
remains of Pangaea continued to break apart. By the beginning of
the Late Cretaceous epoch South America had split off from Africa,
completing the southern half of the Atlantic Ocean. This event had
a profound effect on global climate by altering ocean
currents.
During the Cretaceous, Africa was populated by allosauroids and
spinosaurids, including the largest known carnivorous dinosaurs.
Titanosaurs were significant herbivores in its ancient ecosystems.
Cretaceous sites are more common than Jurassic ones, but are often
unable to be dated radiometrically making it difficult to know
their exact ages. Paleontologist Louis Jacobs, who spent time doing
field work in Malawi, says that African beds are "in need of more
field work" and will prove to be a "fertile ground...for
discovery."
Climate
The Triassic was generally dry, a trend that began in the late
Carboniferous, and highly seasonal,
especially in the interior of Pangaea. Low sea levels may have also
exacerbated temperature extremes. With its high
specific heat capacity,
water acts as a temperature-stabilizing heat
reservoir, and land areas near large bodies of water—especially the
oceans—experience less variation in
temperature. Because much of the land that constituted Pangaea was
distant from the oceans, temperatures fluctuated greatly, and the
interior of Pangaea probably included expansive areas of
desert. Abundant evidence of
red
beds and evaporites such as
salt support
these conclusions.
Sea levels began to rise during the Jurassic, which was probably
caused by an increase in
seafloor
spreading. The formation of new crust beneath the surface
displaced ocean waters by as much as more than today, which flooded
coastal areas. Furthermore, Pangaea began to rift into smaller
divisions, bringing more land area in contact with the ocean by
forming the
Tethys Sea. Temperatures
continued to increase and began to stabilize.
Humidity also increased with the proximity of
water, and deserts retreated.
The climate of the Cretaceous is less certain and more widely
disputed. Higher levels of
carbon
dioxide in the
atmosphere are
thought to have caused the
world
temperature gradient from north to south to become almost flat:
temperatures were about the same across the planet. Average
temperatures were also higher than today by about 10°
C. In fact, by the middle Cretaceous, equatorial
ocean waters (perhaps as warm as 20°C in the deep ocean) may have
been too warm for sea life, and land areas near the equator may
have been deserts despite their proximity to water. The circulation
of
oxygen to the deep ocean may also have
been disrupted. For this reason, large volumes of organic matter
that was unable to
decompose
accumulated, eventually being
deposited as "
black shale".
Not all of the data support these hypotheses, however. Even with
the overall warmth, temperature fluctuations should have been
sufficient for the presence of
polar ice
caps and
glaciers, but there is no
evidence of either. Quantitative models have also been unable to
recreate the flatness of the Cretaceous temperature gradient.
Oxygen levels in the Mesozoic atmosphere were probably lower (12 to
15%) than today's level (20 to 21%). Some researchers have
postulated levels of 12% because that was assumed to be the lowest
concentration at which natural
combustion
could occur. However, a 2008 study concludes that at least 15 % is
necessary.
Life
The extinction of nearly all animal species at the end of the
Permian period allowed for the
radiation of many new lifeforms. In
particular, the extinction of the large
herbivorous and
carnivorous dinocephalia left those
ecological niches empty. Some were filled
by the surviving
cynodonts and
dicynodonts, the latter of which subsequently
became extinct.
Animal life during the
Mesozoic was dominated, however, by large
archosaurian reptiles that
appeared a few million years after the Permian extinction:
dinosaurs,
pterosaurs, and
aquatic reptiles such as
ichthyosaurs,
plesiosaurs, and
mosasaurs.
The climatic changes of the late Jurassic and Cretaceous provided
for further adaptive radiation. The Jurassic was the height of
archosaur diversity, and the first
birds and
placental mammal also appeared.
Angiosperms radiated sometime in the early
Cretaceous, first in the
tropics, but the
even temperature gradient allowed them to spread toward the poles
throughout the period. By the end of the Cretaceous, angiosperms
dominated tree floras in many areas, although some evidence
suggests that
biomass was still dominated by
cycad and
ferns until
after the KT extinction.
Some have argued that
insects diversified
with angiosperms because insect
anatomy,
especially the
mouth parts, seems particularly
well-suited for flowering plants. However, all major insect mouth
parts preceded angiosperms and insect diversification actually
slowed when they arrived, so their anatomy originally must have
been suited for some other purpose.
As the temperatures in the seas increased, the larger animals of
the early Mesozoic gradually began to disappear while smaller
animals of all kinds, including
lizards,
snakes, and perhaps the ancestor
mammals to
primates, evolved.
The KT extinction exacerbated this trend. The large archosaurs
became extinct, while birds and mammals thrived, as they do
today.
References
- British Mesozoic Fossils, 1983, The Natural History
Museum, London.
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