During the Mesozoic, or "Middle Life" Era, life diversified rapidly and giant reptiles, dinosaurs and other

monstrous beasts roamed the Earth. The period, which spans from about 252 million years ago to about 66
million years ago, was also known as the age of reptiles or the age of dinosaurs.

BOUNDARIES
English geologist John Phillips, the first person to create the global geologic timescale, first coined the term
Mesozoic in the 1800s. Phillips found ways to correlate sediments found around the world to specific time
periods, said Paul Olsen, a geoscientist at the Lamont-Doherty Earth Observatory at Columbia University in
New York. 
The Permian-Triassic boundary, at the start of the Mesozoic, is defined relative to a particular section of
sediment in Meishan, China, where a type of extinct, eel-like creature known as a conodont first appeared,
according to the International Commission on Stratigraphy. 
The end boundary for the Mesozoic Era, the Cretaceous-Paleogene boundary, is defined by a 20-inch (50
centimeters) thick sliver of rock in El Kef, Tunisia, which contains well-preserved fossils and traces of iridium
and other elements from the asteroid impact that wiped out the dinosaurs. The Mesozoic Era is divided up into
the Triassic, Jurassic, and Cretaceous periods.

LIFE AND CLIMATE

The Mesozoic Era began roughly around the time of the end-Permian extinction, which wiped out 96 percent of
marine life and 70 percent of all terrestrial species on the planet. Life slowly rebounded, eventually giving way
to a flourishing diversity of animals, from massive lizards to monstrous dinosaurs.
The Triassic Period, from 252 million to 200 million years ago, saw the rise of reptiles and the first dinosaurs,
the Jurassic Period, from about 200 million to 145 million years ago, ushered in birds and mammals, and
the Cretaceous Period, from 145 million to 66 million years ago is known for some of its iconic dinosaurs, such
as Triceratops and Pteranodon.
Coniferous plants, or those that have cone-bearing seeds, already existed at the beginning of the era, but they
became much more abundant during the Mesozoic. Flowering plants emerged during the late Cretaceous
Period. The lush plant life during the Mesozoic Era provided plenty of food, allowing the biggest of the
dinosaurs, such as the Argentinosaurus, to grow up to 80 tons, according to a 2005 study in the journal Revista
del Museo Argentino de Ciencias Naturales.  
Earth during the Mesozoic Era was much warmer than today, and the planet had no polar ice caps. During the
Triassic Period, Pangaea still formed one massive supercontinent. Without much coastline to moderate the
continent's interior temperature, Pangaea experienced major temperature swings and was covered in large
swaths of desert. Yet the region still had a belt of tropical rainforest in regions around the equator, said
Brendan Murphy, an earth scientist at St. Francis Xavier University in Antigonish, Canada.

EXTINCTIONS
The Mesozoic Era was bookended by two great extinctions, with another smaller extinction occurring at the
end of the Triassic Period, Olsen said.
Around 252 million years ago, the end-Permian extinction wiped out most life on Earth over about 60,000
years, according to a February 2014 study in the journal Proceedings of the National Academy of
Sciences (PNAS). At the end of the Triassic Period, roughly 201 million years ago, most amphibious creatures
and crocodile-like creatures that lived in the tropics were wiped out. About 65 million years ago, a giant
asteroid blasted into Earth and formed a giant crater at Chicxulub in the Yucatan Peninsula. 
Because the fossil record is incomplete, it's difficult to say exactly what caused the extinctions, or even how
rapidly they occurred. After all, certain species or traces of catastrophic events could be missing in the fossil
record simply because the sediments may have disappeared over tens of millions of years, Olsen said.
"Nature is very efficient at getting rid of its corpses," Olsen told Live Science.
However, there are a few prime suspects in each of the extinctions.
At the end of the Permian, the Siberian Traps underwent massive volcanic eruptions, which most
geologists believe caused the world's biggest extinction. Exactly how, however, is up for debate.
The volcanic eruptions caused a spike in carbon dioxide in the atmosphere, though the 2014 PNAS study
suggests that the spike was brief. The eruptions may have increased sea surface temperatures and led to
ocean acidification that choked out sea life. And another study published in March 2014 in PNAS proposed that
the eruptions released huge troves of the element nickel, which fueled a feeding frenzy by nickel-munching
microbes known as Methanosarcina. Those microbes may have belched out huge amounts of methane,
superheating the planet.
Most scientists agree that an asteroid impact wiped out the dinosaurs at the end of the Cretaceous Period. The
impact would have kicked up so much dust that it blocked the sun, halted photosynthesis, and led to such a
huge disruption in the food chain that everything that wasn't a scavenger or very small died.
But the Deccan Traps, in what is now India, were spewing massive amounts of lava both before and after the
asteroid impact, and a few scientists believe these flows either directly caused or accelerated the dinosaurs'
demise.
Volcanism may also be to blame for the end-Triassic extinction. Though volcanism in general leads to global
warming, after an initial volcanic eruption, huge amounts of sulfur spew into the air and cause a brief period of
global cooling. Such cooling-heating cycles may have occurred hundreds of times over 500,000 years. Similar
cold snaps have been tied to huge crop failures in historical times, such as in Iceland in the 1700s, Olsen said.
As a result, animals used to constant, balmy temperatures in the tropics were wiped out, while animals that
were insulated with proto-feathers, such as pterosaurs, or that lived at higher latitudes and were already
adapted to big temperature variations, did just fine, Olsen said.
 "When you have these volcanic winters, where temperatures may have dropped even below freezing in the
tropics, it was devastating," Olsen said.

Mesozoic means "middle animals," and is the time during which the world fauna changed drastically from that which
had been seen in the Paleozoic. Dinosaurs, which are perhaps the most popular organisms of the Mesozoic, evolved in
the Triassic, but were not very diverse until the Jurassic. Except for birds, dinosaurs became extinct at the end of the
Cretaceous. Some of the last dinosaurs to have lived are found in the late Cretaceous deposits of Montana in the United
States.
The Mesozoic was also a time of great change in the terrestrial vegetation. The early Mesozoic was dominated by ferns,
cycads, ginkgophytes, bennettitaleans, and other unusual plants. Modern gymnosperms, such as conifers, first appeared
in their current recognizable forms in the early Triassic. By the middle of the Cretaceous, the earliest angiosperms had
appeared and began to diversify, largely taking over from the other plant groups

Mesozoic fossil localities

 Blue Nile Gorge, Ethiopia: Come along on a fossil-hunting trip to Ethiopia with UCMP researchers and see the
first dinosaur fossils found there.

 Clayton Lake, New Mexico: This Cretaceous site has some of the most extensive and best preserved dinosaur
trackways in the United States.

 Ischigualasto, Argentina: The best-known and best-preserved early dinosaurs come from this Triassic locality in
South America.

 Pt. Loma Formation, California: This Cretaceous locality has yielded important fossils for understanding
western North American dinosaurs.

 Karoo Basin, South Africa

 Solnhofen Limestone, Germany: Exquisitely detailed fossils have come from these Jurassic deposits in southern
Germany.

. https://www.livescience.com/29231-cretaceous-period.html

The Cretaceous Period was the last and longest segment of the Mesozoic Era. It lasted approximately 79
million years, from the minor extinction event that closed the Jurassic Period about 145.5 million years ago to
the Cretaceous-Paleogene (K-Pg) extinction event dated at 65.5 million years ago. In the early Cretaceous, the
continents were in very different positions than they are today. Sections of the supercontinent Pangaea were
drifting apart. The Tethys Ocean still separated the northern Laurasia continent from southern Gondwana. The
North and South Atlantic were still closed, although the Central Atlantic had begun to open up in the
late Jurassic Period. By the middle of the period, ocean levels were much higher; most of the landmass we are
familiar with was underwater. By the end of the period, the continents were much closer to modern
configuration. Africa and South America had assumed their distinctive shapes; but India had not yet collided
with Asia and Australia was still part of Antarctica.

One of the hallmarks of the Cretaceous Period was the development and radiation of the flowering
plants. The oldest angiosperm fossil that has been found to date is Archaefructus liaoningensis, found
by Ge Sun and David Dilcher in China. It seems to have been most similar to the modern black
pepper plant and is thought to be at least 122 million years old. It used to be thought that the
pollinating insects, such as bees and wasps, evolved at about the same time as the angiosperms. It
was frequently cited as an example of co-evolution. New research, however, indicates that insect
pollination was probably well established before the first flowers. While the oldest bee fossil was
trapped in its amber prison only about 80 million years ago, evidence has been found that bee- or
wasp-like insects built hive-like nests in what is now called the Petrified Forest in Arizona. These
nests, found by Stephen Hasiotis and his team from the University of Colorado, are at least 207
million years old. It is now thought that competition for insect attention probably facilitated the
relatively rapid success and diversification of the flowering plants. As diverse flower forms lured
insects to pollinate them, insects adapted to differing ways of gathering nectar and moving pollen thus
setting up the intricate co-evolutionary systems we are familiar with today.
There is limited evidence that dinosaurs ate angiosperms. Two dinosaur coprolites (fossilized
excrements) discovered in Utah contain fragments of angiosperm wood, according to an unpublished
study presented at the 2015 Society of Vertebrate Paleontology annual meeting. This finding, as well
as others, including an Early Cretaceous ankylosaur that had fossilized angiosperm fruit in its gut,
suggests that some paleo-beasts ate flowering plants. 
Moreover, the shape of some teeth from Cretaceous animals suggests that the herbivores grazed on
leaves and twigs, said Betsy Kruk, a volunteer researcher at the Field Museum of Natural History in
Chicago. 
Cretaceous Period animals
During the Cretaceous Period, more ancient birds took flight, joining the pterosaurs in the air. The
origin of flight is debated by many experts. In the “trees down” theory, it is thought that small reptiles
may have evolved flight from gliding behaviors. In the “ground up” hypothesis flight may have evolved
from the ability of small theropods to leap high to grasp prey. Feathers probably evolved from early
body coverings whose primary function, at least at first, was thermoregulation. At any rate it is clear
that avians were highly successful and became widely diversified during the
Cretaceous. Confuciusornis (125 million to 140 million years ago) was a crow-size bird with a modern
beak, but enormous claws at the tips of the wings. Iberomesornis, a contemporary, only the size of a
sparrow, was capable of flight and was probably an insectivore. By the end of the Jurassic, some of
the large sauropods, such as Apatosaurus and Diplodocus, went extinct. But other giant sauropods,
including the titanosaurs, flourished, especially toward the end of the Cretaceous, Kruk said. Large
herds of herbivorous ornithischians also thrived during the Cretaceous, such as Iguanodon (a genus
that includes duck-billed dinosaurs, also known as hadrosaurs), Ankylosaurus and the ceratopsians.
Theropods, including Tyrannosaurus rex, continued as apex predators until the end of the Cretaceous.