Project Relife: 2x Isekai System

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.

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.

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.

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.

About 65.5 million years ago, nearly all large vertebrates and many tropical invertebrates became extinct in what was clearly a geological, climatic and biological event with worldwide consequences. Geologists call it the K-Pg extinction event because it marks the boundary between the Cretaceous and Paleogene periods. The event was formally known as the Cretaceous-Tertiary (K-T) event, but the International Commission on Stratigraphy, which sets standards and boundaries for the geologic time scale, now discourages the use of the term Tertiary. The "K" is from the German word for Cretaceous, Kreide.

In 1979, a geologist who was studying rock layers between the Cretaceous and Paleogene periods spotted a thin layer of grey clay separating the two eras. Other scientists found this grey layer all over the world, and tests showed that it contained high concentrations of iridium, an element that is rare on Earth, but common in most meteorites, Kruk said in a class she co-taught on Coursera.org.

Also within this layer are indications of "shocked quartz" and tiny glass-like globes called tektites that form when rock is suddenly vaporized then immediately cooled, as happens when an extraterrestrial object strikes the Earth with great force.

The Chicxulub (CHEEK-sheh-loob) crater in the Yucatan dates precisely to this time. The crater site is more than 110 miles (180 kilometers) in diameter and chemical analysis shows that the sedimentary rock of the area was melted and mixed together by temperatures consistent with the blast impact of an asteroid about 6 miles (10 km) across striking the Earth at this point.

When the asteroid collided with Earth, its impact triggered shockwaves, massive tsunamis and sent a large cloud of hot rock and dust into the atmosphere, Kruk said. As the super-heated debris fell back to Earth, they started forest fires and increased temperatures.

"This rain of hot dust raised global temperatures for hours after the impact, and cooked alive animals that were too large to seek shelter," Kruk said in the class. "Small animals that could shelter underground, underwater, or perhaps in caves or large tree trunks, may have been able to survive this initial heat blast."