The Phanerozoic eon
This post will teach you about the Phanerozoic eon, the “Age of Life”. There are three eras in the Phanerozoic eon, the Paleozoic, Mesozoic, and Cenozoic eras. This post will only discuss two of the eras, the Mesozoic and the Cenozoic.
Earth’s history is split into two large periods, called eons. The two eons are called the Cryptozoic and the Phanerozoic. Eons are split into smaller time periods, called eras. Eras are split up even more, into periods. Some periods are split into even smaller time periods, called epochs.
This post will cover 255 million years of Earth's history, from the beginning of the Mesozoic era to the present day.
The Mesozoic era
The Triassic period started 255 million years ago and ended 200 million years ago. The period is divided into three epochs, Early, Middle, and Late. During this period, all of Earth’s land was gathered into a supercontinent called Pangaea.
Land
One of the biggest land animals of the Triassic period was Postosuchus, an archosaur. Archosaurs were the direct ancestors of dinosaurs. Today’s crocodiles are the last surviving archosaurs.
Coelophysis was one of the earliest dinosaurs. Hundreds of well-preserved skeletons have been found at Ghost Ranch, a fossil quarry in New Mexico. At the start of the Triassic period, there were no dinosaurs.
During the Triassic, there were no flowering plants and no grass, but there were huge trees. In the Petrified Forest, Arizona, fossilized trees dot the ground. They were originally 2000 feet (60 metres) high.
Sky
Flying reptiles called pterosaurs ruled the sky during the Mesozoic era. Pterosaurs first appeared in the late Triassic. One of the earliest pterosaurs was Peteinosaurus, which was barely larger than a modern crow.
The Jurassic period
The Jurassic period lasted from 200 million years ago to 145 million years ago. It is divided into three epochs, Early, Middle, and Late. During this time, Pangaea split into two smaller supercontinents.
Land
During the Jurassic period, dinosaurs reached enormous sizes. New species evolved, including huge long-necked sauropods and the large meat-eating Allosaurus.
Sauropods were huge huge dinosaurs, some of them reaching as long as a tennis court! Scientists think that some sauropods sound like a whip with their tail. A sauropod named Diplodocus swallowed stones (called gizzard stones) to help break down tough plant matter. Some sauropods could even live to be 100 years old!
Sea
During the Jurassic, there were giant monsters living in the seas, which included plesiosaurs and ichthyosaurs. These monsters were discovered by 12-year-old Mary Anning, whose first discovery was in 1811.
Her first discovery was a reptile named Ichthyosaurus. Its skull was lizard-like, but its teeth were like a crocodile. Its legs were flippers, and the tail was like a fast-swimming shark’s.
Her second discovery was a reptile named Plesiosaurus. It had a long neck and flippers like that of a sea lion. Inside the skeleton’s stomach area was hundreds of little hooks from squid, which showed Plesiosaurus’s diet.
Sky
Mary Anning also discovered a dactyl named Dimorphodon. It was the first dactyl ever dug up. Dimorphodon’s wings were like those of a bat. It could swoop down and snatch squid and fish from the sea.
Pterosaurs were still around, and they were getting bigger. Some pterosaurs had fur, like modern bats. They had large eyes, giving them great vision.
The Cretaceous period
The Cretaceous period lasted from 145 million years ago to 66 million years ago. It had lots of dinosaurs, including Triceratops and Tyrannosaurus rex. By then, Pangaea had split up even more, into the continents we recognize today. Flowering plants also appeared. The period is divided into two epochs, Early and Late.
Land
During the Cretaceous period, there were hundreds of amazing dinosaurs, including tyrannosaurs, hadrosaurs, ankylosaurs, and small predators such as Troodon. The first mammals appeared during this period.
The most famous tyrannosaur was Tyrannosaurus rex. It was one of the biggest land predators of all time, the largest skeleton reaching 42 feet (12.8). Its teeth were as large as bananas. They were not meant for slicing through flesh, but for crushing bone. Its main prey was a frilled dinosaur named Triceratops.
Hadrosaurs were large plant-eaters. Most of the hadrosaurs had colourful crests on their head, possibly for mating. Hadrosaurs had very toothy skulls; they had more than 1,000 teeth at any time! There were no teeth at the front of the mouth. Instead there was a beak for stripping leaves from branches. Most hadrosaurs travelled in herds.
Ankylosaurs were large armoured dinosaurs. They had large bony plates fused along their backs. They even had armour on their eyelids! Only their underside was unprotected. Some even had bony clubs at the ends of their tails. When it was attacked, an ankylosaur would lie down on its stomach to protect the unprotected area of its body. Others would swing their club at the attacker.
During the Mesozoic era, mammals were small and nocturnal so that they could hide from dinosaurs. One of the early mammals was Docodon. It probably raided dinosaur nests when it had the chance.
Sky
Pterosaurs reached their largest sizes during the Cretaceous. Tropeognathus and Quetzalcoatlus were huge monsters. Tropeognathus had a wingspan of 20 feet, and it only weighed as much as a seven-year-old child. Quetzalcoatlus had a wingspan of 40-46 feet. It was the largest of all pterosaurs.
But pterosaurs were having competition from the early birds. When pterosaurs went extinct, birds took over the skies. Birds are descended from dinosaurs.
The Extinction
The dinosaurs ruled for over 170 million years, but at the end of the Cretaceous, about 66 million years ago, a mass extinction killed them off. No land animal larger than a dog survived. Pterosaurs also went extinct.
The Cenozoic era
The Tertiary period
The Tertiary period lasted from 66 million years ago to 1.8 million years ago. Mammals survived the extinction that ended the Cretaceous period, and now that the dinosaurs were gone, mammals started evolving into bigger forms to occupy the gaps that dinosaurs had opened.
Now that the large meat-eating dinosaurs were gone, mammals evolved into lions, wolves, dingoes and other species to take their place. Now that plesiosaurs and ichthyosaurs had left the ocean, whales and sharks took over. The mammals were now ruling the Earth.
In the sea, there were hundreds of species. Possibly the biggest carnivore in history was megatooth shark, a huge shark that could swallow a grown man whole. Its jaws were almost 5 times bigger than the living Great White shark.
On land, wooly mammoths and saber-tooth tigers evolved. Wooly mammoths survived to the last Ice Age, but went extinct when early humans appeared and started hunting them. Humans, apes, gorillas, and other species were primates. Primates were smarter than other animals. In time, they learned how to make tools and start fires. Today, there are many species of primates.
Mammals were ruling the Earth, but that didn’t mean that there weren't other animals. Reptiles, birds, amphibians, and others share the planet with the mammals. Amphibians can live in or out of water, which helps them to hide from predators. Birds are the descendants of the dinosaurs, and they rule the sky, along with bats. After the extinction of the dinosaurs, billions of species evolved on land, including us. If the dinosaurs hadn’t gone extinct, we wouldn’t be here.
The Quaternary period
The Quaternary period lasts from 1.8 million years ago to the present day. Over this period, dozens of species of human have evolved and gone extinct, ending with Homo sapiens, us.
Our species appeared about 200,000 years ago, and over time, we learned to make tools and fire. We learned to make huts out of reeds or logs. And then, a couple hundred years ago, we learned to make skyscrapers, ships, cars, and we also learned how to harness electricity.
We are the only species of human alive, but we are not the only primates alive. Apes, gorillas, orangutans, and other primates are still alive today.
In the seas live whales, sharks, fish, squid, and hundreds of others. On land, there are thousands of species, including lions, birds, elephants, tigers, leopards, and us.
But we are causing thousands of animals to go extinct, such as the dodo and the Tasmanian tiger. Scientists estimate that 140,000 species go extinct each year. If we keep this up, scientists estimate that by 2100 25% of all animals will be extinct.
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