In Jurassic Park, the dinosaurs cloned were from the Mesozoic Era.
250 Million Years Ago
Life on land was very well established. Plant life was varied and animal life had developed to fill many different areas of the environment. The largest creatures on land were the Mammal-Like Reptiles, some of which were huge. Predators as big as modern bears and plant-eaters almost as big as modern elephants began to roam the land. In the seas, life was flourishing as well with sharks and sea-dwelling reptiles, ammonites, and many kinds of fish and plants. Overall, life on Earth was exploding. But nature had other things in mind. Something happened, a catastrophic event, which killed off more than 80% of ALL the living things on the Earth. This included plants, fish, and land animals. Many, such as trilobites, disappeared forever. Some, such as sharks and Mammal-Like Reptiles, eventually came back.
Extinction events occur repeatedly throughout Earth's history, and there is much debate about the causes of specific extinctions. There seems to be general agreement that this particular event, because it was so widespread, was caused by massive geologic activities. This could have been a combination of volcanoes and earthquakes. Volcanoes on land would have created great clouds of dust and particles in the atmosphere that could block sunlight for years, killing plants and disrupting the food chain. Undersea volcanoes could have changed the chemistry of water, making it poisonous for sea life. Earthquakes could have changed the courses of rivers, cut off migration routes, and created other barriers that would have drastically changed the Earth's ecosystems. This was the greatest extinction event yet discovered in the history of our planet.
Just recently, scientists have discovered evidence that this extinction was caused by a comet impact, possibly in the single ocean that covered most of the Earth at the time. Evidence of this has been found in layers of rock dating from 250 million years ago. Speculation is that the comet struck the ocean and that the actual crater has disappeared due to plate tectonics.
225 Million Years Ago
Halfway into the Triassic Period things were going pretty well again. Life had made a comeback after the great extinction 25 million years earlier. Mammal-Like Reptiles were once again at the top of the food chain on land, but they were getting some competition from the ancestors of the crocodile family. Sea life, however, was undergoing another minor extinction event. A large number of species disappeared from the oceans at this time, but nothing on the scale of the extinction at the beginning of the Triassic. So what makes this point in time special? A new creature appeared on the scene, an animal that would change the balance of life on land for the next 160 million years - dinosaurs.
What made the dinosaurs different from the other land animals such as the Mammal-Like Reptiles and lizards? Most importantly, they were the first creatures to walk with their legs underneath their bodies. This gave them several advantages - greater speed, greater mobility, and the capability to support a great deal of weight. Dinosaurs are defined by the design of their skulls and their pelvises. These new designs allowed the speedy little hunters to start killing off the competition. They also ate the slower, bigger plant eaters. This created a hole in the environment for plant eaters, so some of the dinosaurs started to eat plants. Now there were speedy carnivores and herbivores. Some of the herbivores, rather than rely on speed to get away, started to get bigger as a form of protection. It wasn't long after this that the last of the Mammal-Like Reptiles disappeared.
205 Million Years Ago
This marked the beginning of the Jurassic and another extinction event was taking place. Up until this time, some of the top predators were the phytosaurs, the ancestors of crocodiles. This extinction killed off many of these creatures, which opened up the door for dinosaurs to quickly move into the top predatory spot in the ecosystem. There were also big plant-eating crocodile ancestors that died off and that created an even bigger opportunity for dinosaurs to assume a dominant role in Earth's ecosystems. In fact, it was at this time that dinosaurs really began to dominate just about every aspect of terrestrial life. Creatures such as Dilophosaurus grew to sizes never before seen on Earth for predatory creatures. And the plant eaters got bigger too! The environment was still pretty much the same as it had been for the previous 100 million years - hot and dry. The land, however, was still clumped together in a large landmass so animals could travel and spread out over most of the Earth's land surface.
180 Million Years Ago
Now we are well into the Jurassic and dinosaurs are extremely well established. Plateosaur-type dinosaurs were the dominant plant-eaters, dilophosaurs (crested dinosaurs) were the dominant meat-eaters. Large sea reptiles ruled the oceans.
Once again, however, another extinction event struck and all of these large creatures disappeared. The only larger creatures that survive were those that lived in freshwater swamps and rivers, such as turtles and crocodiles. The climate was still hot and dry though, so there weren't too many swamps and not many large species survived. The path had cleared, however, for the very biggest creatures the earth had ever seen to develop. It was at this time that the huge sauropods began to appear, as did the very large carnivores such as the Megalosaurs, Ceratosaur, and the Allosaurs. While the sauropods were eating a lot of the tall plant life, plated dinosaurs such as Stegosaurus developed and began to feed on vegetation lower to the ground. This second wave of dinosaur families ruled until the very end of the Jurassic period when another mass extinction hit.
140 Million Years Ago
This is the beginning of the Cretaceous period and a time of great geologic activity. One of the main events was the separation of Pangaeasauropods very hard. Many of the big sauropods died out completely, especially those that lived in the northern hemisphere. Megalosaurs were gone, as were the Ceratosaurs, and almost all the Allosaurs disappeared. New meat-eaters such as the big utahraptors, giganotosaurs, and spinosaurs appeared. In the north, new plant-eaters like the iguanodonts appeared. In the south, some of the huge sauropods still flourished along with carnivores like Carnotaurus. In the sea, changing ocean currents probably upset the balance of life and that also led to another great extinction in which the plesiosaurs were hit hard.
120 Million Years Ago
There were a number of mass extinctions throughout the Cretaceous. Plated herbivores disappeared completely and were replaced by Ankylosaurs with their clubbed tails and wide heads. The first duckbills appeared and the iguanodonts begin to disappear. More advanced raptors like Deinonychus begin to appear.
85 Million Years Ago
Some of the most well known dinosaurs begin to appear. Tyrannosaurs and Ceratopsians (horned dinosaurs) appear in great numbers. Nature seemed to perfect the predator with the Tyrannosaurus rex. All the other large predators disappeared, but the smaller, speedy raptors, Oviraptors and ostrich dinosaurs proliferate. Herds of Hadrosaurs and Triceratops roam North America. In the last part of the Cretaceous, long-necked Titanosaurs move into North America from Central America. A shallow sea covers much of the middle part of North America. In the southern hemisphere, Titanosaurs and huge predators like Giganotosaurus rule the land.
65 Million Years Ago
This is, by far, the most famous, and probably the most debated, extinction event in Earth's history. It marked the end of the dinosaurs, the most successful family of creatures to walk the earth (sharks are the most successful of the vertebrates). It has been well established that a large comet or asteroid hit the Earth off the coast of Central America at this time, and that it certainly would have had a global impact. Huge clouds of dust may have blocked sunlight over the entire Earth for more than a year. It is suspected that, because of the angle and location of the impact, about 75% of all life in North America would have been wiped out by a wave of intense heat. This percentage, however, would not necessarily have applied to other parts of the world. There were other events, though, taking place around the world that could also have led to the demise of dinosaurs. Around this same time there was a great deal of volcanic activity in what is now India, which was still drifting towards its current location, whereby hundreds of thousands of square miles of molten rock was pouring out of volcanoes into the sea. This could help explain why all of the marine reptiles disappeared along with the dinosaurs.
There are also other, more subtle factors to consider. Mammals were becoming more of a factor and may have discovered that dinosaur eggs were a tasty treat. And without eggs, there would have been no babies. Also, it is possible that viral and bacterial infections could have contributed to the end of the dinosaurs. Overall, the world was cooling - the poles were getting colder, oceans were cooling - and much was changing that would all have had an impact on all life on Earth. For whatever reason or combination of reasons, this marked the end of the dinosaurs and the beginning of the age of the mammals.