The Jurassic and Cretaceous periods are known as the time of the dinosaurs. During these two vast eras of time the world was dominated by these giant reptiles. Some of the sauropod herbivores are the largest animals to have ever walked the planet, and the Tyrannosaurus Rex is the largest ever land predator. The periods also saw the development of flowing plants, birds and mammals, all of which are still alive and thriving today.
The start of the Jurassic period was signalled by the Triassic-Jurassic extinction event. With many species dying out, there were large ecological niches that would be filled over time. During the Jurassic the supercontinent Pangea broke up into two separate land masses, Laurasia in the North and Gondwana in the South. The climate at the time was much warmer than that of today, with no land at the North or South poles.
Marine crocodiles, dolphin-like ichthyosaurs and the plesiosaurs dominated the oceans. All reptilian, whales and dolphins would not enter the seas for millions of years to come. The Jurassic also saw a large increase in planktonic species.
During this time many of the ‘classic’ dinosaurs evolved. Long-necked herbivores such as Diplodocus and Apatosaurus were feeding from the abundant ferns, cyads and conifer trees. Their enormous size would have been a deterrent to predators and would also allow them to browse vegetation at levels that other animals could not reach. With such a long and flexible neck an animal such as Diplodocus would have been able to feed on almost all vegetation growing at the time. Large Tyrannorsaur-like therapods were also living at this time, possibly hunting sauropods. Little is known about the behavior of dinosaurs as so many fossil specimens are incomplete. During the late Jurassic the first birds evolved from smaller bipedal dinosaurs.
The Cretaceous period started around 145 million years ago and lasted until the extinction of the dinosaurs, around 65 million years ago. During the Cretaceous period the continents continued to break up and started to resemble that which we see today. South America, Antarctica and Australia all moved away from Africa forming the Atlantic and Indian Oceans. The planet continued to cool in temperature, although remaining much hotter than current climates. Tropical average temperatures would have averaged at around 37 degrees centigrade, with deep sea temperatures sitting around 20 degrees hotter than they do today.
The Cretaceous is notable for the spread of flowering plants, or angiosperms. Along with the development of flowers was the evolution of bees and other pollen-spreading insects. This flourishing ecosystem is an excellent example of where two organisms can greatly accelerate eachother’s development, commonly known as coevolution. Many leafy trees also began to show up during the Cretaceous, and plants started to look much more like they do in modern times.
The dinosaurs would continue to evolve and thrive during the Cretaceous period with many of the most well-known species flourishing during this period. Tyrannosaurus was the top land predator along with smaller bipedal dinosaurs such as Velociraptors. Pterosaurs would face increasing competition from birds and would dwindle significantly in numbers towards the end of the Cretaceous. Mammals were still restricted to smaller nocturnal creatures. At this point in the sea sharks and rays had fully developed and would remain the same even millions of years later.
The Cretaceous period was ended by a huge mass extinction event known as the Cretaceous-Tertiary extinction. It is thought that the extinction was brought on by a huge meteorite impact, the crater of which can be found on the Chicxulub coast in Mexico. The impact would have kicked up a huge dust cloud, blocking much of the sun’s light. The impact crater shows that the meteor struck the coastline and so would have caused massive tsunamis, which would have proved fatal to all animal life caught in them. Evidence of tsunamis is prevalent across the USA, with marine sand found a long distance in land. The impact produced a cloud of sulphur dioxide which would have caused reduced sunlight as well as acid rain. This would have killed plants and plankton, severely harming the ecosystems that depended on them.
The only survivors of the event were omnivores, insectivores and carrion-eaters. Ecosystems that were based on consuming detritus would have been able to survive, and mammals with their warm blood and diet of insects would have been able to live through events that killed the larger reptilian dinosaurs. Crocodiles also survived the extinction event, this is thought to be due to their ability to live as scavengers and go for months without food.
After this mass extinction killed off the vast majority of the dominant life-forms a vast number of ecological niches were left open for mammals and birds to fill. The next era is known as the Paleocene Epoch and would see the growth of mammals across the planet.
Once photosynthesising cells present in the oceans had produced enough oxygen to form the ozone layer conditions on land were much more favourable to life than before. Without exposure to ultraviolet radiation cells were much more likely to survive being washed ashore. After a major extinction event 488 million years ago plants and fungi started to grow in out and around the water. Genetic mutation and evolution would see life adapt to the new environment and go on to thrive. Arthropods are believed to be the first animals to appear on land, around 450 million years ago. These early creatures would have been able to feed from the new plants that were growing on land, albeit around the water’s edge.
440 million years ago there was another mass extinction that would kick start evolution. Extinctions are perhaps naively thought of as bad for life, being that most of it is killed off. While it is bad for the organisms alive at the time for obvious reasons, a mass extinction actually has a major benefit for evolution in the path to intelligent life. Every mass extinction to date has been followed by a surge in new species and evolution. The reason for this is that evolution can get into a rut relatively easily, without a mass extinction it is likely that life would stay the same with little adaptation. Extinctions force adaptations and so accelerate evolution.
The result of the extinction event at the end of the Ordovician period, around 440 million years ago, was fish evolving into land-dwelling tetrapods. As their fins evolved into limbs they adapted to breathing air and eventually would live their entire life on land, only returning to water to lay eggs. These animals were the first amphibians and are essentially the genesis of much of life as we know it. During this period plants would evolve seeds and would spread across land, sparking the full colonisation of land by life. In another 20 million years animals would have evolved the capability to lay amniotic eggs as well as having diverged into many distinct evolutionary paths. Insects, reptiles, fish and bacteria were all present at this point.
The next major event has become known as the Great Dying. The Permian-Triassic extinction event is the most severe in Earth’s history. It is estimated that around 96% of marine life and 70% of land dwelling vertebrates would die out. It is also the only known mass extinction of insects. The causes for this are thought to be numerous, including environmental change and a catastrophic event such as increased volcanism or meteorite impacts.
The result of this extinction event was the Triassic period and the start of the age of the dinosaurs. Extending from 251 million years ago until around 199 million years ago the Triassic age is bookmarked by two mass extinctions. During this time the first dinosaurs evolved and it is believed the first mammals, flying vertebrates and flowing plants evolved as well.
The most common land vertebrate during the early Triassic was the Lystrosaurus. It was a small reptilian herbivore about the size of a pig. It had a horny beak and tusks and accounted for 95% of the population of organisms at the time. The Lystrosaurus is notable for being the only species to dominate the planet in such a great number. It is thought that by surviving the Permian-Triassic extinction that they became free of predators and could reproduce with little resistance.
Seeding plants, conifers and cycads would go on to dominate and flourish in the plant world. This would lead to greater insect numbers and would allow greater biodiversity among vertebrates. Towards the end of the Triassic early dinosaurs had evolved along with pterosaurs in the skies and icthyosaurs in the oceans.
As dinosaurs grew in numbers they would force mammalian ancestors to live nocturnally on a diet of insects. It is thought that living in this way would lead mammals to evolve fur and their higher metabolic rates required to function in the cooler night time temperatures.
The Triassic period came to a close 199 million years ago during the Triassic-Jurassic extinction event. During this time around half the species on Earth died out, leaving vast ecological niches. These niches were quickly filled and gave rise to the domination of dinosaurs. The next two periods of time, the Jurassic and Cretaceous, would be inhabited by some of the largest animals that have ever lived and would set in motion the events that would lead to the evolution of modern mammals.
