Earth's Cretaceous Period

The Cretaceous Period is best known for the mass extinction event that marked its end, and also the end of the dinosaurs. It lasted from around 145 million years ago until 65 million years ago. Unlike many other periods in Earth's history, the Cretaceous did not begin following a disastrous mass extinction. Evolution, especially with respect to the dinosaurs smoothly transitioned from the Jurassic Period to the Cretaceous.

During the Cretaceous, the supercontinent of Pangaea continued to break up and it is during this time that the present-day continents formed. At the beginning of the Cretaceous, North America was already broken away, as was Eurasia. By the dawn of the Cretaceous, the giant southern continent of Gondwana was still intact, but by the end of the eon had fractured into the more familiar continents of Africa, Australia, Antarctica, and South America. The rifting of Gondwana caused huge mountain ranges to push up along the sea bed, thus raising sea levels worldwide. With its breakup, the Southern Atlantic and Indian Oceans were formed. The climate of the Cretaceous Period was considerably warm, though drier than the Jurassic Period.

Plant life on Earth went through a dramatic new phase of evolution. The first flowering plants, or angiosperms appeared on the newly formed continents. These flowers formed a symbiotic relationship with insects that transported pollen from male and female plants, thus facilitating reproduction. This form of reproduction was so effective that angiosperms began to replace conifers as the dominant form of plant life on Earth.

Giant pterosaurs were still common in the Cretaceous and reached their largest size. Some species boasted a wingspan of up to forty feet across and had a body the size of a man. However, pterosaurs were encountering increased competition from birds, which had evolved from small, light-boned dinosaurs. The improved maneuverability of the smaller birds, as well as their hardier wing-construction based around feathered forelimbs allowed them to flourish in the Cretaceous skies.

Sharks and large marine reptiles continued to control the top of the ocean food chains. Giant marine reptiles called mosasaurs were fearsome predators and could grow to lengths of 55 feet. The dolphin-like ichthyosaurs unfortunately could not survive competition from new kinds of fish in the ocean and this family of marine reptiles died out as the Cretaceous wore on. Crocodiles continued to evolve and the first freshwater crocodiles prowled inland lakes and rivers, much in the same fashion as modern crocodiles and alligators. As with earlier periods, ocean life was dominated by smaller creatures such as corals, brachiopods, ammonites, and other mollusks. Cephalopods such as squid continued to evolve and thrive throughout the warm Cretaceous seas.

Insects began to evolve into their present-day groups, including social insects such as ants and termites that we know today. Insects were able to diversify with the coming of flowering plants. This development opened up a brand new ecological niche that affected food chains from the very lowest level.

Dinosaurs continued to rule the land, although many groups that were important in the Jurassic were beginning to die out. The giant sauropods like brachiosaurus and diplodocus gave way to much smaller and less-widespread sauropods. In their place, dinosaurs of the ornithopod type began to dominate the higher level herbivorous food chains. These dinosaurs included giants like the duck-billed dinosaurs and iguanadons. Therapods continued to diversify into a range of shapes and forms. Smaller, more agile pack hunters such as deinonychus hunted alongside giants like the gorgosaurus and tyrannosaurus rex. As the smaller hunters continued to evolve, so did their bird counterparts, creating an intense avian rivalry with the pterosaurs, which they would eventually drive to extinction. As the birds evolved, so did the modern groups of mammals (placentals, marsupials, and monotremes). At this point they were still small and low on the food chains, but with the passing of the dinosaurs, they would start to make it big on Earth.

The Cretaceous came to a violent end in which a mass extinction wiped out the majority of the large life in the oceans and on the continents. About one half of all animal families died out in what is called the Cretaceous-Tertiary Extinction, including the dinosaurs, most marine reptiles, several groups of birds, and the ammonite cephalopods. Mammals and birds were two groups that were hardy and unspecialized enough to survive the extinction, which would ensure their supremacy over the Earth for the next 65 million years. There is no definitive proof to explain any one hypothesis accounting for the cause of the Cretaceous-Tertiary Extinction, but there is evidence supporting a number of them. Geological evidence of an asteroidal impact crater in the Yucatan Peninsula helps to support the contention that a cosmic collision caused catastrophic conditions for life on Earth and helped cause the extinction. Other hypotheses include an increase in volcanic activity that poisoned the atmosphere, radiation from a nearby supernova that did the same, and multiple asteroidal impacts that caused a giant dust cloud that blotted out the sun. Some scientists believe that a combination of any of these possible events could have caused the mass extinction.