The Cretaceous Period
The Cretaceous period or the chalk period as it is referred to in many languages, began after the Jurassic period marking the end of the age of the dinosaurs. This period began 145.5 million years ago and it ended 65.5 million years ago. This period was succeeded by the paleogene period and it was the longest period of the Phanerozoic Eon. The end of the Cretaceous period marked the boundary between the Mesozoic and Cenozoic eras. In this period, there was a relatively warm climate with a high eustatic sea level. The land was populated by dinosaurs and the water bodies by extinct marine reptiles, rudists and ammonites. During the cretaceous period, there were huge carnivores such as Tyrannosaurus rex, triceratops, giganotosaurus among many others. Big and small dinosaurs lived in vast forests of ferns, cycads and conifers.
Marine life in The Cretaceous Period was characterized by rays, modern sharks and teleosts. The marine reptiles were itchyosaurs or fish lizards in the early and mid-Cretaceous period. These later became extinct during the late Cretaceous period and mosasaurs appeared in the late Cretaceous. By the end of the Cretaceous period, the flying reptiles known as pterosaurs had become huge like most of their relatives on land and in the sea. The flying reptiles were gradually crowded out of the sky by the first prehistoric birds which had evolved from land dwelling dinosaurs.
Some of the events that took place during the Cretaceous period were very important to life on earth. Some of those included the development of anthophyta or flowering plants also known as angiosperms. These appeared in the lower Cretaceous at around 125 million years ago. The flowering plants first radiated in the middle Cretaceous which was about 100 million years ago. By the end of the Cretaceous, there were various forms that had evolved which modern botanists would recognize. Around the same time, many modern groups of insects began spreading such as the oldest ants and butterflies. Other insects included aphids, gall wasps, eusocial bee termites and grasshoppers. The eusocial bee played a major role in the evolution and ecology of angiosperms. The first radiation of the diatoms in the oceans was also seen during the Cretaceous period.
During this period, the late Paleozoic to early Mesozoic, the super continent of Pangaea completed its tectonic breakup into today’s continents. However their positions were significantly different from the present day. In the beginning of The Cretaceous Period, the earth had essentially assembled into two continents, Gondwana in the south and Laurasia in the north. The two continents were almost completely separated by the equatorial Tethys seaway and the different segments if Gondwana and Laurasia had already began drifting apart. North America had started to pull away from Eurasia during the Jurassic and South America had begun to drift away from Africa and so was India, Australia and
Antarctica. At the end of the period, Australia remained connected to Antarctica and India became adrift from the Indian Ocean. Madagascar drifted away from Africa in the late Cretaceous period and North America separated from Greenland. One of the key factors that influenced the paleogeography of this period is the high sea levels that existed during most of the Cretaceous. The high sea levels were caused by the displacement of water in ocean basins by the expansion of the mid oceanic ridges. There were three major geographic subdivisions including the southern boreal, the northern boreal and the Tethyan region. The Tethyan region is what separated the northern boreal from the southern boreal and it was characterized by corals, larger foraminiferans, fossilized reef forming rudist bivalve and some ammonites that lived in the Tethyan waters.
Scientists believe that an asteroid impact caused the great extinction, but there are other theories including high volcanism and climate changes that are believed to have caused it. At the end of The Cretaceous Period, there was an abrupt major extinction of many species including dinosaurs. Many forms of life that had played key ecologic roles for many years disappeared. The only things that survived were birds, small reptiles, mammals and many marine creatures. Flowering plants suffered greatly during this extinction period and with most of the species disappearing, the base of the food chain declined significantly causing larger organisms to collapse as well. Most of the species that survived were not dependent on plant life. Insects fed on dead organisms and mammals would feed on insects. This allowed most mammals to survive to this day and they replaced dinosaurs as the dominant species.