Tropes

Livescience_2013 01776.txt

#Cretaceous period: Facts About Animals, Plants & Climate The Cretaceous period was the last and longest segment of the Mesozoic era. It lasted approximately 79 million years from the minor extinction event that closed the Jurassic period about 145.5 million years ago to the Cretaceous-Paleogene (K-Pg) extinction event dated at 65.5 million years ago. In the early Cretaceous the continents were in very different positions than they are today. Sections of the supercontinent Pangaea were drifting apart. The Tethys Ocean still separated the northern Laurasia continent from southern Gondwana. Â The North and South Atlantic were closed still although the Central Atlantic had begun to open up in the late Jurassic period. By the middle of the period ocean levels were much higher most of the landmass we are familiar with was underwater. By the end of the period the continents were much closer to modern configuration. Africa and South america had assumed their distinctive shapes; but India had collided not yet with Asia and Australia was still part of Antarctica. Cretaceous period plants One of the hallmarks of the Cretaceous period was the development and radiation of the flowering plants. The oldest angiosperm fossil that has been found to date is Archaefructus liaoningensis found by Ge Sun and David Dilcher in China. Â It seems to have been most similar to the modern black pepper plant and is thought to be at least 122 million years old. It used to be thought that the pollinating insects such as bees and wasps evolved at about the same time as the angiosperms. It was cited frequently as an example of co-evolution. New research however indicates that insect pollination was established probably well before the first flowers. While the oldest bee fossil was trapped in its amber prison only about 80 million years ago evidence has been found that bee -or wasp-like insects built hive-like nests in what is called now the Petrified Forest. These nests found by Stephen T. Hasiotis and his team from the University of Colorado are at least 207 million years old. It is thought now that competition for insect attention probably facilitated the relatively rapid success and diversification of the flowering plants. As diverse flower forms lured insects to pollinate them insects adapted to differing ways of gathering nectar and moving pollen thus setting up the intricate co-evolutionary systems we are familiar with today. Cretaceous period animals During the Cretaceous period birds replaced the Pterosaurs in the air. The origin of flight is debated by many experts. In the oetrees down theory it is thought that small reptiles may have evolved flight from gliding behaviors. In the oeground up hypothesis flight may have evolved from the ability of small Theropods to leap high to grasp prey. Feathers probably evolved from early body coverings whose primary function at least at first was thermoregulation. At any rate it is clear that Avians were highly successful and became widely diversified during the Cretaceous. Confuciusornis (125 to 140 million years) was sized a crow bird with a modern beak but enormous claws at the tips of the wings. Iberomesornis a contemporary only the size of a sparrow was capable of flight and was probably an insectivore. Image Gallery: Avian Ancestors: Dinosaurs That Learned To fly By the end of the Jurassic the giant Sauropods such as Apatosaurus were becoming extinct. Evidence suggests that by the early Cretaceous they were being replaced by large herds of herbivorous Ornithischians such as Stegosaurus Iguanodon and the Ceratopsians. Theropods including Tyrannosaurus rex continued as apex predators until the end of the Cretaceous. K-Pg extinction event About 65 million years ago nearly all large vertebrates and many tropical invertebrates became extinct in what was clearly a geological climatic and biological event with worldwide consequences. Geologists call it the K-Pg extinction event because it marks the boundary between the Cretaceous and Paleogene periods. The event was formally known as the Cretaceous-Tertiary (K-T) event but the International Commission on Stratigraphy which sets standards and boundaries for the geologic time scale now discourages the use of the term Tertiary. The K is from the German word for Cretaceous Kreide. Â The Chicxulub crater in the Yucatan dates precisely to this time. The crater site is more than 1000 feet across and chemical analysis shows that the sedimentary rock of the area was melted and mixed together by temperatures consistent with the blast impact of an asteroid about 6 miles (10 km) across striking the Earth at this point. There is also evidence of a series of huge volcanic eruptions along the tectonic border between India and Africa that began just before the K-Pg event boundary. It is likely that these regional catastrophes combined to precipitate climatic change. Cretaceous period climate All over North america the highest Cretaceous fossils are found directly beneath a thin layer of sediments that contain an unusual amount of iridium an element otherwise uncommon in Earth s crust. Also within this layer are indications of oeshocked quartz and tiny glass-like globes called tektites that form when rock is vaporized suddenly then immediately cooled as happens when an extraterrestrial object strikes the Earth with great force. These remains are found as far north as Colorado. Temperatures would have reached levels high enough to cause flash fires in many locations and a tsunami inundated much of the Eastern United states. It is hypothesized that debris aerosols from the asteroid impact and the resulting fires may have combined with the ash and smoke from the continuing volcanic activity to significantly reduce global temperatures for a few decades. As a result of suddenly lowered temperatures there may have been a global disruption in the numbers of both land plants and plankton in the oceans evidence of which is part of the K-Pg fossil record. With such a crisis in the photosynthetic communities a cascading crisis would have toppled the food/energy pyramids extant at the time. This hypothesis accounts for the extinction of the largest animals of the time the dinosaurs and giant marine reptiles. It also explains how smaller animals such as the mammals and other animalsâ with more modest energy requirements managed to survive and diversify in the new Cenozoic era e

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