Chapter 2 The Age of Dinosaurs The very first dinosaurs were small bipedal carnivorous animals that first show up in the fossil record 238 million years ago during the Mesozoic Era This time span is also referred to as the Age of the Dinosaurs since dinosaurs were the dominant animals at this time but other animals existed as well There are three major divisions to the Mesozoic Era called periods o Triassic Period 251 200 million years ago The oldest dinosaurs show up midway o Jurassic Period 200 146 million years ago This was the time of the very largest through this era dinosaurs o Cretaceous Period 146 65 54 million years ago This was the final stage for the dinosaurs and the time when the T rex ruled The dinosaurs went extinct 65 54 million years ago at the end of the Cretaceous Period We know the numerical ages for bones coming from Mesozoic age sediments from analyses of the breakdown of radioactive isotopes found in sediments The isotopes decay at very specific rates The best isotope for dating rocks is Uranium 235 which breaks down into Lead 207 o Its half life the amount of time it takes for half of the original material to break down spans an appropriate amount of time to age things in millions of years Uranium 235 has a half life of 704 million years meaning that after 704 million years there will be 50 of the original Uranium 235 and the rest will have become Lead 207 The accuracy of this method is within the range of several million years so we don t know an animal s birth year or anything like that Climate in the age of the dinosaurs o The earth was a very different place during the age of the dinosaurs the worldwide It was arid in equatorial regions and much like Florida in middle latitudes climate was much warmer Close to the poles it was cool wet and rainy like Seattle or British Columbia today Because it was so much warmer there were no ice caps at the poles Consequently there was more free water in the oceans than there is today and a greater portion of the continents was covered by water Paradoxically the dinosaurs had much less land available to them then than we have available today Most of today s mountains had not developed and the land was flatter as a whole However many volcanoes existed sometimes choking the air with ash and contributing to the movement of the continental plates o The continents were not aligned as they are today in the Triassic Period all were adjoined into one super continent known as Pangea Throughout the age of the dinosaurs the continents moved apart this is known as plate tectonics The continental movements were driven by volcanic seafloor spreading in the oceans Dinosaur distributions have played a major role in determining the positions of continents during the Mesozoic Era Dinosaurs of the same species have been found both in the U S and Northern Africa providing evidence that the two continents abutted when they were part of Pangea o By the end of the Cretaceous Period the continents were in approximately the places o The splitting of the continents fueled the diversification of dinosaurs on various they are today continents Flora in the age of the dinosaurs from plants that live today o The types of plants available to dinosaurs for forage were for the most part different o Plant types also changed considerably during the Age of the Dinosaurs leading to diversification Some dinosaurs developed extremely bizarre dentitions in order to consume new kinds of plants o During the Triassic and Jurassic Periods low lying ferns horsetails and club mosses were common in wetter areas Large gymnosperms such as pine trees conifers cypress trees cycads ginkgos and others dominated the world s dryer ecosystems As a whole there was less ground cover than there is today o Fast growing disturbance tolerant angiosperms flowering plants like those dominant today showed up midway through the Cretaceous Period in arid equatorial regions They had diversified to all the world s ecosystems by the end of the Cretaceous Conversely the earlier plants declined The world became more vegetated with ground cover all over and closed forests of deciduous trees became abundant Grasses although they are a type of flowering plant were not very common during the time of the dinosaurs Living with dinosaurs o Crocodilians o ALL DINOSAURS WERE TERRESTRIAL LAND ANIMALS There is not even evidence of any of them being amphibians living on both water and land There were crocodiles and crocodilian relatives during the age of the dinosaurs but there not like those of today They were small agile terrestrial predators Some were even bipedal There were also crocodiles that lived completely in the water and had tails and fins The Deinosuchus terror crocodile was thirty two feet long and fed on large turtles and unsuspecting dinosaurs o Crocodiles such as this one became giant not by growing quickly but by growing at the same rate as living alligators They simply extended their growth phase for an extra twenty years o Turtles o Sea Monsters There were lots of turtles during this time The first ones from the Triassic period were fully terrestrial and even had teeth Turtles that look like the modern ones we see today flourished throughout the Jurassic and Cretaceous Periods There was even a thirteen and a half foot long sea turtle In the sea were ichthyosaurs reptiles that looked and swam like dolphins o Some were forty feet long and deep diving forms had gigantic eyes one foot in diameter o Like mammals they gave live birth o There were also the long necked plesiosaurs the supposed Loch Ness Monsters Their bodies were like those of sea turtles but their necks were long and snake like with up to eighty to ninety vertebrae Mammals like us have seven vertebrae o There there were mosasaurs the sea going relatives of living monitor lizards like the Komodo dragons They ate squid like animals called ammonites and other shelled creatures Some were fifty feet long and used primarily their tails for propulsion o There were also many kinds of fish present in the age of the dinosaurs o Pterosaurs There were no birds until the Late Jurassic Period but bat like predators abounded They were close relatives of the dinosaurs with bat like wings Their fourth fingers supported their wings and they had big sternums breast bones for the attachment of flight muscles indicating that they flapped instead of glided to fly They were covered in fur like mammals and had incredibly thin bones paper thin
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