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AR103 - RN Ch#5 W8.pdf

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Jonathan Haxell

AR103 - Readings pgs. 105-115 - Week 8 Notes - Nov. 1, 2011. Chapter Five: Macroevolution: Processes of Vertebrate and Mammalian Evolution Pages # 105 -115 What Are Fossils and How Do They Form? - Fossils are traces or remains of organisms found in geological beds on the earth’s surface. - The oldest fossils date as far back to 3 billion years ago, because they are remains of microorganisms, they are extremely small and are called mircofossils. - After the organism died, these “hard” tissues were further impregnated with other minerals, being eventually transformed into a stone-like composition. This process is called mineralization. - Mineralization is the process in which parts of animals (or some plants) become transformed into stone-like structures. Mineralization usually occurs very slowly as water carrying minerals, such as silica or iron, seeps into the tiny spaces within a bone. In some cases, the original minerals within the bone or tooth can be completely replace, molecule by molecule, with other minerals. - Sometimes insects were trapped in tree sap, which later became hardened and chemically altered. Because inside hardened amber there was little or no oxygen, the insects have remained remarkably well preserved for millions of years, even with soft tissue and DNA still present. - Leaf imprints in hardened mud, or similar impressions of small organisms, or even the traces of dinosaur feathers are fossils, as well as dinosaur footprints and hominin tracks. - Whether a dead animal will become fossilized and how much of it will be preserved depends partly on how it dies, but even more on where it dies. - Taphonomyis the study of how bones and other materials come to be buried in the earth and preserved as fossils. - Taphonomists try to understand the processes of sedimentation, including the action of streams, preservation properties of bone, and carnivore disturbance factors. Vertebrate Evolutionary History: A Brief Summary - Geologists have formulated the geological time scale - Geological Time Scale is the organization of earth history into eras, periods, and epochs; commonly used by geologists and paleoanthropologists. - For the time span encompassing vertebrate evolution, there are three eras: the Paleozoic, the Mesozoic, and the Cenozoic. - The earliest vertebrate fossils date to early Paleozoic at 500 mya, and their origins are probably much older. - During the Paleozoic, several varieties of fishes, amphibians, and reptiles appeared. - At the end of the Paleozoic, close to 250 mya, several varieties of mammal-like reptiles were also diversifying. - The evolutionary history of vertebrates and other organisms during the Paleozoic and Mesozoic was profoundly influenced by geological events. - The positions of the earth’s continents have dramatically shifted during the last several hundred thousand million years ago. - This process is called the continental drift, and is explained by the geological theory of plate tectonics, which states that the earth’s crust is a series of gigantic moving and colliding plates. - Continental drift is the movement of continents on sliding plates of the earth’s surface. As a result, the position of large landmasses have shifted drastically during the earth’s history. 1 AR103 - Readings pgs. 105-115 - Week 8 Notes - Nov. 1, 2011. - Such massive geological movements can induce volcanic activity, mountain building, and earthquakes. - During the late Paleozoic, the continents came together to form a single colossal landmass called Pangea. - During the Mesozoic, the southern continents began to split off from Pangea, forming a large southern continent called Gondwanaland. - The northern continents were consolidated into northern landmass called Laurasia. - Gondwanaland and Laurasia continued to drift apart and to break up into smaller segments. - By the end of the Mesozoic the continents were beginning to assume their current positions. - Groups of animals became effectively isolated from each other by oceans significantly influencing the distribution of mammals and other land vertebrates. - These continental movements continued in the Cenozoic and indeed are still happening, without such dramatic results though. - During most of the Mesozoic, reptiles were the dominant land vertebrates, and they exhibited a broad expansion into a variety of ecological niches, which included aerial and marine habitats. - The most famous of these highly successful Mesozoic reptiles were the dinosaurs, which themselves evolved into a wide array of sizes and species and adapted to a variety of lifestyles. - Many dinosaurs were “warm-blooded” - Many species became extinct because of major climate changes to the earth’s atmosphere from collisions with comets or asteroids; not all dinosaurs became entirely extinct and have many descendants still living today. - The Cenozoic is divided into two periods: the Tertiary, about 63 million years in duration, and the Quaternary, from about 1.8 mya up to and including the present. - Epochs are categories of the geological time scale; subdivisions of periods in the Cenozoic era, including the Paleocene, Eocene, Oligocene, Miocene, and Pliocene (from the Tertiary) and the Pleistocene and Holocene (from the Quaternary). Mammalian Evolution - Studies using fossils and DNA of living species, suggests that all the living groups of mammals had diverged by 75 million years ago. - Later, only after several million years following the beginning of the Cenozoic, did the various current mammalian subgroups begin to diversify. - Cenozoic could be called the Age of the Mammals because during this era that, along with birds, mammals replaced earlier reptiles as the dominant land-living vertebrates. - Several characteristics relating to learning and general flexibility of behaviour are of prime importance to the growth of mammals in the Mesozoic and Cenozoic eras - To process more information, mammals were selected for larger brains than those typically found in reptiles. - The cerebrum became generally enlarged, especially the outer covering, the neocortex, which controls higher brain functions. In some mammals, the cerebrum expanded so much that it came to comprise most of the brain volume; the number of brain convolutions also increased, creating more surface area and thus providing space for even more neurons. - For such a large and complex organ as the mammalian brain to develop, a longer, more intense period of growth is required. Slower development can occur internally (in utero) as well as after birth. 2 AR103 - Readings pgs. 105-115 - Week 8 Notes - Nov. 1, 2011. - Other forms (fish reptiles birds) lay eggs, and “prenatal” development occurs externally, outside the mother’s body. Mammals with few exceptions, give birth to live young. - There’s considerable variation among the major groups in how mature the young are at birth, and in placental mammals, in utero development goes the farthest. - Placental is a type (subclass) of mammal. During the Cenozoic, placentals became the most widespread and numerous mammals and today are represented by upward of 20 orders including the primates. - A distinctive feature of mammals is the dentition. - Many living reptiles have similar shaped teeth, mammals have differently shaped teeth. - This varied pattern, termed heterodont dentition, is reflected in the ancestral mammalian arrangement of teeth, including 3 incisors, 1 canine, 4 premolars, and 3 molars in each quarter of the mouth. A total of 44 teeth in ancestral mammalian dental complement. - Heterodont is having different kinds of teeth; characteristic of mammals, whose teeth consists of incisors (cutting), canines (grasping), premolars (crushing), and molars (grinding). - The heterdont arrangement allows mammals to process a wide variety of foods. - Regarding teeth, their disproportionate representation in the fossil record. - As the hardest, most durable portion of a vertebrate skeleton, teeth have the greatest likelihood of becoming fossilized, since
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