EASC 218: Introduction To Palaeontology PDF

EASC 218: INTRODUCTION TO PALAEONTOLOGY References 1. Clarkson, E.N.K. (1993). Invertebrate Palaeotology and Evolution. Chapman and Hall, UK 2. Shrock, R.R. & Twenhofel, W.H. (1953). Principles of Invertebrate Palaeontology. McGraw-Hill, New York, London 3. Raup, D...

EASC 218: INTRODUCTION TO PALAEONTOLOGY References 1. Clarkson, E.N.K. (1993). Invertebrate Palaeotology and Evolution. Chapman and Hall, UK 2. Shrock, R.R. & Twenhofel, W.H. (1953). Principles of Invertebrate Palaeontology. McGraw-Hill, New York, London 3. Raup, D.M., & Stanley, S.M (1978). Princilpes of Palaeontology. W. H. Freeman & Co., San Francisco 4. Woods, H. (1958).Palaeontology Invertebrate. Cambridge Univ Press, London  Definition of a fossil. The word fossil comes from the Latin word ‘fossilis’ meaning ‘dug up’. In the early 1800’s when geology was becoming a distinct science, the term fossil was applied to almost any object that was found or dug up. Today most geologists restrict the definition of a fossil to include only the naturally preserved remains or traces of plant and animals that lived in the geologic past.  Perhaps one of the most confusing aspects of the definition of a fossil concerns the geologic past. Many geologist define the geologic past as being any time prior to recorded human history, whereas others set the dividing line between past and present at the beginning of the Holocene epoch, or about 11,000 yrs ago. Such a restriction is unwarranted for the following reasons: 1. a measured age cannot be assigned to the rocks containing the fossil under consideration.  2. there is nothing in the fossilization process relating directly to the passage of time. 3. because there are variations in the animal’s hard parts and composition as well as the chemical parameters of the depositional environments. The vast majority of fossils are found in sedimentary rocks. The concentration and distribution of fossils vary markedly, both vertically and horizontally, within sedimentary units. In some cases the entire sedimentary unit may be composed of fossils; a good example is coquina limestone.  Preservation of plants and animals as fossils are enhanced by two favourable conditions. 1. the organisms possession of a durable internal or external hard part. 2. the organisms rapid burial within the sedimentary environment. Modes of fossil preservation 1. Preservation without alteration. Mummification and entrapment of organisms.  Hard parts – Unaltered shells, bones, teeth, most commonly of calcium carbonate, calcium phosphate, silica, and chitin, cellulose and wood. 2. Preservation with alteration – Leaching, – Carbonaization – Permineralization – Recrystalization – Replacement.  Molds and Casts – Molds: removal by dissolution of organic material buried in sediments: void left in the rock is a mold. Mold can be internal or external. – Casts: Filling of a mold with sediments or mineral material, thus preserving the shape (internal or external) of the organic feature.  4. Trace fossils. – Tracks and trails: foot prints of animals and birds – Burrows: excavations by worms, clams etc – Borings: drill holes bored by shells, snails etc – Coprolites: fossilized animal excrement, may give you evidence of diet, animal size, and habitat. – Chemical fossils: Trace of organic acids such as are found in Precambrian sediments.  Palaeontology is the area of geological study that deals with the examination and interpretation of fossils. The palaeontologist uses fossils in different ways. The most important ways are: 1. Field identification of fossils as an aid in geological mapping. 2. Fossils as indicator of past environments. 3. Study of fossils taxonomic classification and evolutionary development to gain a better understanding of the role as an indicator of geologic time. USE OF FOSSIL ASSEMBLAGE IN AGE DETERMINATION One of the most important task in historical geology is the determination of ages of rocks on the earths surface. Since a high percentage of these surface rocks are sedimentary and fossiliferous, the relative age of such rocks can be determined by some techniques, once a series of index fossils have been identified.  1. The phylum level of each fossil must be identified. Next the specific name of the individual fossils must be determined. 2. Once the genus and species is known, the geologic range for the fossil may be plotted on a geologic time diagram. Each fossil must be plotted separately. 3. Three fundamental assumptions are made in age determination. a. that all the fossils have been collected from the same horizon or formation, which represents one period of deposition. b. that all of the fossil organisms lived together and are representative of life during one interval of geologic time. c. that the complete geologic range of each fossil is known. Therefore from a sample diagram, an interval of geologic time can be determined in which all of the fossils could have mutually co-existed.  GEOLOGIC TIME A B C D E F Jurassic Triassic Permian Pennsylvanian Mississippian Devonian  Palaeontology can be divided into vertebrate and invertebrate palaeontolgy. Micropalaeontolgy deals with fossils which are microscopic and can be studied with the aid of a microscope. Organisms studied are either organic walled or mineral walled. Study of the mineral walled organisms is stilled called the classical micropalaeontology. These include organisms like foraminifera, diatoms, radiolaria, ostracodes, etc  Study of organic walled organisms is called Palynology. Some of the organisms are pollen, spores, acritarchs, dinoflagellates, chitinozoans. Dinoflagellates Spores/pollen Spores/pollen Foraminifera Radiolaria Diatoms  KINGDOM Protista Members of this kingdom consist of a single, eucaryotic cells (cells having a nuclear wall and organelles). Some members are photosynthetic and others must consume other protist. RADIOLARIA These are marine zooplanktons that secrete a test of opaline silica in spherical, helmet- shaped and spiny forms commonly with open pores. Radiolaria became abundant in the Mesozoic era.  As in the foraminifera (forams), the cell is protozoan and pseudopodia extends from openings in the lattice of the test to trap food particulates. Some radiolaria contain algae within their tissue, which supply them with oxygen. Radiolaria is smaller than foraminifera, closer to silt size and are abundant in modern seas. DIATOMS They are a form of algae and are thus photosynthetic organisms.  They first appeared in the early Mesozoic and became abundant later that era. They secrete minute silt-size tests of opaline silica that are usually round or oval shaped. The two valves of the test fit together as a box and lid. They are found in fresh water as well as marine waters. Diatoms and radiolarians are the primary source or components of deep sea siliceous oozes, and after lithification these deposits form one variety of the sedimentary rock chert.  NANNOFOSSILS Calcareous nannofossils are extremely small objects (less than 25 microns) produced by planktonic unicellular algae. As the name implies, they are made of calcium carbonate. Nannofossils first appeared during the Mesozoic Era and have persisted and evolved through time. One extant group that produces "nannofossils" is the Coccolithophorans, planktonic golden-brown algae that are very abundant in the world's oceans.  The calcareous plates accumulate on the ocean floor, become buried beneath later layers, and are preserved as nannofossils. Some chalks, such as those comprising the White Cliffs of Dover, are composed almost entirely of nannofossils. Like the planktonic foraminifera, the planktonic mode of life and the tremendous abundance of calcareous nannofossils makes them very useful tools for biostratigraphy.  FORAMINIFERA GROWTH AND REPRODUCTION These are marine protists in which some members are planktonic and others benthic. They secrete tests of calcite are create tests of cemented silt grains (agglutinated). The structure of the test is a single chamber or series of chambers and the size is about that of a grain of sand.  If the test grows continuously from the proloculus (first formed chamber of the shell or test), it is a simple sphere with a small neck or a hollow tube that is straight, curved, or coiled. Such a test is called unilocular or monothalamus. Periodic growth results in the formation of multichambered test of great variety. These are described as multilocular or polythalamus.  The life cycle consist of an orderly succession of sexual and asexual phases known as alternation of generation. This results in the production of two different kinds of tests which illustrates dimorphism. A smaller shell is produced asexually, with a large proloculus, thus called megalospheric form. The microspheric form has a small proloculus, large shell and is produced sexually.  Megalospheric Microspheric Asexually produced Sexually produced Proloculus large Proluculus small Chambers relatively few Chambers many Tests smaller Test larger Ontogeny incompleteOntogeny more complete Test common/numerous Tests uncommon  Paleoecology Forams are found in different environments. 1. Brackish water : Dominated by arenaceous forms. 2. Reefal and lagoonal shallow environment: Dominated by calcareous forms 3. Tethyan Sea (Open Marine) : Dominated mostly by the aragonitic forms. 4. Very Deep Oceans: Dominated by chitinous and very fine arenaceous forms.