Chapter 17 The History of Life Section 17-1 The Fossil Record Current Estimate of the Age of the Earth Scientific methods of estimating the age of the earth are based on analysis of evidence found in rocks and on radiometric dating . Based on these methods, scientists have concluded that the earth is much more than 6,000 years old. Recent estimates of the age of the earth: 4.6 billion years old (4.6 billion = 4,600,000,000) Geologic Time Scale Developed By Paleontologists To Classify Geologic And Biologic Events Rock Layers Fossils Present Life On Earth Fossils & Ancient Life Scientists Who Study Fossils? • Paleontologists • Infer What Past Life Looked Like What They Ate & What Ate Them Behavior Environment They Lived In What They Looked Like What They Ate Fossils & Ancient Life Key Concept The Fossil Record Provides Evidence About The History Of Life On Earth. It Also Shows How Different Groups of Organisms Have Changed Over Time. Fossils can be used to 1. Show the course of natural selection… how organisms change overtime. 2. Show changing environments and patterns. Fossil: from the Latin word meaning “dug up.” Any organism can leave a fossil. In order to leave a fossil, organic material changes 0.1% of organic from it original remains fossilize. material into a mineral. Bones and shells fossil best, but all things can fossilize. Conversion: Bone to Rock Conversion is a VERY SLOW Process. It can take millions of year to occur. Dirt builds up over a burial. The Pressure from the dirt builds up and drives water out of the bone and dirt and turns it into ROCK. Bone which is made up of calcium Converts into rock. The most common are calcite aragonite, quartz and silica. Burial Burial is important for fossilization. Burial protects a bone. If a bone is not buried, scavengers may eat the remains, or the bone will decay. Without burial, a bone can decay in 10 years or less. Discovery At the same time as fossilization is occurring, other geological processes such as mountain formation may raise the fossil to the surface, where weathering and erosion may eventually expose a fossil. How Fossils Are Formed Most Form In Sedimentary Rock Plants & Animals Die In A Water Environment And Are Covered With Silt Others Covered With Fine Sand or Volcanic Ash Compressed Into Rock 4 TYPES OF FOSSILS MOLDS: are fossils which formed from an impression of the shape or tracks of an organism. 4 TYPES OF FOSSILS CASTS: are fossils which are formed when sediments fill in the cavity left by decomposing organisms. 4 TYPES OF FOSSILS Trace fossils are non-body remains indicating the activity (behavior) of an organism. 4 TYPES OF FOSSILS TRUE FORM FOSSILS: are fossils of the actual animal or plant. Many are trapped in ice, amber or tar. Eggs, Embryos, and Nests- Fossilized dinosaur eggs were first found in France in 1869. Many fossilized dinosaur eggs have been found, at over 200 sites. Sometimes they have preserved parts of embryos, which can help to match an egg with a species of dinosaur. The embryo also sheds light on dinosaur development. The nests and clutches of eggs tells much about dinosaurs' nurturing behavior. 6 ways that organisms can turn into fossils 1. Unaltered preservation -like insects or plant parts trapped in amber, a hardened form of tree sap Unaltered Remains Insects in amber Preservation in tar 6 ways that organisms can turn into fossils 2. permineralization=petrification -in which rock-like minerals seep in slowly and replace the original organic tissues with silica, calcite or pyrite, forming a rock-like fossil - can preserve hard and soft parts - most bone and wood fossils are permineralized 6 ways that organisms can turn into fossils 3. replacement -An organism's hard parts dissolve and are replaced by other minerals, like calcite, silica, pyrite, or iron This brachipod has been pyritized (hard structures replaced by pyrite [fool's gold]). 6 ways that organisms can turn into fossils 4. recrystalization -hard parts either revert to more stable minerals or small crystals turn into larger crystals These brachiopods lived about 375 million years ago. The original organisms decayed, leaving voids in the surrounding sediment, which were then filled by other minerals. In the lithified rock, the void spaces are represented as molds (on the right), and the sedimentary fill is represented by casts (on the left). 6 ways that organisms can turn into fossils 6. carbonization=coalification (When the organism dies most of the elements in its body -- such as hydrogen, oxygen, and nitrogen - - are removed, but the carbon, which appears not only in living bodies but also in diamonds, coal and many other mineral items, remains.) These are carbonized fronds of Mariopteris, an inhabitant of Late Carboniferous swamps. Most animals did not fossilize they simply decayed and were lost from the fossil record. Paleontologists estimate that only a small percentage of the dinosaur genera that ever lived have been or will be found as fossils. Fossils & Ancient Life Each Fossil Is Found Only In It’s Particular Time Period or Layer A Kind Of Fossil Clock Of All The Life Forms That Have Ever Lived On Earth ----- • 99% Are Extinct No Longer Alive Fossils & Ancient Life Fossil Record • Organization Of Fossils From Oldest To Newest Including Supporting Information Of Other Life Forms In Their Environment Interpreting Fossil Evidence Fossils Are Released By • Tectonic Forces • Erosion • Mining • Construction Interpreting Fossil Evidence Most Fossils Disarticulated • Predation • Scavengers • Rot Catastrophic Events May Bury & Preserve Whole Animals and/or Ecosystems Dating Fossils Two Types of Dating 1. Relative Dating Organizes Fossils By Order (1st, 2nd, 3rd, etc.) Tells You Which Organisms Lived In What Order Does Not Give You Actual Years 2. Absolute Dating Gives You Age In Years Before Present e.g. 150 Million Years Ago Relative Dating Using the Law of Superposition (successive layers of rock were deposited one on top the other) scientists can estimate the “Relative” age of a fossil. Fossils are approximately the same age as the layer they are found in. The younger fossils are found closer to the surface, the older fossils are found in the deeper layers. Relative Dating Relative Dating Can determine a fossil’s relative age Performed by estimating fossil age compared with that of other fossils Drawbacks – provides no info about age in years INDEX FOSSILS GUIDE Fossils • Easily Recognized • Existed For A Short Period • Wide Geographic Distribution • Does Not Give Absolute Age Trilobite Absolute dating Can determine the absolute age in numbers Is performed by radioactive dating – based on the amount of remaining radioactive isotopes remain Drawbacks - part of the fossil is destroyed during the test Absolute or Radioactive Dating Key Concept: In Radioactive Dating, Scientists Calculate The Age Of A Sample Based On The Amount Of Remaining Radioactive Isotopes It Contains Radioactive Dating Or Absolute Dating Uses The Half Lives Of Certain Radioactive Isotopes To Calculate The Age Of A Fossil Half Life The Length Of Time Required For Half Of The Radioactive Atoms In A Sample To Decay Carbon-14 Dating By looking at the ratio of carbon- 12 to carbon-14 in the sample and comparing it to the ratio in a living organism, it is possible to determine the age of a formerly living thing fairly precisely. Carbon-14 Dating Carbon-14 Dating The carbon-14 decays with its half-life of 5,730 years, while the amount of carbon-12 remains constant in the sample. Carbon-14 Dating Because the half-life of carbon-14 is 5,730 years, it is only reliable for dating objects up to about 60,000 years old. half-life of 1.26 billion years COMPARING RELATIVE DATING AND RADIOACTIVE DATING OF FOSSILS RELATIVE DATING RADIOACTIVE OR ABSOLUTE DATING CAN DETERMINE AGE OF FOSSIL WITH RESPECT TO AGE OF A FOSSIL IN YEARS ANOTHER ROCK OR FOSSIL (THAT IS, OLDER OR YOUNGER IS PERFORMED BY COMPARING DEPTH OF A FOSSIL’S DETERMINING THE RELATIVE SOURCE STRATUM TO THE AMOUNTS OF A POSITION OF A REFERENCE FOSSIL RADIOACTIVE ISOTOPE AND OR ROCK NORADIOACTIVE IOSTOPE IN A SPECIMEN DRAWBACKS IMPRECISION AND LIMITATIONS DIFFICULT LABORATORY OF AGE DATA METHODS Other useful radioisotopes for radioactive dating include Uranium-235 - 704 million years (decays to Lead-207) Potassium-40 - 1251 million years (decays to Argon-40 and Calcium- 40) Uranium-238 - 4468 million years (decays to Lead-206 and Helium) Rubidium-87 - 48,800 million years (decays to Strontium-87) That’s all folks!
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