What are fossils?
•Preserved remains or traces of animals, plants, and other organism from the remote past •Found in sedimentary rock layers •Can be as small as bacteria or as giant as a dinosaur •Paleontology is the study of fossils. Condition for forming fossils? •Quick burial of organism •Layers of sediments build up over the organism •Left for a long period of time •Most fossil remains are from hard structures such as bones and teeth. Erosion and uplifting exposes them to the Earth’s surface Sedimentary Rocks •Formed in layers by the deposition of weathered rocks •Sediments are pressed and cemented together. •Fossils are found here because sedimentary rocks form at low temperatures and pressure. •Fossils and the rock layer in which they are found are approximately the same age. Law of Superposition •Geologists assume the newest rock layers are on top of the older ones, unless some type of disturbance occurs. •This law helps scientists determine the age of fossils, relatively speaking. •They can say one fossil is older than another because of the strata in which it was found. Fossil Record •History of life as documented by fossils •Tells us when organisms lived and how they changed over millions of years (evolved) •Can also give us a clue as to the environment at that time •Recorded on the Geologic Time Scale Types of Fossilization •Petrification •Replacement •Permineralization •Molds and Casts •Carbonization •Fossil Resin (amber) •Tar and Ice •Trace Fossils Petrification •Two types – Replacement and Permineralization •Both result in organic material converting into stone or a similar substance •Petrified wood is the most well know example of this process. Replacement •Happens when water dissolves the original solid material and replaces them with mineral matter such as calcite, silica, pyrite and hematite •Happens slowly •Bones, shells, and wood are often fossilized in this manner. Permineralization •Groundwater carries dissolved minerals into the pores and cavities of bone, wood, or shells. •The original material is preserved rather than replaced. •Bones, teeth, and shells can be preserved this way. Molds and Casts •Mold – organism dies and there is no filling of the cavity with minerals •Cast – organism dies and the cavity fills with minerals, maybe sand or clay •Many times shells are fossilized through these processes. Carbonization •The process by which all substances of plants and animals decay, except carbon. •This leaves a carbon film on the sedimentary rock. •This process particularly occurs in plants and fish. Fossil Resin •Resin is excreted from certain plants, which is thought to protect them from insects and seals off plant injuries. •The sticky resin captures insects and other invertebrates. •It hardens and they are preserved in the resin including their DNA. •Fossil resin is known as amber. Tar and Ice •An animal can become trapped in tar and the whole body can be preserved. An example is the La Brea Tar Pits L.A. •Freezing can also trap whole animals. Mammoth’s bodies have been found with skin, hair and even organs. Trace Fossils •Provides indirect evidence of life in the past rather than the body of the animal itself. •Examples of trace fossils •Footprints •Tracks •Burrows •Feces •Borings Index Fossils •Special fossils called “index fossils” indicate to geologists the boundaries in geological time. •This is a tool to determine the age of rocks. •Characteristics of Index Fossils •Easily recognizable •Abundant •Wide geographic distribution •Lived a short period of geological time Fossil Record and Rock Strata •Index fossils are used to correlate the age of the rock strata. •If two different rock strata in different areas on Earth contain the same index fossils, then the strata are probably the same age. •Also called ‘relative age’ Absolute Age •Most accurate form of dating, also call radiometric dating •Uses the decay of radioactive elements to find the absolute age of a rock or fossil •This relies on the property of half life, which is the predictable time an element takes to decay. Stomatolites •Rock like structures built by blue-green algae. •First forms of life on Earth - oldest known fossils (3.5 b.y.) •Still found in some remote areas of the world today Fossil Record •By using relative and absolute dating scientists are able to put together the Earth’s geologic history. •The Geologic Time Scale is a chronological representation of Earth’s geologic history going back 4.6 billion years until present day. •Through fossils scientists can see changes in organisms over time. Environment •Fossils can tell us something about the environment at the time the organism died. •Many times marine fossils are found on mountain tops. •When continental plates collide, it causes uplift. •Finding buried marine fossils suggests the rock layers were once covered by water. Environment •Fossil evidence helps support the Theory of Continental Drift. •Fossils of the same plants and animals were found on different continents now separated by oceans. •These organisms had no way of traveling those distances. •Concluding that the continents were once one supercontinent Comments are closed.
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Erik E. Mason
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