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Fossil Formation requires the normal processes of decay to be permanently arrested. This can occur if the remains are isolated from the air or water and decomposing microbes are prevented from breaking them down. Fossils provide a record of the appearance and extinction of organisms, from species to whole taxonomic groups. Once this record is calibrated against a time scale (by using a broad range of dating techniques), it is possible to build up a picture of the evolutionary changes that have taken place.
PR E V ON IEW LY
Fossils are the remains of long-dead organisms that have escaped decay and have, after many years, become part of the Earth’s crust. A fossil may be the preserved remains of the organism itself, the impression of it in the sediment (mold), or marks made by it during its lifetime (called trace fossils). For fossilization to occur, rapid burial of the organism is required (usually in water-borne sediment). This is followed by chemical alteration, where minerals are added or removed. Fossilization
Modes of Preservation
Silicification: Silica from weathered volcanic ash is gradually incorporated into partly decayed wood (also called petrification).
Brachiopod (lamp shell), Jurassic (New Zealand)
Phosphatization: Bones and teeth are preserved in phosphate deposits. Pyritization: Iron pyrite replaces hard remains of the dead organism. Tar pit: Animals fall into and are trapped in mixture of tar and sand.
Trapped in amber: Gum from conifers traps insects and then hardens.
Mold: This impression of a lamp shell is all that is left after the original shell material was dissolved after fossilization.
Limestone: Calcium carbonate from the remains of marine plankton is deposited as a sediment that traps the remains of other sea creatures.
Bark
All photos: RA
Ray structure
Growth rings largely destroyed
Insects in amber: The fossilized resin or gum produced by some ancient conifers trapped these insects (including the ants visible in the enlargement) about 25 million years ago (Madagascar).
The Evidence for Evolution
Ants
Polished amber
Petrified wood: A cross-section of a limb from a coniferous tree (Madagascar).
Rock phosphate matrix
Sand and tar matrix
Shell
Stone interior
Wing bones
Ammonite: This ammonite still has a layer of the original shell covering the stone interior, Jurassic (Madagascar).
Shell and chambers replaced by iron pyrite
Bird bones: Fossilized bones of a bird that lived about 5 million years ago and became stuck in the tar pits at la Brea, Los Angeles, USA.
No Cla t fo ssr r o Us om e
Shark tooth: The tooth of a shark Lamna obliqua from phosphate beds, Eocene (Khouribga, Morocco).
Soft mudstone
Carbon
Impressions of leaf veins
Cast: This ammonite has been preserved by a process called pyritization, late Cretaceous (Charmouth, England). Š BIOZONE International 2006-2012 ISBN: 978-1-877462-98-6 Photocopying Prohibited
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Fossil fern: This compression fossil of a fern frond shows traces of carbon and wax from the original plant, Carboniferous (USA).
Periodicals: Meet your ancestor, The quick and the dead
Sub-fossil: Leaf impression in soft mudstone (can be broken easily with fingers) with some of the remains of the leaf still intact (a few thousand years old, New Zealand).
Related activities: The Fossil Record
Weblinks: Getting into the Fossil Record
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