Fossils: Formation and Classification

Editor's Notes

1. 1. Sedimentary rocksWhen organisms die and become buried by sediment, or when organisms travel over or through sediment and leave marks. Rocks created from sediments deposited under anoxic conditions in quite water preserve particularly fine sediments2. Volcanic ash depositsSettles like sedimentsFossils cannot form in intrusive igneous rockOccasionally lava flows preserve trees, because lava cools before it would totally be incinerated
2. Frozen or dried bodiesIn a few environments, whole bodies of organisms may be preservedFairly young in geological standardsMeasured in thousandsEX. Wholly mammoth (permafrost)Egyptian mummiesFossils preserved in an amber or tarInsects land son bark of tress become trapped in the sticky sap (golden syrup)The syrup envelops the insects over time and hardens into amberA semi-precious stone40 million years or more
3. Preserved or replaced bones, teeth and shellsThe hard parts consists of durable minerals which may survive in the rockEventually, the minerals recrystallizes or are replaced by new minerals that precipitate from groundwater solutionBut the shape of the bone remains in the rockPetrified (premineralized) organisms Petrified (turn into stone)Typically forms when volcanic eruption rapidly buries a forest in siliceous ashPetrified wood
4. Mold and castAs sediments compacts around a shell, it conforms to the shape of the shellWhen it disappears because of mechanical weathering.. forming a moldCarbonized impressionsImpressions are simply flattened molds created when soft or semi soft organisms get pressed between layers of sedimentChemical reactions eventually remove the organic chemicals that composed the organisms, leaving only a thin film of carbon on the surface of the impressionTrace fossilsInclude footprints, feeding traces, burrows and dung that organisms leave behind in sediment
5. Fossil preservationNot all living organisms become a fossil when they die, in fact , only a small portion percentage do for it takes special circumstances  Death in an anoxic (oxygen free) environmentFlesh that has not been eaten or does not rot reacts to oxygen and turn to CO2Remaining skeleton weathers in air and turn to dustHow?Carcass should settle in an oxygen free environment where redox does not occurBottom of stagnant lakes, deep ocean or in tar/amberRapid burialIf organism dies in depositional environment where sediment accumulates rapidly, it may be buried before it has time to rot, oxidize or be eaten even if oxygen is presentStorm occurrencePresence of hard partsPalaeontologist know how much more about the fossil record of bivalves than they know about the fossil record of jellyfishLack of diagenesis/metamorphism
6. Classifying fossilsPalaeontologists traditionally distinguish different fossil species from one another according to its morphology aloneNaming:Compared fossil forms from known organismsNature of skeleton (internal/external)Symmetry of organisms (bilaterally/fivefold symmetry)Design of the shell (invertebrate)Design of jaws and feet (vertebrate)Ex. Fossil organize with spiral shell that does not contain internal chamber is classified as a member of class gastropoda
7. Trilobites- 3 segmented shell that is divided across its width into 3 parts (arthropod)Gastropods (snails)- spiral shell, does not contain internal chamberBivalves (clams & oysters) – shells that can be divide into 2 similar halves
8. Brachiopods – resemble screen like grid of cellsCrinoides – looks like a flower, stalks consist of numerous circular plates stacked on top of each otherGrapholites – tiny carbon saw blades in a rockAmmonites – spiral straight shell that contains internal chambers and has ridge surfacecorals
9. Brachiopods – resemble screen like grid of cellsCrinoides – looks like a flower, stalks consist of numerous circular plates stacked on top of each otherGrapholites – tiny carbon saw blades in a rockAmmonites – spiral straight shell that contains internal chambers and has ridge surfacecorals