Fossils Flashcards
What is a fossil?
-The evidence of prehistoric life older than 10,000 years.
What are the two types of fossils?
-Body fossils=the remains of an organism whether it is unaltered or altered, e.g., bones, teeth, skull etc.
-Trace fossils=evidence of the organism activity, such as burrows, footprints, coprolites, bite marks etc.
What are the fossil preservation factors?
-Hard parts (bones, teeth, cartilage, exoskeleton, cellulose etc).
-Rapid burial (no scavengers, no erosion and no bacterial decay).
-Low-energy environment (prevents abrasion and destruction through no transport).
-Anaerobic conditions (much slower decay).
-Fine-grained covering sediment (silts and muds ideal for covering sediments as less likely to break up fossils and fill in fossil texture to preserve greater detail).
-Favourable tectonic setting (less chance of destruction due to metamorphism, faulting or igneous activity).
What is lagerstatten?
-Exceptional preservation.
-Generally occurs in hostile environments:
amber, peat, tar, ice, anoxic lakes etc.
-Also rapid burial by turbidity currents or deltas.
What is petrification/mineral replacement?
-Mineral solutions percolate the soil.
-They pick up more minerals along the way.
-Most common: calcite, silica, iron pyrite or haematite.
-Infill pore spaces.
-Eventually whole organism will be turned to stone.
What is carbonisation?
-Affects organic substances such as cellulose.
-Made up of Carbon, hydrogen, nitrogen and oxygen.
-Rapid burial and heat at depth cause some of the elements to be lost.
-Original plant material is abundant in carbon, so that remains.
-This leaves a blackened imprint of the fossilised plant, also called a carbon film.
What are moulds and casts?
-Moulds=imprint of shell in fine-grained sediment.
-Casts=a mould is infilled with sediment leaving an impression.
-Firstly buried in sediment.
-Original material, especially calcium carbonate, dissolves to leave an exact mould of itself.
-New minerals may infill the mould to create a cast.
What is a life assemblage?
-Organisms die and are fossilised in their living positions.
What is a death assemblage?
-Organisms die and are transported.
-They are then fossilised.
-The fossils are usually fragments due to transportation.
-They are not in their living positions and not with organisms that they once lived with.
What is a derived fossil?
-A reworked fossil.
-An organism that becomes fossilised but gets eroded and incorporated into another rock later.
-For example: an organism such as solitary coral living in a warm shallow tropical sea may become fossilised into a limestone rock. Millions of years later the limestone rock may be eroded by a river and a chunk of the limestone containing the fossil coral may be transported and deposited to form another rock such as a conglomerate (sedimentary).
-When found by a geologist, the fossil would appear to be unbroken and unaffected by transportation, making it a life assemblage. However, it would be very obvious that it did not live in the high energy and turbulent environment of a fast-flowing river, therefore it is known as a derived fossil.
Why is the fossil record biased?
-Biased in favour of marine organisms as hard parts resistant to decay, living in low energy environments and suffered rapid burial.
Why is the fossil record incomplete?
-Natural processes can distort or destroy fossil evidence, such as predation, scavenging, diagenesis, bacterial decay, weathering, erosion and metamorphism.
What is the principle of uniformitarianism?
-“The present is the key to the past”.
-Present day processes operate the same way that they did in the past.
-E.g. ammonites are millions of years old and extinct so we can take a similar organism and assume that certain body parts were for certain functions and we can also infer things for example about its behaviour etc.
What are sedimentary facies?
-Characteristics of a rock/sediment after deposition, used to distinguish from another rock/sediment.
What can sedimentary facies tell us?
-The sedimentary environments can be more easily compared an interpreted by grouping characteristics of the rock into facies.
