Modes of preservation (canvas) Flashcards
What type of rock does Mouldic preservation occur most commonly in?
Highly porous rocks like san
How does mouldic preservation occur?
shell is dissolved away leaving impressions of internal and external surface in rock
What are the ways that the original skeletal material can be preserved?
Sometimes the calcium carbonate shell material but more commonly organic skeletons
Bones preserved in peat tar
What are some examples of preservation of of original skeletal material?
early graptolite growth stage (freed from a limestone matrix by dilute acid) and the insect entombed in amber
How is bone more commonly preserved in rocks?
Permineralisation- (impregnation petrefaction)
What is permineralisation?
original cavities in the bone are infilled by an inorganic substance
How can permineralisation fossil be distinguished from the rock?
Greater weight
Loss of fine detail
What bone structure is incredibly resistant and will remain unchanged for long periods? (with example)
Vertebrae teeth - Tooth of the extinct Great White Shark Carchardon megalodon
When does recrystallisation usually occur?
During fossilization process (even where no change in skeletal minerology)
What is recrystallisation of organisms with original calcite shell structure?
involves no change in external form but may destroy original shell microstructure
What is recrystallisation like when the shell in wholly or partly aragonite?
Recrystallisation to calcite almost invariable most visible if original shell had Nacre (parallel sheets of aragonite)
Where doe pyrite replacement most commonly occur?
Black shales (source of iron and sulphur)
What process do pyritised fossils tend to be subjected to?
Pyrite disease
What is pyrite disease?
pyrite is oxidized (and the fossil destroyed) by bacterial action
How can pyrite disease be prevented?
This process can be prevented if the specimen is exposed to an ammonia atmosphere to destroy the bacteria, and then varnished to exclude both the bacteria and the air
What is the process of silicification?
original calcareous skeleton of these fossils has been replaced by silica
How can silicification be selective?
sometimes only affecting certain taxonomic groups or only a particular range of size
What is carbonization?
Loss of volatiles so as to leave a film of carbon
What is a typical example of carbonization?
The fossil Gingko leaves from the Jurassic of Yorkshire
What might carbonization affect?
any organic skeleton
What can occur if carbonisation has not proceeded too far?
the excess carbon can often be removed by treating the specimen with concentrated nitric acid and potassium chlorate
What are trace fossils?
preserved remains of the activity of once living organisms
What are the most common type of trace fossils?
tracks and trails
How are trace fossils usually preserved?
at the interface between two different types of sediment
What is exceptional preservation?
occurrences in which organisms are preserved much more completely than usual
What is an example of exceptional fossils?
entirely soft-bodied animals may be found and those with skeletons may show traces of the gut and other soft parts
