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
