Lecture Twenty Two - Fossils Flashcards
What is a fossil?
Remains of once living organisms (including plants):
1) Original material.
2) Replacement or alteration of original material.
3) Tracks, trails, burrows made by once living organisms.
How are fossils made?
Most fossils are found in sediments and sedimentary rocks.
Organisms living in to near the environment due and their remains are preserved in the sediments.
Energy of sedimentary environment helps to determine the quality of the fossil.
Where are fossils found?
Sedimentary rocks:
Ideal original environment.
- Anoxic = no decay of body.
- Quiet water.
Fossils occasionally found in low grade metamorphic rocks or in volcanic ash deposits.
Never found in igneous rocks or high grade metamorphic rocks - too hot.
What process must an organism go through to be fossilised?
Animal alive - living assemblage - biocoenosis.
Animal dies - soft parts lost.
Preservable parts - lost to scavengers or scattered or weathered.
Parts actually preserved - lost to erosion immediately.
Parts not eroded - lost by being buried too deep or cooked or crushed.
Fossils now at surface - lost by not being collected or is actively destroyed.
Fossil assemblage - lost to science in private collection or kids bedrooms.
Collected and studied fossil assemblage - death assemblage - thanatocoenosis.
How are ‘original material’ fossils created?
Fossilisation of, or part of, the organism its self.
No chemical or other changes to preserved material:
- Amber.
- Desiccation/mummification in caves.
- Foraminifera and diatoms.
- Pollen.
- Some shells.
Not the normal occurrence for macros coping organisms.
How are ‘permineralised’ fossils created?
Original hard parts (shells/bones) preserved.
Additional intercellular materials added such as mud, sand, calcite, etc.
E.g.
Most vertebrae bone fossils.
Mose invertebrate shell fossils.
How are fossils that have replacement of original material created?
Original materia is removed and is replaced by another material.
May be entirely new material, or recrystallisation of original dissolved material.
May be detailed cell by cell replacement, or just external or internal structures.
Calcite (common).
Silica - E.g. Quarts (common) or opal (rare).
Aragonite (unusual).
Pyrite (fairly common).
How are fossils formed via distillation (carbonisation)?
More volatile (oxygen, hydrogen, nitrogen) chemicals 'burned off,' or organic substances (cellulose, chitin) 'burned off' during fossilisation process. Residual carbon imprint of original organic material left behind.
Common in fossilisation of: Plants. Graptolites. Some soft bodied animals (burgess shale). Rarely found with large animals.
What are trace fossils?
Not the original animals or plants - but evidence it was there.
- Body impressions (moulds, casts, imprints).
- Burrows (both for feeding and/or living in).
- Feeding traces.
- Escape structures (mainly in turbidites).
- Footprints.
- Poo (otherwise known as coprolites).
How are body impressions formed? (Trace fossils).
Hard parts of organisms dissolved.
Mould = hole left in rock after fossil destroyed.
Cast = Hold filled in with secondary material to form replica of outside of original fossil.
Imprint = Fossil leaves impression in sediment (outside body). Fossil removed. Then soft sediments infill the impression.
How are tracks, trails and burrows formed? (Trace fossils).
Formed as organisms move about on sediments or though them.
- Later can be filled by sediments of a different nature.
- Important because in situ and only remains that soft bodied forms may leave.
- These and known as trace fossils or ‘ichnofossils,’ and are classified in the same way as true organism - can be confusing.
How are coprolites formed? (Trace fossils).
Fossilised faeces.
The best evidence for what the organism ate, its health and lifestyle.
How are chemical fossils formed? (Trace fossils).
Traces of organic acids or ratios of isotopes reflecting organic processes present at the time.
- Usually lacking in visible traces of actual organisms.
- Sometimes the only evince that life existed at the time.
