The nature of the Fossil Record Flashcards
In order to become a useful fossil an organism must:-
(long answer)
- Die and avoid destruction by biological (microbial decay, scavenging, trampling) or physical (weathering, erosion) processes (wind, rain, snow etc)
- Be transported into an environment where it is deposited, buried and thus incorporated into sediment. Fossils can only form where sedimentary rocks are deposited.
- The organism can then be fossilised by a variety of processes (taphonomy). How an organism is fossilised is called taphonomy.
4a. That sediment must avoid diagenetic or metamorphic processes that alter the rock and destroy the fossil.
4b. That sediment must avoid later destruction by erosion. - The fossil must become exposed at the surface and discovered by someone who will study it.
Why is the fossil record incomplete and biassed?
The fossil record is incomplete and biassed due to the need of all the preservation processes needed
Why is there incompleteness in the fossil record?
- Sediment only accumulates over a very small area of the earth
- Only a tiny fraction of the organism that live will be fossilised
- Most will be destroyed
- Certain parts of organism are preserved - bones etc - soft tissue is unlikely to be preserved
- Jelly fish are very unlikely to be preserved - only soft tissue.
- Certain environments are preferentially preserved - organism in bogs more likely than mountains
- Older rocks more likely to be destroyed
- Collector bias - wealthier countries have more
What kind of organisms leave a fossil record?
- Bacteria: certain bacterial sheaths and structures built by bacteria (stromatolites).
- Protists: Those that form exo or endoskeletons.
- Plants: Woody tissues (lignin), Cuticle (cutan), Spores (sporopollenin)
- Fungi: Chininous spores and hyphae
- Animals: those with recalcitrant exo and endoskeletons
What organisms are more likely to be preserved?
A fish in a lake is more likely to be preserved than a bird (however a bird can die and fall into the lake).
Organisms with a recalcite exoskeleton preserve really well.
What is meant by ‘the pull of the recent’?
- Older rocks / fossils are much more likey to be destroyed.
- More records from recent times
- fossil record gets worse the further back in time you go.
Why is the continental shelf where most fossils are found?
- Washed onto
- Where most sediment ends up
- Don’t get deep ocean sediments
- Best preservation on the continental shelf
- Most life lives on continental shelf (good amount of lights etc)
What are the 4 types of continental shelves?
- Tropical
- Warm, temperate
- Cold, temperate
- Polar
4 different types of continental shelves where sediment is being deposited and fossils are being formed today
Biass against different areas for where fossils will form.
What is a braciopod?
- Sessile marine filter feeders
- looks like a clam
- Good fossil evidence
How fossils form (diagram)
see doc
What are the large-scale biasses related to the earth system?
- Sea level changes (ice-house world / greenhouse world) long term climate trends and or continental. configeration.
- Continental configuration (e.g. more of less shelf)
- Atmospheric composition (e.g. oxygen levels)
Ice-house world and greenhouse world
Earths history has largely been made up of greenhouse world with more continental shelves.
How have sea levels changed over the history of the earth?
- Only over once been lower than it is at the moment.
- Much more continental shelf area for fossil preservation.
What is continental configuration ?
How the continental shelves are configured e.g. how they have moved
Do the patterns we see in the fossil record reflect biological signals or geological signals or both (common cause hypothesis)?
When sea level is high, fossils are easier to preserve but also biodiversity is higher because there is more continental shelf for organisms to live on.
