Fossils Flashcards
What is a Lagerstatte?
- a fossil site exhibiting extraordinary preservation
- and often faunal or floral diversity
Why are Lagerstatten important?
- the worlds they span when combined cover some billion years of geological time
- they complete the most important portion of the fossil record for understanding evolution
What is the Lagerstatte for the Precambrian?
Ediacara
- the Ediacaran Hills, Australia
- named after the Ediacaran geological period
How old is Ediacara?
700 million years old
- oldest fossil Lagerstatte
What is ‘Ediacaran biota’?
the collective referral for the preserved soft bodied organisms, representing the earliest known complex multicellular organisms
What is the Lagerstatten for the Cambrian?
the Burgess Shale
- high in the Canadian Rockies
- named after the nearby Burgess Pass
How old is the Burgess Shale?
505 million years old
How are Burgess Shale fossils preserved?
as black carbon films on black shales
Why are the Burgess Shales important?
a record of early Cambrian life and its early diversification
- preserves soft-bodied organisms that would otherwise be missing from the fossil record and would contribute to BIAS
What is the Silurian Lagertstatte?
Wenlock Limestone
- in Herefordshire
How old is Wenlock Limestone?
420 million years old
- one of the first fossil communities discovered and studied
What does Wenlock Limestone tell us about it’s surroundings?
The environment in which the Wenlock Reef formed
- a reef formed in shallow tropical seas
Why is Wenlock Limestone important to palaentologists?
- shows how many different sorts of organism were living at the time/in this environment
- gives clues to the relationships between these organisms
What is the Jurassic Lagerstatte?
Solnhofen
- a warm shallow sea studded with islands covered a lot of where Germany is now
How old is Solnhofen?
around 155 million years ago, towards the end of the Jurassic
How did Solnhofen become a site for preservation?
sponges and corals grew on rises in this sea, forming reefs that divided up the sea into isolated lagoons
- cut off from the ocean and terrestrial runoff
- warm and isolated = salinity rose, became anoxic/toxic in some places
- nothing could survive bar cyanobacteria/small protists like foraminifera
How did Solnhofen preserve remains?
any organism that fell/drifted/washed in were buried in soft carbonate muds, and so many soft bodied creatures were not eaten by scavengers or torn apart by currents
What is a fossil?
Any evidence of a living organism 10,000 years or older
What are some general fossil types?
- Body fossils: the actual shell, tooth, bone, spine
- Imprint of creature
- footprint
- burrow
- poo: coprolites
- borings
- gastroliths
What are the two categories of fossils?
Give examples in each one
BODY FOSSILS
Remains of an animal, including poo
- bones
- teeth
- shells
- exoskeleton
- coprolite
TRACE FOSSILS
Traces left behind from an animal
- footprints
- burrows
- boring
- scratches
What are the most common types of body fossils?
How does this influence the fossil records?
Hard parts (shells, exoskeletons, internal skeletons, teeth, woody material from plants)
Fossil record is BIASED, if only hard parts can be (commonly) found
What are the methods for preservation?
- Carbonisation
- Petrification by mineral - - replacement
- Mould
- Unaltered remains
How does carbonisation take place?
What is most commonly carbonised?
during compaction, chemical changes plus heat plus pressure drive off oxygen, hydrogen and nitrogen, leaving the remaining part rich in carbon
- the result is a type of blackened imprint
often left by fossil plants
What is the most common method of fossiliation? (petrification by mineral replacement)
permineralisation
- the replacement of the original organism by minerals
What are the three types of mineral replacement?
Calcification
Silicification
Pyritisation
How does calcification or silicification work?
pore water rich in dissolved calcium carbonate or silica precipitate minerals into pore spaces
PYRITISATION
a) how (simple)
b) what preserved
c) where
a) involves deposition of iron and sulfur into pores by pore water
b) both solid fossils and preserved soft tissues
c) anoxic marine environments, when organisms are buried in sediments containing high concentrations of iron sulfides
How does pyritisation work?
- decaying organisms release sulfide, which reacts with iron in the surrounding water
- the reaction between iron and sulfides forms pyrite (FeS2)
- the carbonate shell material of the organism then dissolves
due to acidic groundwater. soil contains CO2, and the water percolating makes carbonic acid - the carbonate shell is then replaced with pyrite
What are moulds?
three-dimensional impressions in which the surface contours of an organism are preserved, when the remains of an organism are completely dissolved or destroyed
What are the two types of mould?
external moulds
internal moulds
What are external moulds?
Give an example
moulds of the outside of something (a living organism)
e.g. footprints
What are internal moulds?
Give an example
moulds of the insides of something
e.g. shells
How does a mould and cast form?
- Fossil is buried in sediment. Soft parts decay, hard parts remain
- Fossil is dissolved by percolating acidic groundwater. This leaves behind a cavity in the rock. This forms an external mould.
- Further percolating water precipitates minerals into the mould left behind. This cast is an exact replica. It could also be infilled with sediment. This would produce a cast.
- BUT internal detail is lost. (if you cracked a fossil in half you would only see rock or minerals)
What are unaltered remains?
body fossils that have undergone very little physical or chemical damage
Where can unaltered remains be found?
- GLACIERS (skeletal material)
- AMBER (small animals are kept intact as they are not strong enough to pull free)
- TAR SUBMERSION (preserves body fossils and is instrumental in preserving body fossils and is instrumental in conserving soft tissue as well as bones)
What is preservation potential?
- the likelihood of being preserved
- some places and organisms are more likely to result in preservation
What aspects of an organism lend it to high preservation potential?
- hard parts
- small
- abundant
- widespread
What areas lend themselves to high preservation potential?
- low energy
- with a high chance of being covered rapidly with fine grained sediment
- marine
- aren’t restricted to one environment (facies dependent) e.g. coral
What are the time periods in order, oldest to youngest?
Precambrian
Cambrian
Ordovician
Silurian
Devonian
Carboniferous
Permian
Triassic
Jurassic
Cretaceous
Tertiary
Quaternary
What is wrong with the fossil record?
biased and incomplete
How is the fossil record biased?
Give some examples of what it is biased towards
towards some creatures and environmental conditions
- hard parts
- low energy environments
- marine/aqueous
- small
- abundant
- widespread
How is the fossil record incomplete?
Give some examples
some is missing
- eaten
- scavenged
- decomposed
- metamorphism
- melting
- weathering
- erosion
- not found (by the right people)
What are the two types of assemblage?
Life
Death
What is a life assemblage?
Give some characteristics
fossils preserved in their living location, with no transport involved
- variety of species
- variety of sizes (juvenile to adult)
- less broken
What is death assemblage?
Give some characteristics
the organisms have been transported away from their living position
- broken/fragmented
- sorted
- 1 species
- alignment
What are time zones?
when we break up geological history times into zones
How is a time zone diagram set up?
Youngest at the top, oldest at the bottom
When a fossil is only is one zone, it is the most useful, as it tells us when that creature lived, tells us loads. If it is in more than one, it is more general and less helpful
What are zone fossils?
an organism that lived for a geologically short period of time so can be used to date the beds they’re found in
What is a derived fossil?
a fossil that has been eroded out and redeposited at a later date
What does it mean if a fossil is abundant?
There were lots of them so the chances of finding them is high
What does it mean if a fossil is widespread?
Found all over the world so we can use them to date rocks all over the world
What does it mean if a fossil is easily identifiable?
It has a recognisable shape that we won’t confuse with something else
What does it mean if a fossil is readily preserved?
Had hard parts, lived in marine environments
What does it mean if a fossil is facies independent?
Give an example
They lived in a variety of different environments, more likely to be found in a variety of rock types
e.g. A fossil which is only found in shale is only useful if you have shale!!!
What is relative dating?
The age of something compared to something else
What features can we use to relative date something and why?
- CROSS CUTTING FEATURES
- if a unit or structure cross cuts another it must be younger - INCLUDED FRAGMENTS
- if fragments of one rock are included in another, the included fragments must have come from the older unit: they need time to have weathered out - PRINCIPLE OF SUPERPOSITION
- the youngest rocks are at the top unless they have been overturned
What is radiometric dating?
a method of dating geological specimens by determining the relative proportions of particular radioactive isotopes present in a sample.
What is the theory behind radiometric dating?
Unstable elements break down to form a new element
- parent atoms decay into daughter atoms
- different radioactive elements decay at different rates
- the rate they decay at is called the half-life
What is half life?
TIME IT TAKES FOR:
- mass to halve
- rate of reaction to halve (clicks per second)
- for the percentage to half
What is an example of radiometric decay element?
Potassium (K) decays into Argon (Ar)
with a half life of 1250 million years (ma)
Potassium is found in mica and hornblende
What rocks can we radiometrically date?
only igneous or metamorphic
- when something crystallises, they have helpful and necessary isotopes
cannot date sedimentary because you would only be dating the mineral/incl. fragment formation
What are the problems with radiometric dating?
- closure point
- argon
- different ages for different minerals
- different ages at different parts of the igneous body reflect when it crystallised
RADIOMETRIC DATING - what is closure point and why is it a problem?
the point at which the temperature has cooled enough for the crystal lattice to close. Up until this point, elements can leave and enter the lattice
RADIOMETRIC DATING - why is argon a problem?
Because it is a gas and can more easily leave the crystal lattice: it will seem there are fewer daughters and we will date it as younger than it actually is
Why is different ages for different minerals a problem for radiometric dating?
Give an example
They form at different times
E.G. olivier crystallises much faster than biotite
Why does different ages at different parts of the igneous body reflecting when it’s crystallised a problem for radiometric dating?
The edge will crystallised before the middle, so whole thing has different dates
How does carbon dating work?
UV turns N14 into C14. This is unstable and decays back to N14
- usually we would look at the P:D ratio (C14:N14) but N14 is very common so we can’t tell what was there before already
- we therefore look at % of C12 and C14
- when something is alive it is continually taking in C12 and C14. The % C14 taken in is 0.033%
- upon death, no more is taken in and the C14 decreases. We can therefore C14 date things which were alive.
How long does it take for C14 to decay back into N14?
5730 years
What is the oldest sample we can date with carbon dating?
50,000 years
What can we use to date materials?
Radiometric dating
Carbon dating
Dating with samarium and neodymium
Are samarium and neodymium stable or unstable?
143 Nd = stable
144 Nd = stable
147 Sm = unstable
What is the relationship with decay of Samarium and Neodymium?
147 Sm is the parent, decays into 143 Nd over 106 billion years
What are the ratios of Parent: stable and stable:daughter in Hornblende, Plagioclase and Auguite?
Stable:daughter = 1:1
PARENT TO STABLE
Hornblende 1:1
Plagioclase 2:1
Augite 5:1
They all start with same ratio of daughter to stable
They all start with different ratio of parents to stable
