Chapter 5- Fossils, Preservation, and Dating Flashcards
1
Q
Fossil
A
describes any trace of past life, may be parts of/whole organisms or traces such as burrows and footprints
2
Q
organism
A
individual life form, animal/plant/fungus/bacteria, may be single or multi cellular. can be preserved as fossils
3
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Body Fossil
A
hard parts of an organism, ie skeleton or shell
4
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Replacement
A
atom by atom substitution of one mineral for another
5
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Dissolution
A
the process whereby minerals that make up the fossils are dissolved away and removed in solution by groundwater
6
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taphonomy
A
study of the entire process of fossilisation to when it’s discovered
7
Q
Mould
A
the impression of the outside or inside of a fossil
8
Q
cast
A
an in-filled fossil void, usually with another mineral
9
Q
Factors affecting fossilisation (8)
A
Original composition, energy levels, transport distance, rapidity of burial, amount of oxygen, size of sediment, diagenesis, compaction
10
Q
Silicification
A
occurs when percolating groundwater is rich in silica dioxide
11
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Carbonisation
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occurs during burial
12
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Pyritisation
A
occurs in an anaerobic environment containing live sulfur bacteria
13
Q
Benthonic
A
organism lives in or on sediment substrate of the sea floor
14
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Infaunal
A
organism lives in the sediment, usually in a burrow and typically filter feeds
15
Q
Example of a benthonic infaunal organism
A
Bivalve clam
16
Q
Epifaunal
A
organism lives on the sediment substrate
17
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Example of a benthonic epifaunal organism
A
bivalve oyster
18
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Vagrant
A
organism moves around on the sea floor, typically a scavenger or predator
19
Q
Example of a benthonic vagrant organism
A
regular echinoid
20
Q
sessile
A
organism does not move, some are attached to the substrate, others lie on the sea floor, most filter feed
21
Q
example of a benthonic sessile organism
A
bivalve mussel
22
Q
pelagic
A
organism lives in the water column, typically in the surface layer
23
Q
planktonic
A
organism that floats in the water current usually a filter feeder
24
Q
Pelagic Planktonic orgaism
A
graptolite
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Nektonic
organism that actively swims in the water column, typically scavenger or predator
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example of a pelagic nektonic organism
ammonite
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extant
organisms are still alive today
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extinct
organisms are no longer alive today
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Death assemblage
collection of organisms found in a different place and position than they occupied in life, ie collection of disarticulated shells
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Disarticulated
organisms found as fragments, ie seperate shells or parts of trilobites
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Life Assemblage
a collection of organisms found within sediments in the same position they would have occupied in life, in bivalve in a burrow
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Geopetal structures
allow us to see the way up of a rock, ie coral or bivalve in life position
33
Derived fossil
fossil that has been weathered out of one rock and deposited in another, different fossils may give conflicting dates
34
Ornament
expressed on the surface of the fossil, such as ribs, tubercles, spines, and growth lines
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Robustness
ability of a fossil to resist abrasion, robust forms are more likely to be preserved whole or with minimal damage
36
Articulated
organisms found whole or connected, as in life
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Littoral zone
high energy area between high and low tide
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anoxic/ anaeorbic conditions
lack oxygen and are unsuitable for life
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Trace fossils
formed by benthonic infaunal and epifaunal organisms, aquatic or terrestrial. preservation potential is poor
40
Tracks
footprints of an organism makes when it moved along the sediment
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Trails
impressions of animals which were travelling, could have formed due to part or all of the animal dragging along the substrate
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Resting traces
a type of trail formed by the animal at rest
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terrestrial
refers to anything formed on land
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bioturbation
refers to burrowing or working the sediment in a way that disrupts the bedding. caused by activity of living organisms
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substrate
name given to sediment/rock on the sea floor
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Vertical burrow
skolithos
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u-shaped burrow
diplocraterion
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branching burrow
thalassionoides
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trilobite resting trace
rusophycus
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trilobite walking trace
cruziana
51
borings
structures formed in rock or wood
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Catastrophism
theory that changes in the earth's crust have been due to sudden violent and unusual short-lived events
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Gradualism
assumes that change comes about gradually, or that variation is gradual in nature, especially in evolution
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uniformitarianism
geological processes observable now were acting in the same way in the past
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Absolute dating
gives specific dates for rock units or events in Ma
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Relative Dating
putting units or events into sequential order by saying one event is older or younger than another
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Half-life
the time taken for half the unstable parent atoms to decay and form stable daughter isotopes
58
closure temperature
the temperature at which a system has cooled, so there is no diffusion of isotopes in or out of the system
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Closed system
when a mineral neither gains nor loses atoms. the higher the temperature, the more likely exchange of atoms or ions will be
60
40K: daughter isotope, found in, half life, uses
40Ar, mica, 1250Ma, igneous rocks older than 10,000
61
238U: Daughter isotope, found in, half life, uses
206Pb, Zircon, 4470Ma, can be used to date igneous rocks older than 10Ma
62
Potassium- Argon dating
89% of K40 decays to 40Ca, which is indistinguishable from rock forming calcium. 11% decays to form 40Ar, which is not formed any other world. The disadvantage is that it is a gas and can escape the mineral lattice, especially when heated
most widely used method
63
Rubidium-Strontium dating
87Rb is the most useful isotope, it decays to form 87Sr which is solid and hard to lose, making it most useful for metamorphic rocks
Uncertainty over the length of the half life makes dating more difficult
64
Problems with radiometric dating (5)
1-radioactive isotopes are found in very small quantities
2- as the number of of half lives increases, the difference becomes smaller, (ie 0.8% between 6 and 7)
3- major intrusions take millions of years to cool, minor intrusions close to the surface are more reliable
4- sed rocks are made up of clasts of different rocks, older than the rock itself
5- weathering can alter the parent daughter ratio
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Eras of the Phanerozoic Eon (Oldest to youngest)
Palaeozoic, Mesozoic, Cenozoic
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Age range of the Phanerozoic Eon
541Ma - now
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Age range of the Palaeozoic Era
541Ma- 252Ma
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Age range of the Mesozoic Era
252Ma- 66Ma
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Age range of the Cenozoic Era
66Ma- now
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Peroids of the Palaeozoic Era (oldest to youngest) (6)
Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian
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Periods of the Mesozoic Era (oldest to youngest)
Triassic, Jurassic, Cretaceous
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Periods of the Cenozoic Era (oldest to youngest)
Palaeogene, Neogene, Quarternary
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Original horizontality
assumed that beds have been laid down horizontally and if they are tilted they have been moved from their original horizontal position
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Principle of superposition
assumed that the rocks at the bottom of a sequence were laid down first and are the oldest, assumes they have not been inverted. Can be checked using way up criteria
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Included Fragments
fragments from an older rock can be found within a younger rock, ie xenoliths, derived fossils, or pebbles within a conglomerate.
76
Cross cutting relations
features which cut though a rock must be younger than the rocks they cut
77
Unconformities
Us represent a break in deposition and erosion of the succession. There may be folding of the older rocks before new sediment is deposited
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Lithostratigraphic Correlation
relies on recognising rock types/sequences of rock types to align laterally
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Biostratigraphic correlation
uses assemblages of fossils to find rocks of the same age
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chronostratigraphic correlation
relies on finding an actual age for rocks, ie counting varve layers or radiometric dating
81
lateral variations
means there are changes in thickness or lithology in beds laid down at the same time
82
marker horizon
shows a bed or bedding plane with a change of lithology easily distinguished which covers a wide geographic event
83
varve deposits
alternating light and dark layers of sediment in glacial layers of sediment in glacial lakes, each pair representing one year. Summer deposits vary based on amount of meltwater produced and can be correlated
84
diachronus beds
have the same lithology but vary in age along their extent
85
Characteristics of cephalopods
chambered shells to change bouyancy and move up and down in the water column, tentacles for swimming and grabbing prey, aperture, septum, siphuncle, septal neck, protoconch, ribs, umbilicus, keel, sulcus
86
Charcteristics of coral
colonial or solitary, calice, columella, dissepiments, septa, epitheca, tabula, corallite, growth lines
87
Charcteristics of Bivalve
two similar valves, symmetrical through the plane of the hinge. Cardinal teeth and socket (hinge line), ligament, adductor muscle scar, pallial sinus(posteria side), pallial line, umbo(anterior side), growth lines, shelf margin
88
Charcteristics of a trilobite
Arthropods, chitin exoskeleton, three sections (cephalon, thorax, pygidium). Glabella, compound eye, genal angle, pleuron, legs and gills
89
Characteristics of a Brachiopod
larger pedicle valve, smaller brachial valve, umbo, growth lines, delidial planes, commisure where valves meet, pedicle valve and forearm. Pedicle attaches the brachiopod to the substrate
90
Problems of bristratigraphic correlation
- many fossils are restricted to just one environment
-some fossils are long ranged with slow evolutionary tracts
-many are delicate
derived fossils can confuse the true sequence
-not all sed rock contain fossils
-changes in benthonic species take a long side to spread around the globe,
-first and last appearence can be difficult to pinpoint
91
what makes a good zone fossil
-rapid evolution
-abundant in rocks where they are found
-easily identifiable
-found in many different rock types
-wide geographical distribution
-strong hard skeletons/shells
92
ammonites as zone fossils
wide distribution, strong calcareous exoskeletons, evolved rapidly
93
microfossils as zone fossils
found in very large numbers, preserved in mud chippings from boreholes, silica skeletons survive CCD
