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LECTURE 2: Taphonomy and Time
OUTLINE
Fossils: Definition, Types
Taphonomy
Preservation: Modes and Biases
Depositional environments
Preservation potential of dinosaurs
Geologic Time Scale: Relative and Absolute
FOSSILS
Definition
any remnant of ancient life
from Greek fossilis, meaning "dug up"
fossils do not have to be fossilized, only preserved
Body fossils: eggshells, bones, cells, DNA, proteins, organs, etc…
Soft parts
Soft parts: most commonly preserved as impressions (sometimes also in
amber, ice, etc…) or as mineralized bacterial films
Keys to exceptional preservation
Hard parts: bone, teeth, and shells
Shell composition - CaCO3
Bone and tooth mineralogy
Living bones and teeth
Apatite mineral dahllite (carbonate hydroxyapatite):
Ca10(PO4)6(OH, CO3, Cl)2
Fossil bones and teeth:
Apatite mineral francolite (calcium fluorapatite):
Ca10(PO4)6(F, OH, Cl)2
Bone vs. dentin vs. enamel
enamel: ~97% mineral, 3% soft tissue
dentin and bone: ~70% mineral, 30% soft tissue
Durability of bone vs eggs, shells, carapaces, etc…
Trace fossils: tracks, trails, burrows, borings, nests, etc…
Dino trace fossils: tracks, gastroliths, coprolites
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TAPHONOMY
The study of everything that happens to an organic body between the time the original
organism dies and the time it is found by a collector.
Surface taphonomic processes
Predation/scavenging
Trampling
Aqueous transport
Bloat and float
Exposure
Invertebrate colonization/vertebrate gnawing
Subsurface processes
Burial compaction
Cementation and Concretion formation
Bone-pore water interactions
Bioturbation/erosion
Post-collection processes
Collecting bias
Loss during extraction and transport
Decomposition during storage
PRESERVATION: Most living things are not preserved when they die.
Styles
Permineralization - pores of original skeleton are infilled with minerals
Recrystallization - original skeletal crystals reorganize into larger crystals
Replacement - parts or all of original material replaced with a new mineral
Carbonization - carbon residue from soft parts or unmineralized hard parts
Mold/Cast/Impression - the original material is gone, but an impression remains
(mold) or the impression has been filled with a new substance (cast)
Original material - little or no alteration of original material
Fossil longevity
Oldest fossils = 3.8 (body fossils of cells preserved as carbon ghosts)
Oldest hard parts = 600 Ma (Vendian explosion)
Oldest bones = 510 Ma
Oldest DNA = about 60,000 years (sloth)
DEPOSITIONAL ENVIRONMENTS
All dinosaurs lived on land, but they inhabited a variety of environments from
mountainous highlands to shorelines
Only certain environments accumulate sediment, and therefore preserve fossils
Major depositional environments
Fluvial (rivers and floodplains)
Coastal (beaches, deltas, swamps)
Lakes
Bogs
Deserts (rare)
Shallow marine (rare)
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PRESERVATION POTENTIAL OF DINOSAURS
Dinosaur remains are typically fragmentary and mixed.
Complete skeletons are rare, and found in either low-energy environments or in
deposits that resulted from catastrophic processes (floods, ash flows, etc…).
Because of all of the above, dinosaurs displayed in museums are often cobbled
together from the remains of more than one individual.
GEOLOGIC TIME SCALE
There are two general approaches to reconstructing geologic time.
Relative dating: Determine the age of an event relative to other events. Put events in a
chronological order.
Absolute dating: Use different methods (largely geochemical methods) to assign
actual ages to specific events.
Beginning around 1800, time scale put together using relative methods.
Absolute dating began in the 1920s
Learn Eons, Eras, Mesozoic Periods, Major boundary dates
Construction of the Relative Time Scale of Rocks
Most sedimentary rocks are layered.
They come in discrete beds or strata.
Nicolaus Steno (1638-1686) fundamental principles of stratigraphy.
Steno observed the behavior of flooding streams, and how sediment was
deposited after floods. He realized that the hard, layered rocks he could observe
all around him in Italy had originally formed as soft, squishy sediment. This
recognition, that strata are made of sediment that was initially soft, led to the
following principles.
The Principle of Original Horizontality: as soft sediment flows and forms beds
under the influence of gravity, the layers must initially be deposited in
horizontal sheets.
The Principle of Lateral Continuity: at the time of deposition, these soft
sediments must spread out laterally in all directions until they either thin
out (due to a lack of material) or run into a barrier, such as the edge of a
geologic depression.
The Principle of Superposition: for undisturbed layers, the oldest beds are on
the bottom, and the youngest are on top.
Other important rules/observations were made by folks other than Steno.
The Principle of Cross-Cutting Relationships: any structure that cuts across
another feature must be younger than that feature.
The Principle of Inclusions: fragments in a rock are older than the rock itself.
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Strata often contain unconformities.
An Unconformity is a surface that separates older from younger rock, with some
amount of time missing between the older and younger units
Unconformities occur because sediment is not deposited for a time period,
perhaps because the sediment supply is low, or sediment representing the
time period was once present, but is was removed by erosion.
These rules allow us to put a stack of strata in relative temporal order, even when
it is slightly disturbed. We still have a major problem, however. How do we
recognize the same time period in different temporally-ordered stacks of rock that
are sometimes separated by 1000s of kilometers?
Correlation
Lithostratigraphic correlation: based on similarity of rock type and
position.
This approach can work on a small geographic scale, but rarely over vast
distances because:
Most strata are not that distinctive.
Different sediment types deposited simultaneously in different
areas.
Most beds do not represent time planes.
Late 18th and early 19th century workers in England, Europe and North
America noticed that different life forms were present in different layers.
When these fossil-containing strata were put in temporal order using
Steno's principles, it was clear that different life forms were present at
different times in Earth history. This Principle of Fossil Succession is
one of the first hints that organisms may have evolved, but none of the
folks making these observations believed in evolutionary transformation.
Most thought that different layers had different animals and plants
because they were placed there, in succession, by the creator. However
this basic observation of fossil succession has allowed the correlation of
strata based on biological similarity, a method we call biostratigraphic
correlation.
Biostratigraphic Correlation
If: life forms have varied through time, and fossils assemblages from
different times are distinctive, and the relative ages of assemblages
can be determined by superposition.
Then: the occurrence of the same fossil assemblage in rocks from
different regions indicates that the rocks formed at the same time, and
a relative time-scale based on fossil assemblages can be constructed.
The method assumes that:
No two species are identical.
Species don't reappear after becoming extinct.
First and last appearances are rapid and synchronous around the globe.
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Absolute Time Scale of Rocks
Dates have been determined in the last 100 years through the application of
dating techniques based on the decay of radioactive nuclei. Some atomic nuclei
(parent nuclei) are unstable; they undergo spontaneous decay (spitting out
particles and energy) until they reach a stable state, which we call daughter
nuclei.
Dating is possible because decay occurs in such a way that a constant
proportion of atoms decays per unit time. The half life of a radioactive parent is
the amount of time it takes for the abundance of this material to drop by 50%. If
we know this half life, and we know the relative proportions of parent to daughter
nuclei in a rock, we can calculate how long it has been since the rock
crystallized. Described in much more detail in Chapter 2. Read it.
Date rocks, not fossils
Time is deep - we are shallow
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