Modes of Preservation and Depositional Environments (1)
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Texas A&M University *
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207
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Geology
Date
Feb 20, 2024
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6
Uploaded by GrandProton748
Modes of Preservation and Depositional Environments
Part I: Fossil Identification and preservation
There are numerous ways by which an organism may be preserved. The following are some of the more
common modes of preservation:
Unaltered Body Fossil
●
Soft parts
- preserved given the slow rates or cessation of bacterial decay.
●
Hard parts
- the skeletal material may be composed of calcium carbonate (calcite or aragonite),
silica, calcium phosphate, or organic compounds
Altered Body Fossils
●
Permineralized
(petrified) - the process of filling in the spaces within the original material with
inorganic mineral substances. It does NOT replace, but adds to, the original material. This
process is common with plants and bones.
●
Replacement
- the process of replacing the original biomineral or organic matter with a
substance such as calcium carbonate, silica, or iron compounds. ●
Recrystallization
- the process of changing the microstructure of the original biomineral and is
caused by solution and reprecipitation of the material. The primary chemical composition of the
shell is unchanged, but the mineral structure is changed.
●
Carbonization
(distillation)- the process involving the loss of the volatile constituents: oxygen,
hydrogen, and nitrogen, leaving a carbonized residue.
Concretionary Preservation
●
Concretions
are hard, well-mineralized, portions of a sedimentary layer. They are often ovoid or
spherical in shape and sometimes contain a fossil at their center. Because they are more
resistant to erosion and compaction than the surrounding less-mineralized sedimentary rock,
they can enhance the preservation of body fossils.
Body Fossil Impressions
●
Molds
- impressions of the exterior surface (external mold) or interior surface (internal mold) of
an entire organism or the portion of one. Both types make negative impressions of the objects
(such as fingerprints in clay). The interior molds of closed shells are often called steinkerns.
●
Casts
- formed by filling an external mold and duplicating the shape and ornamentation of the
exterior of the original object. These may form from external molds or between external and
internal molds. Trace Fossils
record the
actions
and
behavior
of an organism rather than a part of the organism itself. A
single organism can make many different types of traces and similar looking traces can be made by
different organisms that have similar behaviors. Thus, trace fossils are named and classified based on the
behavior that made them rather than on the trace maker.
Part 1: Fossil Identification and Preservation (12 questions, 1 pt each)
1.
Dinosaur footprint:
https://skfb.ly/KRU8
. Using the terms on the first page, what type of fossil is this?
Explain your choice.
This fossil type is a trace fossil, because it is not recording the actual organism itself but rather a trace
action or behavior of an organism.
2.
Internal mold of a gastropod:
https://skfb.ly/FCPx
. Describe how this fossil could have formed.
This internal mold of a gastropod could have been formed by sediment filling up the shell of a sea snail
which died creating a mold of this creature’s internal features. Then after some period of time the
outer shell would’ve gotten broken off or eroded leaving this internal mold.
3.
Fossil seed fern:
https://skfb.ly/6YHZS
. Using the terms on first page, what type of fossil is this? Explain
your choice.
Since this is a plant fossil I would think this to be an altered body fossil, specifically one made due to
the process of carbonization. The plant left a carbonized residue.
4.
Mosquito in amber:
https://skfb.ly/6xWCO
. Using the terms on first page, what type of fossil is this?
Explain your choice.
This type of fossil would be considered an unaltered body fossil. The mosquito was preserved
completely in an organic compound, amber, and it was unaltered by any elements. Since amber is tree
resin, this mosquito may have gotten stuck in it, eventually leading the resin to harden, turn into
amber and preserve its remains.
5.
Therizinosaurus cheloniformis
https://skfb.ly/BXSQ
. Using the terms on first page, what type of fossil is
this? Explain your choice.
This type of fossil would also be considered an unaltered body fossil. There was an actual part of the
animal that was preserved, the claw. This claw could have been preserved in many ways including a
combination of slow rates of decay, sediment burial, and mineralization. This could have been because
the claw was covered in sediment after the dinosaur’s death which in turn provided a great
environment to prevent decay.
6.
Permineralized wood:
https://skfb.ly/6AMpB
. Examine the fossil and read the annotations. What
features of the fossil allow you to recognize it as wood?
Some features that allow you to recognize this fossil as wood include there being bark like ridges
around the side of the fossil and what seems to be tree rings on either side of it as well.
7.
Dinosaur skin impression:
https://skfb.ly/oHqGL
. Is this impression in positive or negative relief? Is this a
mold or a cast? Describe how this fossil could have formed.
I think this impression is most likely to be a negative relief and most likely a mold in my opinion, the
fossil could have been formed by a layer of sediment that covered the dinosaur’s skin and preserved
the impression.
8.
Plesiosaur Bone Fossil:
https://skfb.ly/6RwSD
. Examine the fossil and read the annotations. Describe how
this fossil was preserved.
This fossil is a concretion preservation. It was preserved through processes such as mineralization,
sedimentation, and replacement of organic materials with other minerals. It was covered by a
sandstone concretion which was cemented into the ground.
9.
Cephalopod: Gunnarites sp. (PRI 61543)
https://skfb.ly/6RRsD
. Using the terms on the first page, what
TWO types of fossil preservation does this specimen represent? Explain your choice.
This fossil preservation represents both replacement and permineralization. With replacement the
original material gets substituted by different minerals. Permineralization means the holes/pores in
the fossil get filled with different minerals as well. The shell of this cephalopod has been replaced by
minerals such as silica or calcium carbonate and it has been permineralized by them as well.
10. Dinosaur egg:
https://skfb.ly/6YVAS
. The scale bar in the image is marked off in centimeters. Estimate
the completeness of preservation.
The egg looks close to perfect preservation with having a similar size to a real-life dinosaur egg.
11. Petrified palm wood:
https://skfb.ly/6V8r8
. Using the terms on the first page, describe how this fossil
was preserved.
This fossil seems to be preserved using permineralization where the original organic compounds of the
palm tree were replaced by minerals such as silica or calcium carbonate.
12. Carcharodontosaur Tooth:
https://skfb.ly/OLpA
. Examine the specimen. What do you notice about the
edges of the tooth?
The edges of the tooth seem to be serrated and have tiny sharp edges which could help the dinosaur
cut through the flesh of prey easily.
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Part 2: Depositional Environments and Fossil Preservation (13 total points)
1.
Examine these different dinosaur footprints below and the illustration of how dinosaur tracks
preserve on the next page. Note the preservation of skin impressions in the right image. Think about
the grain size of the sediment the dinosaur was walking across.
1a. Would skin impressions more likely be preserved if the true track was made in clay or sand?
Explain your reasoning. (2 pt)
I think skin impressions would likely be preserved better through finer grains that are
associated with the true track being made in clay. Clay has finer particles as compared to sand
thus allowing it to capture and refine those finer details.
1b. Would you expect skin impressions be preserved in the natural cast and/or the undertrack?
Explain your reasoning and the circumstances under which the skin impression would be
preserved. (3 pts)
I think skin impressions would be made in the natural cast. The natural cast refers to an
impression left by a creature or object in a soft material or substrate that over time hardens.
When the dinosaur comes into contact with a soft substrate material like clay it can leave an
impression that hardens and preserves the track over time. This is because the finer particles
and adhesive quality of clay allows it to capture the details of the skin effectively.
True tracks, natural casts, undertracks, and track infills are all types of fossil tracks that can be created
from a single step. Figure adapted from Lockley and Hunt (1995).
Images from:
https://ucmp.berkeley.edu/science/trackways/trackways2.php
2.
Compare these two ichthyosaur fossils. What do you think differed between the environments in
which they died? Explain the fossil evidence that led you to your answer. (4 pts)
Fossil 1:
https://3d.si.edu/enter-sea-dragon
Fossil 2:
https://skfb.ly/6U9MD
These fossils show that these two ichthyosaurs lived in very different environments, fossil number one
seems to be more fit for an environment with shallow waters where it was a fast swimmer hunting
down its prey. It also doesn’t seem to be very well preserved meaning it was in an environment with
more oxygen. With fossil 2 when this animal finally died they were most likely buried in mud at the
bottom of the ocean floor and eventually these skeletons were preserved as fossils encased in rock
containing them. Fossil 2 seems as though it would be more in deep sea conditions because of the
condition of the fossil, it is well preserved without much oxygen disturbing it.
3.
Examine this fossil of
Coelophysis
:
https://skfb.ly/HuWQ
. Imagine how this dinosaur might have met
its demise and become part of the fossil record. Based on the way the fossil was preserved, describe
how you think the dinosaur died. Are there modes of death that you can rule out? What are two
features of the environment that must have been true for this dinosaur to be preserved in this way?
(4 pts)
One possible explanation for the way this dinosaur died could be because of some sudden natural
disaster that swept it away such as a flash flood. This is because the fossil record looks mostly intact
meaning nothing was able to scavenge away at it and pick apart the bones and carry them to other
places. It could have been quickly buried in sediment and the way its neck is bent also suggests that it
was dragged away fast from its original location. We can rule out death by disease because it doesn’t
look deformed or being prey to other creatures because it has all of its bones and doesn’t seem to be
scavenged. One feature that is true for this preservation to be true is that it must be in an oxygen poor
environment so the body did not fully decompose, it could also be in a marine environment such as a
flash flood where quick sediment burial is possible.
After completing the lab assignment, take the quiz on Canvas. The quiz will ask you to report some of
your answers from this lab and will provide you with feedback. You should feel free to correct any
answers on your laboratory assignment you found were wrong after taking the quiz before submitting
this assignment to Canvas. You may retake the quiz twice and you will receive your highest quiz score.
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