Modes of Preservation and Depositional Environments.Completed (1)
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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 first page, what type of fossil is this? Explain your choice.
This fossil is a mold. The footprint made a negative impression into the ground (an external mold) allowing us to see that a dinosaur had previously walked in that spot. 2.
Internal mold of a gastropod: https://skfb.ly/FCPx
. Describe how this fossil could have formed.
This fossil formed from recrystallization. An internal mold is similar to a shell; the shell being the original chemical composition. The shell is filled with various inorganic materials changing the mineral
structure of the fossil. Shells are a common fossil to recrystallize. 3.
Fossil seed fern: https://skfb.ly/6YHZS
. Using the terms on first page, what type of fossil is this? Explain your choice.
This fossil is a carbonization. There is a carbon residue in the shape of the fossil that remains. You can
tell that this is the case due to the detail in the shape of fern as well as the patterns on the leaves.
4.
Mosquito in amber: https://skfb.ly/6xWCO
. Using the terms on first page, what type of fossil is this? Explain your choice.
This fossil is a concretion. There is a rounded spherical layer of amber that protected the mosquito in the middle and enhanced the preservation. 5.
Therizinosaurus cheloniformis
https://skfb.ly/BXSQ
. Using the terms on first page, what type of fossil is this? Explain your choice.
This is a permineralized fossil, something that often happens to dinosaur bone (what we can see in the image – a bone/claw). Nothing of the original inorganic material is replaced, but permineralization does add to the existing material. 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?
There are 2 distinct features that allow us to see if the fossil is wood – the growth lines and the knot that appears on the fossil. 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 believe this was an external mold that was used to create a cast. This would make the fossil a positive impression (or positive relief) of the external orientation (in this case the skin). 8.
Plesiosaur Bone Fossil: https://skfb.ly/6RwSD
. Examine the fossil and read the annotations. Describe how this fossil was preserved.
This fossil was preserved through concretion. A layer of sandstone formed around the bone and cemented it to the ground. Because of this the fossil is more resistant to erosion and compaction making the bone less vulnerable to its environment. 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. I believe this specimen can best represent a mold and a cast. The left side would be the external mold
as there is a negative impression of the outside of the fossil. The right side would be the cast as it is a positive impression that duplicated the shape and pattern of the external mold. 10.
Dinosaur egg: https://skfb.ly/6YVAS
. The scale bar in the image is marked off in centimeters. Estimate the completeness of preservation.
There are no soft parts that are preserved. However, the hard parts seemed to have fossilized. Primarily the shell of the egg. This means that the completeness of this preservation is small. 11.
Petrified palm wood: https://skfb.ly/6V8r8
. Using the terms on the first page, describe how this fossil was preserved.
If the palm wood was petrified (or permineralized), the spaces that existed in the original material is filled with an inorganic substance. It adds to the fossil and preserves the existing organic material/inorganic material. 12.
Carcharodontosaur Tooth: https://skfb.ly/OLpA
. Examine the specimen. What do you notice about the edges of the tooth?
The edge of the tooth looks like they would have originally been jagged. Now they look as though they have been slightly smoothed down from weathering and other external elements.
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)
The skin impression is more likely to be preserved if the true track was made in clay. This is because the grain size of the sediment is smaller and can become more easily compact. 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)
If a skin impression is present it depends highly on the type of sediment the impression was originally made in. Sediment with smaller grain sizes will be more easily compacted and hold their shape. This makes a skin impression more likely. Additionally, if this foot print was quickly shielded from the elements (natural or not) there is a better chance that the footprint
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would retain some skin impression. The true track would host the skin impression if one is present. 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
It is clear that fossil two died in an environment that was exceptionally well off for the preservation of
fossils. In fossil two we are able to see a clear outline (shadow) of soft tissues that normally would not be preserved. Places like the bottoms of lakes where a dead animal is easily buried (little to no weathering and scavenging) and there is little oxygen (this means no decomposers) to aid in the decay
of the dead animal. The preservation also indicates that fossil 2 was preserved in fine grain sediment as they hold detail and imprints better than large grain sediments. Fossil 1 seemed more similar to preservation of only hard parts. There were different bones that were identifiable, but as they were found in current day Nevada it is likely the creature died where it could easily be decomposed. This means that it likely took longer for the body to be buried (more exposure to elements and weathering
as well as scavenging). The link to Fossil 1’s website also states that the fossil was incomplete and the skeleton had fallen apart during the decaying process. There were no impressions indicating that the animal fossilized in larger grained sediment. 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)
This fossil was well preserved, so much so that the remains of its final meal were uncovered alongside the dinosaur. This means we can rule out starvation as a cause of death. Furthermore, the preservation (you can see some soft parts and the hard parts - it lays in a death position) indicates that the dinosaur was buried quickly and in a sediment that was fine grained; Like clay, mud, or tar. There were no signs of scavenging, meaning its remains were difficult to find and/or reach or immediately buried. With all of these indicators, it would not be unlikely that the dinosaur died because it was stuck in tar or mud and unable to move and died from lack of oxygen
or water. 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.