Modes of Preservation and Depositional Environments

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Apr 3, 2024

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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 is a mold fossil. This is because it is an impression in the ground made by a dinosaur footprint. 2. Internal mold of a gastropod: https://skfb.ly/FCPx . Describe how this fossil could have formed.  The sea snail could have been buried in the ocean and its organic material was replaced by a material similar to calcium carbonate and hardened. 3. Fossil seed fern: https://skfb.ly/6YHZS . Using the terms on first page, what type of fossil is this? Explain your choice.  This type of fossil is a carbonized fossil. This pteridosperm fossil is carbonized because it looks like it died and left an impressed on a rock that took its form. The details of the plant are still present as well which makes me think that this fossil is a carbonized fossil. 4. Mosquito in amber: https://skfb.ly/6xWCO . Using the terms on first page, what type of fossil is this? Explain your choice.  This is an unaltered body fossil because the mosquito is preserved extremely well due to the amber. 5. Therizinosaurus cheloniformis https://skfb.ly/BXSQ . Using the terms on first page, what type of fossil is this? Explain your choice.  This is an unaltered body fossil (hard part). The skeletal material appears to be replaced by some organic compound that has preserved its shape and form. 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?  The feature that allows for this to be recognized as wood is the rings within the fossil. Further more the knot of the fossil is the size of a branch. 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.  This impression is a positive relief fossil. This fossil is a cast. The dinosaur could have died and sedimentary material could have filled the decaying space of the organism growing and hardening on the external side of the dinosaur leaving a fossil

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 minerals cementing themselves to the bone. Over time more and more minerals do this until it eventually creates a protective cast. 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 is a body fossil impression that is both a mold and a cast. If you look at the right side you can see the negative impression made by the fossil. On the right side you can see the positive impression made by the fossil. It was both surrounded by and made an impression on sediment. 10. Dinosaur egg: https://skfb.ly/6YVAS . The scale bar in the image is marked off in centimeters. Estimate the completeness of preservation.  I estimate that this is partially preserved. I say this because it look very good but there is chipping and damage taken off the egg. Overall it looks like its spherical shape. 11. Petrified palm wood: https://skfb.ly/6V8r8 . Using the terms on the first page, describe how this fossil was preserved.  The tree died and was buried in sediment. Over time the wood got permineralized and its organic material was replaced by mineral preserving the wood. 12. Carcharodontosaur Tooth: https://skfb.ly/OLpA . Examine the specimen. What do you notice about the edges of the tooth?  I notice that the edges of the teeth are rigid, almost like a side of a knife.

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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)  Skin impressions would be more likely to be preserved in clay. This is because clay grains are finer than sand. This would allow it to create a close mold relative to sand grains. Clay is also a lot finer and more impressionable than sand. Sand grains are large due to their course texture. 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 would expect for skin impressions to be preserved in the natural cast rather than the undertrack. This is because the natural cast will be finer and easier to mold than the undertrack. However, it is still vulnerable to erosion. I think the best circumstance which would preserve the best would be a rapid burial.

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  Firstly, the biggest difference between these fossils is that one is significantly better preserved than the other one. Secondly, fossil 1 is preserved in what seems to be a group of the same species. Fossil 2 is preserved as an individual. I think the environment in Fossil one was more exposed than in the environment of fossil 2. This is because the fossils are significantly harder to see which can be due to weathering and exposure. I believe that Fossil 2 was buried because of how well it was preserved. It looks like it was buried, sheltering the bones from weathering and erosion. 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)

 I believe that this dinosaur met its demise naturally and not due to any sort of predator. I say this because the body is very well preserved and does not look like it was eaten. I think that this dinosaur died of some sort of natural cause, whether it be thirst, infection, or age. I think after it died it was buried since the fossil is preserved very well. It was sheltered from erosion and corruption. It also had to be buried in some sort of clay like material, some mineral with very fine grains. This is because the fossil is very detailed to a point that could not be achieved by a bigger grain like sand. The environment would also have to be free from scavengers in order to preserve this fossil to the degree that it is preserved to. 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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