Modes of Preservation and Depositional Environments

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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. The dinosaur footprint is a trace fossil because it records the actions and behavior of an organism rather than a part of the organism itself, in this case, the dinosaur’s movement. 2. Internal mold of a gastropod: https://skfb.ly/FCPx . Describe how this fossil could have formed. The internal mold of a gastropod is a mold, classified under body fossil impressions. It could have formed when the "shell" of the gastropod was filled with various inorganic materials such as sediment and when that “shell” dissolved or was removed, left behind the mold. 3. Fossil seed fern: https://skfb.ly/6YHZS . Using the terms on first page, what type of fossil is this? Explain your choice. This is a plant fossil, which falls under the category of an altered body fossil. The fossilized remains of the seed fern have been preserved due to carbonization. 4. Mosquito in amber: https://skfb.ly/6xWCO . Using the terms on first page, what type of fossil is this? Explain your choice. The mosquito in amber is an altered body fossil. It was preserved through carbonization, which is the process involving the loss of the volatile constituents: oxygen, hydrogen, and nitrogen, leaving a carbonized residue. This carbonized residue ended up being 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 as it is the actual preserved part of the organism; it is a claw. It could have been preserved given the slow rates or cessation of bacterial 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? The permineralized wood is an altered body fossil. It was permineralized, which is 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. The features of the fossil that allow me to recognize it as wood would be the growth lines and the knot labelled on the 3D model.

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 in negative relief. It is a mold, an impression of the exterior surface. Molds can form if the dinosaur was buried or decayed, leaving behind an impression of its body like 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, as it is spherical in shape and contains a fossil at their center. They are more resistant to erosion and compaction than the surrounding less-mineralized sedimentary rock, so it could have been preserved through burial in sediment and/or other organic material and minerals. 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 specimen represents permineralization and replacement. Permineralization involves the filling of within the original material with inorganic mineral substances, while replacement involves replacing the original biomineral or organic matter with a substance such as calcium carbonate, silica, or iron compounds. The cephalopod shell has been permineralized with minerals like silica or calcium carbonate, and in some areas, has been replaced by these minerals. 10. Dinosaur egg: https://skfb.ly/6YVAS . The scale bar in the image is marked off in centimeters. Estimate the completeness of preservation. According to the scale bar, the completeness of preservation of this dinosaur egg is fully complete. 11. Petrified palm wood: https://skfb.ly/6V8r8 . Using the terms on the first page, describe how this fossil was preserved. This fossil was preserved through because as you can tell by the holes on the model, the spaces within the original material have been filled with inorganic mineral substances. 12. Carcharodontosaur Tooth: https://skfb.ly/OLpA . Examine the specimen. What do you notice about the edges of the tooth? I notice the edges of the tooth are rough and jagged, meaning the dinosaur was most likely a carnivorous predator as those edges would have helped the animal tear though flesh.

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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 more likely be preserved if the true track was made in clay rather than sand because the grain size of the sediment is a lot finer and much more cohesive in makeup. 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) Skin impressions would not be preserved in the undertrack. They can be preserved in the natural cast and true track, but according to Chapter 3 of the textbook, “the preservation of delicate organic structures, such as skin or eggs, requires a special type of sedimentary environment in which these structures will not be damaged or destroyed.”

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 • One environment was accessible by scavengers (Fossil 1) and the other was not (Fossil 2). • Fossil 2 must have been undisturbed after death. o This may be because it died in a low oxygen environment where scavengers are bioturbators were excluded and physical processes like waves could not disturb 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) This dinosaur might have met its demise and became part of the fossil record by dying in a sudden, catastrophic event seeing as the bones are still completely together when fossilized. They have not moved and were not scavenged. While there was evidence of potential cannibalism among theropods, this was not the case as the bones are completely intact rather than in fragments as if the animal was preyed upon. Two features of the environment that must have been true for this dinosaur to be preserved this way is that the environment was a low oxygen environment where scavengers could not disturb it and neither could physical processes. Also, the environment was probably dry which caused sediment to quickly kick up and bury the dinosaur’s body, resulting in an undisturbed fossilization.

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Modes of Preservation and Depositional Environments

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