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LECTURE 2: Taphonomy and Time OUTLINE

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LECTURE 2: Taphonomy and Time OUTLINE 1 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 1 2 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) 2 3 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. 3 4 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. 4 5 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 5 .
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