GEO 302D: Age of Dinosaurs Laboratory 1: Introduction to Paleontology

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1 GEO 302D: Age of Dinosaurs Laboratory 1: Introduction to Paleontology Welcome to Age of Dinosaurs! In this course, dinosaurs will be used as a means of introducing you to the methods and practice of natural science, particularly geology and evolutionary biology. We could not address the interesting questions about dinosaurs and their place in the history of Life without finding their remains. We would be ignorant of the very existence of dinosaurs were their fossilized bones and traces not preserved within particular rock types within the Earth! We also could not begin to interpret their great age, evolution, and environments in which they lived without studying surrounding rocks and the different geologic processes through which preservation occurs. Indeed, knowledge of rock types and geologic processes are fundamental to paleontological work. This lab will serve as an introduction to paleontology (the study of ancient life and their remains) by reviewing to the major types of rock and how they form along with the concepts of geological time, stratigraphy, and fossilization. Rock Types Before discussing the types of rock, it is important to first understand what a rock is. A rock is a naturally occurring cohesive solid comprising one of more minerals. Rocks are classified on the basis of how they are formed. Igneous rocks are formed by the cooling of molten material. These are classified by the environment in which the molten material cooled: volcanic igneous rocks cooled on the surface of the Earth while plutonic igneous rocks cooled underground. Very few fossils are found in igneous rocks, except for volcanic ash which can cool and preserve fossils (like the former residents of Pompeii). Metamorphic rocks are formed under conditions of intense heat and pressure, where pre-existing rock can be physically and chemically altered, transforming ( metamorphosing") into an entirely new type of rock. They have not been re-melted, but the conditions get harsh enough to break down the bonds of many of the minerals making up the rock. When heat and pressure finally subside, entirely new minerals are formed, such as garnet. Fossils are less commonly found in metamorphic rocks and if they are found they are usually distorted. Sedimentary rocks are formed in one of two ways. One way is from the erosion, subsequent deposition, and lithification of pre-existing rocks. Sedimentary rocks formed by this process are called clastic sedimentary rocks. They are composed of grains ranging in size from very coarsegrained cobbles and pebbles to extremely fine-grained clay particles. Common clastic sedimentary rocks are sandstone, mudstone, breccia, conglomerate, and siltstone. The other way to form sedimentary rocks is by precipitation of minerals from water. This may be accomplished by evaporation (often yielding halite [rock salt]) or through the action of organisms in the water (limestone). Paleontologists pay special attention to sedimentary rocks as these are the most likely to contain fossils, therefore we will study them in more detail. All three categories of rock are useful for understanding the age of rock layers, either with radioisotopic dates (by analyzing igneous and metamorphic rocks) or relative dates (through the deposition of sedimentary layers lying one on top of the other). 2

2 An interesting feature about these three rock types is that any rock type can turn into any other rock type, including new versions of the same type, under the correct conditions. This interrelationship is called the rock cycle, first proposed by James Hutton almost 200 years ago. For example, suppose a particle of quartz forms deep in the earth as part of a large underground mass of igneous granite. Over millions of years, the Earth s crust is uplifted, the overlying rock erodes away, and our little quartz crystal is exposed to the air. A pounding rainstorm knocks the quartz particle off the granite, it rolls down a small stream, and is buried with millions of other rock fragments. After years of burial, dissolved minerals carried in groundwater cement the rock fragments together. Our quartz crystal is now part of a sedimentary rock. Later, the sedimentary rock containing the quartz crystal is shoved deep under the earth s surface by movements within the earth s crust. As the rock is shoved deeper, heat and pressure increase greatly. Eventually the particles within it are physically and chemically changed, resulting in a metamorphic rock. The quartz crystal may contribute its chemical composition to the formation of completely new minerals. If the metamorphic rock continues to be heated, it may melt entirely so that the atoms making up our original quartz crystal are released to reform another igneous mineral. This is just one example of the many different combinations and inter-relationships found in the rock cycle. Sedimentation and Depositional Environments Sedimentary rocks have special importance in the field of paleontology because they are the only rock type that regularly preserves the remains of organisms. By the time you finish this lab, you should be able to identify basic sedimentary rock types and their environments of deposition. All sedimentary rocks are originally deposited in horizontal layers, or beds. You can usually see the lines of contact (bedding planes) distinguishing different beds. However, sedimentary rocks are formed in several ways. 3

3 Evaporites are formed by the evaporation of water. As the water disappears, the minerals in solution begin to precipitate out and form layers on the bottom. Rock salt and gypsum are relatively common evaporites. Carbonates include all limestones and dolomites and are generally formed in warm water environments. They are called carbonates because they contain carbonate ions (CO3). Limestones are made of calcium carbonate (CaCO3: calcite) and are usually biogenic (made by living organisms). Many life forms, particularly marine organisms, secrete calcium carbonate in the form of shells or skeletons. Their remains build up large layers of carbonate sediment, which eventually lithifies (turns to rock) into limestone. Dolomites are similar to limestones in having carbonate ions, but they have some of their calcium replaced by magnesium. They are rarer, and often form as evaporites, or as altered limestones. Nearly all of the bedrock in the Austin area is some form of carbonate. Clastic sedimentary rocks are formed by the erosion, transport, deposition, and lithification of pre-existing rocks. They are categorized on the basis of their grain size and the degree of grain roundness. A grain is a particle or a larger clast (fragment) from an original source rock. For example, very large, well-rounded grains (e.g. cobbles, pebbles) cemented together make a conglomerate. Very large, angular clasts cemented together form a breccia. Other clastic rocks defined in order of their grain size are: sandstone, siltstone, and mudstone. Shale is formed from a very fine mud that is highly fissile, meaning it splits very easily along its thin bedding planes. 4

4 Fossils and Fossilization The odds of an organism making it into the fossil record are not very good. Flesh rots away, hair falls out, bones are weathered and crumble, and organisms can be eaten or otherwise destroyed. There are many biases against preservation, which gives us a largely incomplete fossil record. Today s lab is an overview of the basic methods and forms of fossilization. By the end of the lab, you should be able to view hand specimens and determine the type of fossilization into which it falls, and its most likely mode of preservation. Additionally, you should have an understanding of some of the basic biases that affect the fossil record and our ability to study specimens within it. A fossil is any evidence of past life. There are two broad categories of fossils, body fossils and trace fossils. Body Fossils: As the name implies, body fossils preserve some part of the organism or the shape of some part of its body. 1. Unaltered or actual preservation: This is an exceptional and rare form of preservation. Some organisms have mineralized components in their structure that do not need to be altered in order to exist for long periods of time (e.g. bivalve or gastropod shells that are made of calcium carbonate). In other cases, organisms die in unusual environments that prevent the processes of bacterial decay from taking place. A. Freezing: This occurs only in perennially cold climates. Even a short thaw will allow bacteria to decompose a corpse. Numerous mammoths known from the northern latitudes of Siberia are preserved in this fashion. B. Trapped in resin: This usually only happens to small organisms, such as insects, although at least three fossil lizards and two frogs were preserved this way. Once buried and hardened by various processes, tree resin becomes amber (of Jurassic Park fame). The amount of actual tissue remaining in a specimen varies over time and conditions. C. Asphalt impregnation: Tar seeps are known from a few places around the world; Rancho La Brea in present day Los Angeles is probably the most famous. The oily fluids work into the bones, preventing them from being broken down by microorganisms. D. Unaltered: Some body parts of organisms are already heavily mineralized and do not easily break down. Calcite skeletons of corals, bryozoans, and oysters are good examples. The phosphatic shells of inarticulate brachiopods and insects, and the chitin skeletons of graptolites and arthropods also preserve well. 2. Desiccation: This occurs when the tissues of an organism dry out, losing all of their fluids. They become mummified through natural processes. Several specimens of large Pleistocene- aged (approx. 120,000 to 2.59 mya) mammals are known from caves in the North American Southwest and in high dry mountain caves in South America. 3. Carbonization: As organic remains decompose under water, the volatile elements that make up tissue (oxygen, hydrogen, and nitrogen) are slowly lost in a process called distillation. These elements can also be lost when 5 organisms are buried quickly and placed

5 under pressure. When distillation occurs, carbon concentrations are left behind as a thin film that preserves the shape of the otherwise soft body structure. Plants, graptolites, and fish are sometimes preserved in this manner. On very rare occasions, parts of other vertebrates, such as ichthyosaurs and the feathers of birds and dinosaurs, are also carbonized. 4. Alteration by mineralizing solutions: Clastic sedimentary rocks are often porous; these pores often contain a large amount of fluid. As these fluids move through rock, they tend to either dissolve minerals from their surroundings or deposit dissolved minerals they already carry with them. This can result in several forms of alteration. A. Replacement. The original skeletal material is replaced molecule by molecule by another mineral. As the skeleton is slowly dissolved and carried away, the space left behind is filled in totally, by a different mineral. Fine detail is usually preserved, unlike the condition of recrystallized fossils. Common replacement minerals are silica, calcite, pyrite (called pyritization in this case), limonite, and glauconite. B. Permineralization. There is no destruction of the original material. Instead, mineralized water flows through the porous remains and precipitates mineral deposits within the pores. Eventually they are filled with minerals. This process occurs mainly in wood and vertebrate bone, both of which are very porous. C. Recrystallization. The original skeletal material alters in place into another more stable mineral. This occurs mainly with alteration from aragonite (a form of calcium carbonate) to calcite (another form of calcium carbonate). It usually results in the destruction of fine details, because the alteration process yields larger crystals of the new mineral than the original substance. Recrystallization is common in corals, certain molluscs, and other invertebrate groups. D. Concretions. Fossils may be preserved in spherical or ellipsoidal nodules of hardened material. The decomposing organism provides a localized chemical environment that favors the precipitation of mineral around it. Often the degree of preservation within a nodule is exquisite, but the mineral is very hard. 6

6 E. Molds and Casts. Sometimes, after burial and lithification of the surrounding sediment, the skeleton of an organism is completely dissolved away, leaving an empty cavity in the rock. This is a natural external mold. If sediments or minerals later fill the mold, a three-dimensional copy of the external surface of the object is created. This is a cast. Sometimes, a skeleton such as that of a clam or snail, may be filled with minerals or sediments. THEN the skeleton is dissolved, leaving a copy of the internal surfaces of the shell. This is an internal mold, also called by its German name, a steinkern. 7

7 Trace Fossils: The other major category of fossil is trace fossils. Trace fossils are indirect evidence of an organism or evidence of the activity of past life. 1. Tracks and trails: These are obvious traces of past activity. They can provide valuable insight into the behavior or locomotion techniques of organisms. 2. Burrows: Burrows are usually the traces of feeding behavior in soft, aquatic sediments. Many marine bottom dwellers make their living by burrowing through the soft ooze of the sea floor, ingesting sediment, and passing the digested material behind them. This alters the chemistry of the material enough that the burrow will stand out in a rock unit. Sometimes we find burrows left by vertebrates (like lungfish, gophers and other small, burrowing mammals) in terrestrial sediments, though this is much rarer. 3. Borings. Do not confuse borings with burrows. A boring is a hole drilled into a relatively hard surface, such as a rock, hard bottom sediment, or skeletons of other organisms. Several types of gastropods (snails) specialize in boring into other shelled invertebrates. Some sponges chemically bore holes into objects. 4. Coprolites. These are everybody s favorite trace fossil. Coprolites are fossilized feces. These can yield information as to the shape and features of the soft tissue of the digestive tract, and diet of extinct organisms. 5. Gastroliths: Highly polished, rounded stones sometimes found in association with archosaur skeletons. These are gizzard stones that aided in the physical breakdown of swallowed food items. They have been found with some sauropodomorph dinosaurs. Today gastroliths are used by crocodilians and birds. When paleontologists are out in the field it is important that we have an understanding of the age of the layers of rock from which we are collecting. Rocks and structures are dated in either relative or chronometric terms. Chronometric dates, or numerical dates, are determined primarily through the use of radiometric dating techniques. Relative dating involves the use of basic stratigraphic concepts to figure out the relative sequence of events that occurred to a particular rock layer or structure. Basic stratigraphic principles and concepts: Principle of Original Horizontality: Sedimentary rocks are originally deposited as horizontal layers. Strata that are not horizontal must have been disturbed at some point in the past by movements of the crust. Principle of Lateral Continuity: Sedimentary rocks are initially laterally continuous over large distances. Principle of Superposition: In an undisturbed sedimentary sequence or sequence of interbedded sediments and extrusive igneous rocks, the oldest beds are at the base and the youngest beds at the top of the sequence. 8

8 Principle of Cross-Cutting Relationships: Geologic structures (e.g., faults and folds), intrusive rock bodies, and erosional surfaces are younger than the beds and structures which they cut or affect. Principle of Inclusion: A body of rock that contains inclusions (i.e., unmelted rock fragments) of pre-existing rocks is younger than the rock from which the inclusions were derived. Principle of Faunal Succession: Fossil organisms succeed one another in a definite and recognizable order. Fossil content can then determine the relative ages of rocks. Unconformities: Unconformities are intervals of time during which deposition ceased, erosion removed previously formed rock, and then deposition resumed. In the early days of geology, these principles were among the only means of determining the sequence of events observed in geologic sections throughout the world. Eventually, a Geologic time scale was developed. This arranged the geologic record of the Earth into a standard sequence of units in relative age order. The boundaries were originally based on significant changes in lithology and fossil content. As stated above, fossils were used extensively to set up the geologic time scale. Places in the geologic record where large numbers of fossil forms disappeared, or where entirely different assemblages of organisms suddenly appeared in rocks were used to set the boundaries of various divisions of geologic time. A fine example of this is the boundary between the Cretaceous (K) and Tertiary (T). In marine rocks it was observed that many species of marine organisms went extinct at this time, to be replaced by new, very different species. In terrestrially deposited rocks, non-avian dinosaurs disappeared from the geologic record. This shift in fossil forms was observed in rocks world-wide, and made an obvious choice for setting up a geologic time boundary. Relative dating As mentioned above, relative dating predates chronometric dating. However, relative dating is still used today and one method of relative dating, biostratigraphy, is critically important in paleontology. Biostratigraphy is the correlation of geographically separate rock sequences using fossils in conjunction with lithology. Biostratigraphy is built upon a few basic concepts. First, each species has its own, unique, geologic range. This range corresponds to the time when the species was extant. Ideally, the rocks in which fossils of a species are contained should have been deposited at the actual time the species existed. The next important concept is the Principle of Faunal Succession. Species appear in the geologic record, persist for some length of time, then disappear, and are replaced by something new. This succession is most easily observed in marine rocks and can be traced over large geographic areas, in some cases even world-wide. Finally, the same species cannot evolve twice; extinction is forever. Unrelated organisms may look similar, but this is usually due to convergence upon a similar lifestyle or environment. Index fossils are fossils that have a short temporal (time) range, but a wide geographic distribution. They are also abundant, appear in a wide range of lithologies or paleoenvironmental settings, and have a distinctive anatomy. Index fossils are extremely useful in biostratigraphic correlation. Many planktonic (free floating, marine) organisms make good index fossils. Other widely used index fossils are ammonites (shelled, squid-like creatures), ostracods (tiny, shelled crustaceans), and graptolites. It is relatively easy to correlate beds that retain the same lithology from one geographic locality to another. Take a look at the figure below. In section 1, a limestone layer is sandwiched between two sandstone layers. In section 2, taken several miles away, the 9

9 limestone layer is also sandwiched between two sandstone layers. The correlation of these beds is quite simple. To graphically depict correlation between sections, simply draw a solid line from the contacts (boundaries) of units hypothesized to be equivalent. Now look at the two stratigraphic columns below (3 and 4). In these sections there is only one bed in each that appears to be lithologically equivalent, bed B and bed Y. Based on lithology alone, the surrounding layers cannot be determined to be equivalent. This is where fossils come in handy. Notice the presence of identical fossil species in layers A and W. Even though the lithology is not the same, they contain the same fossils, indicating they were laid down at the same time. Solid lines are drawn to their contacts. These lines indicate equal points in time, and are called isochrons. The same goes for units C and Z. When contacts are inferred, dashed lines are drawn. This occurs when a unit is at the top or bottom of a section and you cannot determine the true point of contact. It also applies to beds or units that do not have equivalents in other sections. Take a look at unit X. It does not share lithology with any units in column 1. It does share fossils with B. This suggests that it is equal in age to B, but that whatever environment deposited layer X did not reach geographically as far as column 1. 10

10 Absolute Dating Absolute dates are calculated from the decay of radioactive isotopes found in certain rock types. The ability to accurately measure radioactive decay rates has only existed since the latter part of the 20th Century. Radiometric dating provides us with an actual numerical value for the age of rock units. For instance, in the example above, the sudden disappearance of a large number of species prompted early geologists to create the division between the Cretaceous and Tertiary Periods in the Geologic Time Scale. However, the absolute age of the extinction event at the boundary remained uncertain until the advent of radiometric dating techniques. We now set the K/T boundary at 65 ± 3 million years ago. You may occasionally see a geologic date given followed by Ma or Ga. This is the abbreviation for millions of years ago (Ma) and billions of years ago (Ga). Some definitions which you ve probably seen before, but may need a reminder: Isotope a variety of an element with a different atomic weight. (For example: carbon-12 is the most common variety of carbon, but there are also carbon-13 and carbon-14) Daughter isotope the new element created when unstable, radioactive atoms release radioactive particles, changing their atomic number. Half-life the amount of time required for one half of a given sample of a radioactive element to decay into its daughter isotope. Radiometric dating is possible because radioactive isotopes decay at measurable and constant rates. As magma or lava cools, mineral crystals begin to form. The crystals incorporate atoms from their surroundings, some of which are unstable isotopes of various elements. Once the crystal has formed, the radioactive material in it begins to break down into daughter isotopes. Given knowledge of the length of the half-life of a particular isotope, measuring the ratios between the amount of remaining parent and daughter isotopes will yield the time that has elapsed since the mineral crystal formed. There are limitations on the usefulness of radiometric dating. First, readings become inaccurate if the sample being tested is younger than 1/10 of a half life, or if it is much older than ten times a half life. This is because our current technology is unable to detect the incredibly small amounts necessary for greater accuracy. Second, a real handicap is the fact that radiometric dating cannot be used to directly date most sedimentary rocks. This is because many sedimentary rocks are composed of minerals that were formed in igneous rocks before being incorporated into a sedimentary rock. This means that the dates obtained are not for the formation of the sedimentary rock. 11

11 Exercise 1. Identify the following rock types and briefly describe how each was formed. a. b. c. 2. Which of the rocks in question 1 is most likely to contain a fossil? 3. What mode of preservation is represented by this specimen? a. b. c. d. e. 12

12 4. The diagram below shows two stratigraphic columns, separated by great geographic distance. Use both lithologic and temporal evidence to correlate the beds. 13

13 2. In lab and in class you have used anatomy of organisms to understand their relationships to each other. Another way to think about organizing these taxa is by their time ranges. The following list contains various extinct and extant taxa with their associated age ranges. Trilobite: ~526 Ma 250 Ma Dinosaurs: ~230 Ma 65 Ma Crocodile: ~200 Ma Present Shark: ~450 Ma Present Horse: ~55 Ma Present The stratigraphic column below represents a great expanse of time. You find fossils in different layers throughout that you think can help you elucidate the ages of select layers. You also use radiometric dating on an ash layer (unit B), finding it to be 70.7 Ma. Using the stratigraphic ranges listed above for the fossils you found, give the possible age range for each of the following units: A. B. C. D 14