Here we have an environmental scene with some areas that have the potential to produce sedimentary rock. The type that might be able to preserve fossils. We have mountains, a river and its flood plain, a forest, a lake and a desert off in the distance. Okay. Now let's pinpoint some dead dinosaur carcases. We want to know which of these dead dinosaurs have the good chance of being preserved as a fossil. Well, we know the bones need to be buried in order to have a chance of being fossilized. So let's introduce a natural phenomenon called rain. And see what might happen to these carcasses. So it rains up in the mountains. The water flows down and causes the river to flood, and fills up the lake. Looks like the carcasses that got buried were ones near the river and the one on the lake shore. The carcasses in the forest, desert, and on the mountainside did not get buried. The desert is dry, and sediments in this area must be deposited by wind, which takes a lot longer than being covered by mud and debris during a flood. As for the carcass in the forest, there's also a good chance that plant roots and microbes in the soil will decompose the skeleton of the dinosaur before it has a chance to fossilize. And, unfortunately, the dinosaur up here on the mountain side, will likely not get covered up by anything. In fact, that carcass will likely rot, then dis-articulate, and weather away to dust. So far, we've mostly been talking about sedimentary environments on land, but there are also sedimentary environments in the ocean. Naturally, most of the fossils that are preserved in marine sedimentary environments are marine animals, but are dinosaur fossils ever found in marine sediment? Although all dinosaurs lived on land, many lived near the coast. On occasion, the carcass of the dinosaur was washed out to sea, where it eventually sank and could be preserved in marine sediments. Although rare, there are several known instances of dinosaur skeletons preserved in rocks that were once an ancient seafloor. So the answer to our question is A, Yes. Different kinds of sediments characterize different sedimentary rock types. Sand is typically deposited in rivers, on beaches, and in dunes. Rocks made out of sand are called, you guessed it, Sandstone. Lake deposits are usually made of mud or silt. We call rocks made of mud, Mudstone or sometimes Shale. Swampy areas with a lot of vegetation can make a special kind of sedimentary rock called Coal. Coal is the fossilized compressed remains of plants. Lagoons and shallow seas often deposit chalky sediments composed of plankton exoskeletons. These rocks are called Limestones. Limestones are really characteristic of marine environments. So, we've talked about environments where we might find complete skeletons, like flood plains, lakes, and collapsed sand dunes. However, more often than not, we find dis-articulated portions of skeletons, as well as single bones. Identify the valid reasons why we may not find a complete articulated skeleton. The skeleton is in a flowing river. The carcass is scavenged. The dinosaur dies in a forest. Or, that the bones are scattered randomly by other animals There are many reasons that we might only find a few isolated bones or groups of jumbled up bones. Predators are really good at breaking up large animal carcasses. A theropod that came upon a dead dinosaur could pull off a leg, or head, or break up the ribs and other bones while feeding. Other dinosaurs may step on the skeletal remains trampling the bones and breaking them apart. So, A, B, and D are correct. As we've already pointed out, bones deposited in fast flowing water will break apart. The further a bone travels in the water, the more worn down it will become. Sometimes we can tell if the bones were deposited in a fast flowing river because of something called long-bone alignment. This means the long bones, like the bones of the arms and legs will be oriented in the same direction because of the physics behind how water flows. Once a bone has been chewed on, trampled, transported, broken apart, abraded and finally buried, fossilization can occur. There are a couple different ways fossils can form, and we call these Preservational Style. For dinosaurs there are two main categories of preservational style, Permineralizaton and Replacement. Here I have two plant fossils, they go through the same process of fossilization that bones would. Permineralization occurs when the internal spaces of tissue and bones are filled with dissolved minerals carried by water. This plant has been filled in with minerals, so it shows permineralization. Replacement occurs when the original material is replaced by minerals. This is no longer the original material. It is a cast, or impression of the original material. This is a fossilized tree root called Stigmaria. To explain what would have happened to this tree root, think about when a dentist creates a mold of your teeth when he's about to make a set of braces. He then creates a cast of your teeth by filling that mold with plaster. Essentially, that's the end result of the process that we're seeing here. There's one last tafonomic effect we're going to talk about. Do you notice anything weird about this skull? This Tyrannosaurus skull is asymmetrical. It looks like somebody sat on it, and kind of squished it to the side. But the bones aren't broken. This is called Plastic Deformation. And occurs after the bones have been fossilized. The weight of the sediments and rocks above the skull and the pressure of the rocks below and to the sides cause a lot of fossils to be squished. Let's take a moment to summarize some of the ideas we've just covered. Let's specify the order of major events that happen between the death of a dinosaur. And its skeleton eventual excavation by paleontologists. Of the following, which specifies the correct order of events? Is it A, death, fossilization, burial, erosion, and excavation? B, death, burial, fossilization, erosion and excavation. Or, C, death, fossilization, erosion, burial and excavation. The correct order is first death, second burial, third fossilization, fourth erosion and finally fifth excavation. So B is the correct answer.
