Condensed Lecture Notes 10-17

Condensed Lecture Notes 10-17 - LECTURE 10: FOSSILIZATION...

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LECTURE 10: FOSSILIZATION AND EARLY METAZOANS A fossil (which means ―dug up‖) is any sign of previous life preserved in rocks, whether a remnant of an organism itself or some record of its activity. We can divide fossils into two broad categories: body fossils, the remains of an organism, and trace fossils, something else the organism left behind. Trace fossils include footprints (such as the dinosaur footprints on display outside the Texas Memorial Museum), burrows, coprolites (―dung stones‖), glauconite (green mica sand from fecal pellets of marine organisms) and gastroliths (stones kept in the stomach to help with digestion or to serve as ballast). Whole-body fossils are rare. Famous examples include insects in amber (i.e. the mosquitos that supposedly preserved dinosaur blood Jurassic Park) and entire mastodons caught in tar pits or frozen into the tundra. More common body fossils are hard parts like bone, teeth, or shell, which have undergone permineralization , in which the natural pores are filled with minerals by percolating water. Fossils also may undergo wholesale replacement , in which the hard material is replaced atom-by-atom by minerals in fluids. Bone and shell are often to some extent replaced, and the older they are, the more likely it is that there have been one or more episodes of replacement. Replacement can also result in preservation of less durable materials, such as in the creation of petrified wood. Another way to preserve soft material is by flattening it between layers of sediment. If a carbon-rich film remains, we call such a fossil a compression . If all that’s left is an imprint of its form, we call it an impression . In another version of replacement the organism entirely dissolves away while the surrounding matrix remains intact, leaving a mold , or void in the rock in the shape of the organism (or more likely its shell). Later fluids can fill the void with another mineral, creating a cast . A special type of cast, in which the shell of a gastropod is lost but the sediment filling in the shell is preserved, is called a steinkern . The fossil record is very biased – but that’s not a bad thing, just something we need to keep in mind. If we consider paleontology as the study of ancient life, a bias is something that somehow skews our picture, making some organisms more likely to be preserved and/or studied than others. Natural biases include anatomy, depositional environment, and geological history. Human biases stem from geography (how accessible is a fossil locality, and how many paleontologists are nearby); and observer/collector competence, motivation, and research interest. The earliest-known metazoans (―advanced animals‖) are the soft-bodied Ediacaran fauna, which are found in strata from the late Precambrian. Lacking any hard parts, they were probably mostly passive feeders. The Cambrian Period marks the beginning of the Phanerozoic (―evident life‖) Eon, the most recent 1/8 of the history of Earth when signs of life are abundant in the geological record due to the
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Condensed Lecture Notes 10-17 - LECTURE 10: FOSSILIZATION...

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