Fossils Help (page 2)
There are few places more interesting or awesome to visit than a museum of natural history. These institutions that preserve the past can open up a window on the world of the prehistoric Earth and its inhabitants. Dinosaurs, mammoths, ancient whales, and other extinct creatures are displayed for all to see. Fossil bones and replicas of immense, extinct creatures are amazing and scary at the same time. These creatures are the subject of wild imaginings as well as best-selling books and movies.
What is a fossil anyway? Are they really rocks? Where do you find fossils? How do fossils form?
In this chapter, we’ll look at fossils with the view of how they fit into the Earth’s geology and history. The books listed in the references provide a good start to the study of dinosaur types and lifetimes. For a complete study of dinosaurs, marine life, and other prehistoric creatures and fossils, the following Internet websites provide more information:
It has only been within the past 200 years that fossils were recognized for what they are. Before that, people thought strange-shaped rocks that looked like shells, wood, lizards, and other creatures were just oddities of nature, like a rock formation that looks like a bear or the face of a person.
Since most people today read about and play with toy dinosaurs from childhood, it’s hard to imagine a time when no one really knew what fossils were or how they were formed. Usually, depending on where they were discovered, creative explanations thrived. It was not until Darwin published, On the Origin of Species by Means of Natural Selection in 1859 (a best seller at the time), that the idea of extinct ancestors of modern plants and animals seemed possible.
The field of paleontology began about this time. A paleontologist is a scientist who studies fossils. Paleontology includes all fossils, not just dinosaur bones.
A fossil is the remains of a once-living plant or animal whose hard parts (those resistant to decay, like shells, bones, claws, and teeth) have been preserved within the Earth. Clams and mammals, then, have a better fossil record than soft creatures like worms or slugs.
A fossil is formed from the remains or traces of plants and animals preserved by natural causes in the Earth’s crust.
The key to fossil preservation and dating is decomposition. If a living organism is buried quickly before oxygen and microorganisms can break it down, it has a good chance of being preserved. Since shells and bones are fairly porous, minerals from surrounding sediments are deposited in open areas where the once fleshy parts have dried or disintegrated. Geologists find the age of fossils by radioactive dating. The better preserved the sample, the more accurate the dating within certain margins of error.
Skin imprints, tracks, and feces (called coprolites ) from ancient organisms are also considered to be fossils. Can you think of any other profession where the rock-hardened waste from ancient animals is considered a real find?
By studying petrified stomach contents and coprolites (measuring from less than a centimeter to over 30 cm) from dinosaurs, paleontologists have gotten information about the diet of dinosaurs. Because some plants are tough and resistant to digestion, some coprolites are pretty intact. Several plant-eating dinosaurs had a wide diet by today’s salad standards.
Tooth shape also gives clues as to diet. Scientists look at modern animals’ tooth shape for eating habit and diet comparisons. Broad, grinding teeth seem to point to plant eaters, while jagged, sharp teeth are used by meat-eating predators. Figure 10-1 gives you a sampling of different dinosaurs that have been categorized into plant eaters, meat eaters, or omnivores (eats both plants and animals).
Fossil Rock Types
Highly sensitive instruments like mass spectrometers are used to find the chemical makeup of fossil bones and coprolites. From these data, daily diet and mineral composition can be retrieved and applied to plants containing high levels of the same minerals.
Sedimentary rocks that contain fossils were laid down as sediments on floodplains by rivers, streams, lakes, and inland seas. Sediments are formed from layered sand, mud, and plant and animal materials. This preserves and protects prehistoric remains from air and deterioration. Over time, sediments harden and compress into sedimentary rock.
Since igneous rock is formed from super hot magma, fossils are not likely to be found in igneous rock. Metamorphic rock that was originally sedimentary rock contains fossils, but because of change and compaction, fossils are destroyed and distorted.
Fossil-containing rock can also be heated during metamorphism. This heating destroys the smaller, more delicate fossils first, followed by the heavier, sturdier ones. However, like other exceptions in nature, some fossils have survived high heat and pressure depending on specific circumstances.
Tectonic processes destroy fossils as well. Rock pressures stretch and distort fossils just as they do to other types of rock. Cleavage that develops perpendicular to parallel layers of sedimentary rock fractures fossils as well. Shearing, shifting, and uplifting of landmasses smash fossil-containing rock beyond recognition, leaving little behind for geologists to study.
When dating fossils and rock layers, the oldest layer will be at the bottom, while the newest layers are on top. This is true unless a sedimentary rock layer has been compressed or uplifted by tectonic forces. This oldest-to-youngest layering is called superposition .
It is possible for a part of a rock to be incorporated into another sedimentary rock layer that is older. This can be seen when younger rocks or other geologic features cut across older, preexisting rocks. When this happens, it is known as an inclusion .
Sometimes, geologists find that erosion has erased a large part of a rock layer. There are missing pieces. These erosion or nondeposition surfaces represent a certain time period and must be accounted for in other ways.
Significant breaks in the geologic record due to erosion or a lack of sedimentary deposition are called unconformities.
The same thing can happen with fossils. Fossils at the bottom of a sedimentary rock stack are older than the layers above it. The gradient of change in fossil types is also oldest to youngest.
Fossil succession is when older fossils are found in the lowest levels of sedimentary rock layers while more recent fossils are found closer to the surface.
Practice problems of this concept can be found at: Fossils Practice Test
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