Explanation:
The type of rocks in which dinosaur fossils (and almost all other fossils) are found is called sedimentary rock. Sedimentary rock generally occurs as flat layers called strata (single layers called stratum). Imagine you have a handful of both coarse and very fine sand and a bucket of water. If you were to drop the sand into the water two things would happen. First, the sand would settle to the bottom of the bucket. The coarse sand would settle faster and then the fine sand would slowly settle out. Secondly, when the sand reached the bottom of the bucket it would spread out forming two flat layers, a layer of coarse sand covered by the layer of fine sand. The process of settling is called sedimentation from the Latin, sedimentum, meaning the act of settling. The layers formed across the bottom of the bucket are called strata, from the Latin word stratum, meaning covering.
Virtually the identical process occurs in nature. Imagine a river carrying lots of sand and other material worn from rocks emptying into a large body of water like a lake or ocean. Flat layers are created on the bottom of the body of water. Dinosaur bones carried by the river would also settle out and be found in one of the layers on the lake or ocean bottom. Eventually it would be covered by subsequent layers. Over time, the soft layers would be turned into rock, the bones would be preserved within the layers, and finally they might be raised to the surface where a lucky paleontologist might recover them for a museum.
Looking for bone layers is much like playing detective. Using 4.6 billion years as the age of the Earth and 164 million years for the tenure of dinosaurs on the planet, it is easy to determine that dinosaurs existed on Earth for only a very short part of the Earth's history (only 3.6% of the time). Holmes (see References below) lists a maximum total thickness of 452,000 feet of rocks that are known to contain fossils of animals with hard parts; only 125, 000 feet of which were deposited when the dinosaurs existed. If only 28% of the fossil-bearing rock thickness may contain dinosaur fossils, how do we search for them?
The first thing we must do is narrow the search. We must first locate the 28% of sedimentary rocks in which dinosaur fossils may be found. Let's consider an ideal situation. A paleontologist from Texas went on vacation to Montana where he discovered dinosaur bones in a particular layer of rock. He wondered if the same layer existed in Texas so he could find bones closer to his home. If that particular rock layer appeared on the Earth's surface, continuously, all the way from Montana to Texas he would be able to walk along the layer to follow it. That would be easy, but, unfortunately, the easy way never seems to work. In many cases the bonelayer would be covered by layers of younger rocks, and in other cases the bone layer could be worn away by erosion. Both covering and erosion would make it impossible to follow the layer from Montana to Texas. Clearly, another method must be found. One of the most reliable ways to locate fossils is with other fossils. Certain widely found fossils represent animals that only lived on Earth for a very short time. Such fossils are called index fossils. When found in two geographically separated layers, they allow scientists to say, with some confidence, the layers are the same. Index fossils can be used to track a layer from Montana to Texas, or from Montana to France. The process of determining if layers in different places are the same is called correlation.
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Once the layer is found, more work needs to be done. Dinosaurs probably did not live everywhere in that layer. We know that dinosaurs did not live in the oceans. We can avoid searching in areas that fossils tell us were oceans at the time of the dinosaurs. Fish and marine reptiles (such as plesiosaurs) did not live in the same places as dinosaurs. If paleontologis ts find the former, they won't find the latter. Index fossils allow us to not only determine what layers contain other fossils, but also where in that layer certain fossils may be found
