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He began to identify rock layers by the fossils they contained, and he even noticed that the general order of strata was identical over many different parts of the country.Smith was the first person to understand the principle of fossil succession.The upper layer had scallop fossils, and the lower layer had trilobites.Smith would have brought these two arrangements together, overlapping the common scallop layer, to produce a larger succession of three rock strata!Stegosaurus and Triceratops were not part of the same assemblage because they lived at different times.Obviously, the fossil assemblages change from period to period.Fossil succession is based on the observation that certain assemblages, or groups, of animals and plants have lived during certain time periods over geologic history.For example, human beings and modern elephants are part of the same assemblage because we live in the same time period.
We wouldn't want to use a horseshoe crab fossil, because horseshoe crabs have existed for over 400 million years and are still alive today!
One outcrop shows layers from one geologic time period, while the other outcrop represents a different time. Can he put the pieces together to make the story more complete? Let's find out how scientists deal with this common problem by using the fossils inside the rocks.
Back in 1793, there lived a land surveyor named William Smith.
We want fossils of plants and animals that lived for a relatively short amount of time, like a few hundred thousand years or so.
I know that doesn't seem like a very short time span, but it is when we're talking about geologic time.