Stegosaurus and Triceratops were not part of the same assemblage because they lived at different times.

process of relative dating-51

We talked about relative dating of rocks and how scientists use stratigraphic succession to compare the ages of different rock layers.

You should already understand that the lower rock strata are generally older than the strata found higher up in the rock.

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.

Watch this video to find out how we use index fossils to establish the relative ages of rocks.

In previous lessons, we talked about the Geologic Time Scale and how scientists use it to piece together the history of the earth.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!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.Index fossils help us to distinguish between rock strata from different time periods, so it's important that they don't cover too much historical ground.