How is sedimentary rock important to paleontologists

Sedimentary rocks made from mud, sand, gravel or fossil shells and volcanic lava flows are laid down in layers or beds. They build up over time so that that the layers at the bottom of the pile are older than the ones at the top. Geologists call this simple observation the Principle of Superposition, and it is most important way of working out the order of rocks in time. Ordering of rocks and the fossils that they contain in time from oldest to youngest is called relative age dating.

Once the rocks are placed in order from oldest to youngest, we also know the relative ages of the fossils that we collect from them. Relative age dating tells us which fossils are older and which fossils are younger. It does not tell us the age of the fossils. To get an age in years, we use radiometric dating of the rocks. If there were no fossils and you could only use the characteristics of the rock layers it would be even harder!

This is because at any given time, very different types of sediments can be deposited in different places. It is these sediments that will eventually become the sedimentary rock layers making up the stratigraphic sections. At any given time, mud may be slowly collecting in some places while in other places sand is piling up rapidly.

In other places, maybe there is nothing collecting at all! So you see, very different looking rock layers may mark the same time interval in different stratigraphic sections! The process of matching up equivalent "time layers" of rocks in different places is called stratigraphic correlation.

One of the best and oldest tools for correlating strata around the world is the use of special fossils called index fossils. Index fossils of organisms have two important characteristics. First, they must have been widely distributed around the world. Second, they must have only existed for relatively short periods of geologic time before becoming extinct. Consider a fossil of an organism that only lived in one place, or that existed for very long periods of geologic time. They are searching for early human hominin fossils.

Hominin fossils are the key to our Homo sapiens' history. Knowing the environments animals lived in millions of years ago helps scientists know how the animals evolved. In fact, fossils document the evolution of animals and their environments as far back as million years ago.

This huge collection of fossils is known as the fossil record. Fossils leave clues to the past. They are remains or traces of organisms that have been preserved in the rocks. Fossils are typically found in sedimentary rock, a type of rock that was deposited by wind or water. You can find sediments at the bottom of lakes, rivers, and oceans.

They cover hills, make up sand dunes and beaches, and fill in valleys. By being buried in sediments, bones are protected from things like animals stepping on them, or even rain and wind slowly breaking them down into pieces. In this way they are preserved to become fossils. The fossils came from within silty-sand below a thick sandstone layer. The sandstone sand cemented into rock tells us there was a river nearby because you need flowing water to move and deposit the sand grains.

The sediment the fossils were found in tells us that maybe the river flooded and the fossils spilled out of the river and were buried quickly. Also, the bones did not show any weathering marks from sitting on the surface for a long time. If they had not been deposited and buried like they were, these bones may have never become fossils. The fossil record is not only about bones. Geology is the study of the Earth: Rocks, sediments, and the processes that shape mountains, create oceans, and move lands.

From this field of study we can improve what we know about a fossil site or group of fossils. The sediments associated with the fossils can help date them, tell us what kind of environment the organisms lived in, where and how they died, how fast they were buried, and how long they were exposed to the elements.

When a volcano erupts it creates a huge ash cloud that can be carried around the world by wind. And as we all know, what comes up must come down. This volcanic deposit is called tephra. One way geology is used in the fossil record is to date fossils through tephra. Looking at the amount of two isotopes and comparing them against each other, geologists can date sediments. Geologists use this method on crystals that are found in tephra. These crystals give us an age that we use to date the tephra and the eruption that it came from.

This age is then used to tell us how old fossils are that are related to that tephra. Most animals need to live in certain habitats because of the kinds of food, shelter, and climate that habitat provides. A hippopotamus needs slow-moving rivers or lakes. A cheetah needs open grassland to run and catch prey.