Ask-a-Geologist #10: What Are The Remains Of Once-Living Organisms Found In Sedimentary Rocks?

Hi everyone! We have another great Ask-a-Geologist question from one of our readers. Today’s question is about the fossils in sedimentary rock.

What are the remains of once-living organisms found in sedimentary rocks?

Shane wrote to us and asked about the terms geologists use for fossils.

Hi Shane, Fossils are the remains or impressions of once-living organisms that you can find in sedimentary rocks. There are two basic types of fossils that geologists and paleontologists talk about. These are body fossils and trace fossils. Body fossils are a real body part of an animal such as a dinosaur bone or a shark tooth. A trace fossil is something that shows evidence that something was there such as tracks, burrows, trails, molds, casts, and impressions.

Body Fossils Shark Teeth and Snail

Examples of Trace and Body Fossils in Sedimentary Rock

The shark teeth and snail shells that I show in this video are examples of body fossils. These types of fossils can either be the original shell, tooth, or bone material or the fossil could be altered somewhat but the shape and details of the original are intact.

Trace Fossils in Limestone

The rocks that I show here are fossiliferous limestone. This rock gets its name from the abundant fossils that are a part of the sample. Some are bits of shells but some are trace fossils such as the impression of a shell and areas which appear to be burrows from animals.

You can also see a fossil that looks like a snail shell that was replaced by limestone but holds onto the shape of the original part. Other trace fossils that you might see in books or collections include leaf and fish impressions that are common in sedimentary rocks.

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