What did I find? A guide to fossil types and preservation

A lot of fossil discovery and identification is about pattern recognition. Take a look at the images below to learn more about which parts of creatures are preserved, and what those parts are called.

You will see also see links that will take you to videos about the process of fossilization interspersed throughout this page of the exhibit. The links will take you to an external YouTube video created by the museum.

TRILOBITE (Arthropoda):

The name Trilobite means "three part body" because the creature is laterally split into three separate lobes; the left pleural lobe, the axial lobe (center), and the right pleural lobe. Further, the animal is splite into three parts called the cephalon (head), the thorax (body), and the pygidium (tail).

Trilobites were marine organisms that are first found in the Early Cambrian, when they were small and eyeless. They diversified rapidly in Paleozoic Seas, developing complex, multi-faceted eyes and complex ornamentation on their exoskeletons until their extinction at the end of the Permian Period.

CRINOID (Echinodermata):

Crinoids may look like flowers, but they are actually animals that are most closely related to sea urchins (Echinoderms). Crinoids commonly disarticulate (fall apart) before they are preserved as fossils, so you are more likely to find pieces of stems or calyx rather than whole specimens. There are multiple species of crinoid still living today, but their diversity is much less than it was during the Paleozoic.

SNAIL (Gastropoda):

Snails are mollusks that live in a variety of environments from marine, to freshwater, to land. Just as their habitats are diverse, so are the shapes and sizes of their shells. Here we have chosen to show a genus that is commonly found in the Late Cretaceous strata of Mississippi as well as in today's oceans, the Turritellid.

CLAM (Bivalvia):

Clams, also known as bivalves, are mollusks with two shells. There is no symmetry in clam shells, which makes it easy to tell them apart from another unrelated animal with two shells, the Brachiopod.

The animal itself was soft-bodied, and rarely preserved, but we have plentiful articulated and disarticulated valves, as well as steinkerns (internal molds) in the fossil record to study.

CEPHALOPODS (Cephalopoda):

Cephalopods (mollusks) include Ammonites, Nautiloids, Cuttlefish, and Squids. Ammonites were tentacled, soft-bodied animals that lived in the oceans from the Devonian (400Ma) until the K-Pg extinction at the end of the Cretaceous (65Ma). Their size varied greatly from one species to another, and their shells were divided into chambers by walls called septa. The animal lived in the largest chamber. They moved through their environment by shooting water out of a tube called a hypernome, and had very good vision.

Nautiloids have similar form to the Ammonite, and some are still living today.

Squids are also very similar, but do not have an external shell. Instead, they had a hard, internal "pen" which is what is most commonly preserved in the fossil record.

BRACHIOPODS (Brachiopoda):

Brachiopods had two shells, like the bivalve mollusks, but are completely unrelated to clams and mussels. Unlike the bivalve, Brachiopods have bilaterial symmetry across the center of the shell. Clams, if they are symmetrical at all, show symmetry between the two valves. Rather than being a soft-bodied muscle, brachiopods are a pair of spiraled lophophores and


The word Mosasaur comes from a combination of the Meuse River in the Netherlands (where they were first described) and the Greek word for lizard, "sauros". Because they were marine reptiles they were not dinosaurs.

Mosasaurs were formidable predators in Cretaceous seas (65Ma-100Ma) that are known to have hunted fish, ammonoids, large clams, turtles, and even other Mosasaurs. Some mosasaurs were quite small, measuring only up to 4 feet in lenght, while others grew to nearly 60 feet!

It is relatively common to find parts of Mosasaurs, such as individual back bones (vertebra) or teeth, in Cretaceous sediments. It is much less common, however, to find articulated (whole) specimens! If you ever find an entire, articulated skeleton of a creature, please call your local paleontologist to document the find!


The chances that an organism will be preserved is called "Preservation Potential" and heavily depends on the type of creature (i.e. does it have hard parts,) its life habits (i.e. does it live underground,) and its living environment (i.e. does it live part or all of its life in or near water). Because so many variables have to be just right for an organism to be preserved, you can imagine that there are many more creatures that are NOT preserved than there are creatures that are preserved as fossils.

There is an entire sub-discipline of paleontology, called taphonomy, that studies everything that happens to an organism from the time it dies and is preserved, all the way until the time it is discovered. Taphonomy can help us understand the life habits and preferred living environments of an organism, how some creatures are preserved while others are not, and even how an individual creature may have died.

Sedimentary rocks that form underwater are the most common rock types to preserve fossils. Below we will discuss some of the ways in which fossils form, and some of the common "pseudo-fossils (rocks that appear to be fossils but are not) that can be found in Mississippi.

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Thumbnail image of Fossilization Video.


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Fossilization - Permineralization

With permineralization, existing pore spaces and cells are slowly filled with minerals over time. After an organism is buried by sediments, mineral-rich ground water begins to saturate and pass through the remains, which leads to permineralization. 


Mineral replacement is similar to permineralization, but instead of filling in existing pore spaces the actual bone or shell material is replaced by minerals. Mineral rich ground water dissolves the original material while replacing it with minerals at the same time. This preserves the form of the organism's hard parts. Common replacement minerals include quartz (silica), calcite, opal, and pyrite.


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Steinkerns in snail and clam

Steinkerns (internal molds) or impressions (casts of the outside portions of an organism) form when the inside of a cavity (such as a shell or a brain case) is filled with compacted sediment and hardens over time. The shell or bone then dissolves away, leaving only an internal mold behind.

Impressions (external casts) are the opposite of internal molds. In this case, the outer portion of a shell, bone, or plant is pressed into sediment, which leaves behind a record of the shape and ornamentation of the outside of the organism.


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Thumbnail image of Insect Fossil Video

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Compression Fossils - Shale, formed at lake or sea bottom

Related to external casts, compression fossils are formed in very fine-grained sedimentary rocks, called shales, at the bottom of lakes or seas. An organism sinks to the bottom of a lake or sea and is quickly covered by fine sediment. As sediment accumulates, the overlying weight compresses the organism and only a thin film remains. To discover these fossils you must hit the side of a piece of shale with a hammer. The rock will automatically split along the plane where the fossil is preserved, revealing a part and counterpart (mold and cast).

This type of fossilization most often preserves insects, plants, and small vertebrates such as fish and birds.



Trace fossils include any evidence that an organism existed that is not part of the organism itself. For instance, you may find burrows or living chambers, footrpints or trackways, evidence of feeding or eating, evidence of predation or scavenging, or fossilized poop (coprolites). These fossils can tell us a lot about the life habits or physiology of the organism that made them.


Pseudofossils are any rock or mineral that looks like a fossil but is actually just a rock. Don't feel bad! Even paleontologists can be fooled by pseudofossils at times because they are very interesting, beautiful, and have forms that mimic organisms! Here are a couple of the most common pseudofossils that can be found in Mississippi.


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Septarian Concretion (nodule)


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Concretions - iron and calcium carbonate


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Fossil Field Guide