Fossils: Window to the past

Coprolites & Gastroliths

Both coprolites and gastroliths are trace fossils. These fossil types deal with the behavior of the organism from which they came rather than the actual organism itself.

Coprolites - What are they and how are they formed?

Coprolites are the fossilized or preserved remains of the contents of the intestine and the excrement of organisms: fossilized feces. Coprolites are quite rare because they tend to decay rapidly. They are most commonly found among sea organisms. Coprolites of fish and reptiles are especially common. Typically nodular or contorted in appearance, coprolites are composed of pulverized indigestible remnants of the organism's food, such as portions of scales, bones, teeth or shells. Hence, they are also very phosphatic in composition. These pulverized remains of food, otherwise known as dung-stone, are preserved by process of petrification or cast and mold.

What do coprolites tell us?

As fossilized feces, coprplites indicate the former presence of organisms in the area where they found. They cannot indicate exactly what organisms were present (e.g., the specific animal species). However, there are exceptions. Sharks have a distinctive character of spiral valves in their intestines. Coprolites of sharks show grooves made by those spiral valves. When the organism shows such distinctive characteristics, it would be possible to identify the organism via coprolites.

Coprolites also aid in the interpretation of the diet of the organism. There are also often a good indicator of the ecosystem around that organism. By examining the coprolites and their contents, one can tell where the organism lived (such as in the case of the Ichthyosaurs: extinct, aquatic fish-like reptiles). In one instance, coprolites containing the internal skeletons of two hundred belemnites (a conical fossil shell of an extinct cephalopod) as well as fish scales and bones were found in the Ichthyosaur. This indicates they mostly lived among fish and cephalopods.

Coprolites are worth more than their paleontologic value. There are people out there who treasure them as gifts!

Gastroliths - What are they and what do they tell us?

Gastroliths are fossilized gizzard stones and are usually only applicable in the fossil study of reptiles. These stones, which are found in the gizzard of the organism, help to grind food to pieces as the stomach muscle (gizzard) squeezes to and fro. They have rounded edges and smooth polished surfaces, but they can only be called gastroliths when they are found in presence of reptilian remains as pebbles are also round and smooth. However, unlike gastroliths, a pebble's appearance is a result of erosion by running water or wind-blown dust. A gastrolith's smoothness is a result of the chafing between various gizzard stones. Another indication of the authenticity of gastroliths (i.e., actual gastroliths rather than pebbles) is if they are found among fossils bones where the stomach once was. Gastroliths help identify feeding habits and areas where ancient reptiles lived as mainly ancient reptiles shared this habit of grinding food with gizzard stones.