::Bollinger County Museum of Natural History::

NATURES NATURAL FORGERIES
By Mike Fix

Concretions, such as this one, are commonly found in rocks world wide and resemble the shapes of animals or bones. This specimen is from Jefferson County, Missouri.

Many people, including fossil experts on occasion, have been fooled into thinking they have found a fossil when in fact it is a natural mineral deposit called a nodule or concretion. These geologic curiosities are called "pseudofossils" which means false fossil.

The shape of a nodule or concretion can be highly variable and therefore they can resemble all sorts of fossils including bones, shells, gourds, nuts and dinosaur eggs. Even those of us who have spent years collecting and studying fossils have been momentarily fooled by these things. Sometimes the accidental resemblance to an actual fossil is truly amazing, but the forgery is always revealed upon a closer look. Some pseudofossils have such amazing features that they are highly prized by collectors.

This concretion has taken on the shape of a skull, it was found in Bollinger County, Missouri and donated to the museum's collection.

These concretions have taken on the shape of large eggs. They were found in Jefferson County, Missouri, and frequently erode from the rocks along Highway 55. Many are picked up by amateur collectors and thought to be dinosaur eggs.

A nodule is a local mineral concentration that is totally different from what the surrounding rock is made of, such as a chert (quartz) nodule in limestone (calcite).

A concretion is a local concentration of the mineral cement that binds the particles in certain rocks. For example, a sandstone concretion is just a part of the rock that has more mineral cement than the rest of the rock, and so it is much harder and more resistant to weathering. So as the softer rock weathers away, the concretion erodes free from the rock.

Probably the most commonly mistaken pseudofossils are chert nodules, which are abundant in many parts of Missouri and Illinois. These lumpy masses of microcrystalline quartz are typically found imbedded in, or weathered out of layers of limestone. They are frequently egg-shaped which causes many people to mistake them for dinosaur eggs.

Here is a pseudofossil that looks like a bear track with foot pad and toe imprints, but is actually parts of small, globular concretions that have eroded out of the surrounding rock.

When viewed under a microscope, the surface of a real dinosaur egg shows a complex pattern of tiny, organized structures that varies from place to place on the shell. Whereas a nodule or concretion looks grainy or rocky across the entire surface.

When in doubt about the genuineness of a "fossil" it is always best to check with a paleontologist - a fossil expert. But don't be too disappointed if your dinosaur egg discovery turns out to be a natural forgery.

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Bollinger County Museum of Natural History
207 Mayfield Drive • Marble Hill, Missouri 63764 • 573.238.1174