Pyritized Fossils at Penn Dixie

By Jay Wollin, Educator

While the Penn Dixie site is world renowned for its excellent trilobites, and perhaps even for its incredibly abundant corals, there are small, relatively underappreciated areas of the site which can offer unique and exciting treasures for those willing to take a closer look.

The site is broken up into several main areas which are frequented by our visitors. Among the most popular are our “trilobite beds” and the aptly-named “brachiopod pit”. Different areas of the site represent different exposures and layers of strata. While large portions of the 54-acre site are teeming with fossil horn corals, brachiopods, and trilobites, there are are other sections of the site which represent entirely different conditions.

This location, just to the left of the main parking lot entrance is home to some of Penn Dixie’s pyritized prizes.

During fossilization, there a process known as permineralization. During this process, empty spaces that were present in the living organisms are filled with groundwater, rich in minerals leached from the surrounding materials. This process can fill in very small spaces, even those within cell walls. Depending on the types of minerals present and the conditions during fossilization, this process can result in many interesting types of preservation. One of such types is known as pyritization.

An example of pure iron pyrite, or Fool’s Gold.

Pyrite is an iron sulphide and is often lustrous and gold in color, giving cause for its common name, Fool’s Gold. Under rather specific circumstances it can coat or fill gaps during the fossilization process. Typically, in order for pyritization to occur, organisms must be deposited in seawater areas that are low in both organic matter, and dissolved oxygen. This oxygen-deficient water create what is known as an anaerobic environment. In this environment, certain bacteria are able to survive and flourish. When combined with reactive iron, these bacteria convert the sulphates into sulphides which results in a pyrite mineralization in the remains.

At the Penn Dixie site it is possible to uncover beautiful, golden pyritized specimens, however, the vast majority have exhibit a higher iron content and have weathered out and oxidized to present with a metallic rust colored finish. These fossils are generally rather small in size—averaging roughly 5mm in diameter—and tend only to appear in the site’s “Pyrite Beds.”

Pyritized fossils and pyrite nodules in situ in the Penn Dixie Pyrite Beds.

To the untrained eye, these diminutive fossils could easily be mistaken for pebbles or debris. Upon closer examination however, one can find many interesting examples of Devonian critters. Pyritized goniatites, gastropods, brachiopods, ambocaelia and even trilobites can be found with some effort.

While the “Pyrite Bed” at Penn Dixie represents a rather small and unassuming portion of the site, those willing to spend the time and effort to examine the surface closely can find a veritable treasure-trove of unique and interesting fossils. Don’t be fooled, all that glitters is not gold… sometimes it’s pyritized fossils!

Clockwise from upper-left: Ambocaelia sp.; Greenops boothi; Goniatites sp., possibly G. uniangularis; Nuculites sp.; Loxonema sp.; and an unidentified brachiopod. Click fossils for larger images. All photos courtesy of Jay Wollin.