English: Labechia sp. - fossil sponge from the Ordovician of Ohio, USA. (Jack Kallmeyer collection)

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Exhibit info.: What are stromatoporoids? For many years, paleontologists weren't certain how to classify fossils called stromatoporoids. These fossils all had shared characteristics with each other but no modern analogue was known. Early classifications grouped these fossils with coelenterates, the group of animas that include the corals in the Phylum Cnidaria. In the 1960s, divers found a different kind of sponge called a sclerosponge that closely resembles the stromatoporoids. New classifications place the stromatoporoids as a class in the Order Sclerospongiae in the Phylum Porifer (the group containing the sponges).

The stromatoporoids secrete a calcareous skeleton that forms mounds, columnal forms, and even encrusting forms. These were not soft sponges like bath sponges. The actual living tissue is only at the surface of the structure and at most a few millimeters below the surface. The rest of the structure is the older "dead" part of the colony. Like other sponges, the stromatoporoids were filter feeders moving water through the living tissue in order to trap tiny food particles. Some fossil stromatoporoids have small ridges radiating in a starburst patter (astrorhizae) away from raised surface bumps (mamelons). These radiating patterns are thought to be the remnants of the canals used to move water through the animal. In forms where these structures appear absent (like those shown in this display), it is thought that the astrorhizae canals existed only in the soft tissue of the animal and thus were not preserved.

In examining these specimens, you will note that stromatoporoids grow in layers. Stromatoporoids should not be confused with stromatolites, even though both are layered. The organisms that create stromatolites are various types of bacteria.

The massive forms of stromatoporoids were significant contributors to reef formation. In some parts of the Ordovician south towards Lexington, Kentucky, large stromatoporoids can be seen in roadcuts along Interstate-75 as light colored oval boulders in the gray matrix. These forms can be several feet in diameter. While the specimens displayed here were not these large reef formers, they appear to have covered a significant area of the sea floor.

A stromatoporoid required a hard surface to being growing and any hard surface, regardless of size, was apparently sufficient. They may have begun growing on fragments of shells on the sea floor. Considering the ultimate size of these sponges, they could quickly outgrow a small substrate and become supported by the sea floor itself. On muddy bottoms, the stromatoporoid could sink in to some degree as it grew. Stromatoporoids also grew on top of other stromatoporoids.

This display contains stromatoporoids from a single locality in the Elkhorn Formation south of Dayton, Ohio. All of the specimens are classified in the genus Labechia, which has an internal structure of pillars and cysts.

While all specimens are in the same genus, they exhibit different growth habits. This could be an indication of different species being represented or it could alternately mean that three was an environmental change that altered the growth habit of a single species.

In general, we find two forms in this locality: 1) a lighter colored form with prominent mamelons and broad growth bands; and 2) a dark form with very low mamelons and narrow growth bands. The dark form occurs in a restricted band of dark shale.

One of the unique features of this occurrence is the preservation. Unlike stromatoporoids from other formations in the Cincinnatian that are calcitic, these stromatoporoids are silicified.

[Shown above is a] stromatoporoid showing prominent mamelons. This is is the lighter colored form.

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Classification: Animalia, Porifera, Stromatoporoidea, Labechiida, Labechiidae

Stratigraphy: Elkhorn Formation, upper Richmondian Stage, upper Cincinnatian Series, upper Upper Ordovician

Locality: undisclosed site south of Dayton, southwestern Ohio, USA