File:Petroxestes pera bivalve borings on limestone hardground (Turkey Track Layer, Waynesville Formation, Upper Ordovician; Flat Fork Arm of Caesar Creek Lake, Warren County, Ohio, USA) 9.jpg

English: Petroxestes pera Wilson & Palmer, 1988 - bivalve borings in the Ordovician of Ohio, USA. (hand lens for scale)

These slit-shaped structures are borings made by fossil clams. Borings are one of many categories of trace fossils - any indirect evidence of ancient life. Other examples of trace fossils include burrows, tracks, trails, footprints, and bitemarks. Traces (also called ichnofossils) record the behavior of ancient organisms. Trace fossils are given scientific names in a similar style as living organisms or body fossils.

The Petroxestes borings shown here near Cincinnati, Ohio were made by the bivalve Corallidomus scobina. Bivalves are bilaterally symmetrical molluscs having two calcareous, asymmetrical shells (valves) - they include the clams, oysters, and scallops. In most bivalves, the two shells are mirror images of each other (the major exception is the oysters). They occur in marine, estuarine, and freshwater environments. Bivalves are also known as pelecypods and lamellibranchiates.

Bivalves are sessile, benthic organisms - they occur on or below substrates. Most of them are filter-feeders, using siphons to bring in water, filter the water for tiny particles of food, then expel the used water. The majority of bivalves are infaunal - they burrow into unlithified sediments. In hard substrate environments, some forms make borings, in which the bivalve lives. Some groups are hard substrate encrusters, using a mineral cement to attach to rocks, shells, or wood.

The fossil record of bivalves is Cambrian to Recent. They are especially common in the post-Paleozoic fossil record.

Petroxestes borings very rarely have a bivalve body fossil still occupying the slit. The type specimen of Petroxested pera is a boring incised into a solid, calcareous, bryozoan colony. The examples shown here were drilled into a carbonate hardground, which is a synsedimentarily-cemented surface directly exposed on the seafloor. Some hard-substrate seafloor surfaces are composed of other rock types - such as basalt - and are called rockgrounds. Many marine hardground environments were in moderately high-energy, relatively shallow water. Organisms occupying such settings frequently physically attached themselves to the hardground seafloor (= encrusters) or drilled into the substrate (= borers).

Petroxestes pera borings were sometimes so abundant on ancient Cincinnatian hardgrounds that early stratigraphers called the surfaces "turkey tracks", in reference to their similar appearance with certain bird footprints (e.g., see: www.flickr.com/photos/jsjgeology/26798916929).

That the term "turkey track" referred to Petroxestes hardgrounds has been forgotten over the decades. Modern Cincinnatian geologists misidentify Trichophycus venosum-burrowed surfaces as the "turkey tracks" of old - I've seen this in the literature and in museum exhibits.

The best true turkey track layer I know of is the one shown here - it's an extensive horizon at Caesar Creek Lake State Park in Ohio. Exposures and talus blocks can be seen in the emergency spillway and along the Flat Fork trail and at creek cuts along Flat Fork itself. Some of the best examples in the spillway have been vandalized. The only occurrences I have seen for certain are in the upper Waynesville Formation. I have also encountered this, or a similar, horizon in the Waynesville Formation near Dayton, Ohio.

Stratigraphy: Turkey Track Layer, upper Waynesville Formation, Richmondian Stage, upper Cincinnatian Series, upper Upper Ordovician

Locality: trailside gully along the Flat Fork Arm of Caesar Creek Lake, northeastern Warren County, southwestern Ohio, USA