Devonian Mollusks at the Falls of the Ohio and Surrounding Areas

by Alan Goldstein

                      External Cast (Shows shell structure)                Internal Cast (Lime mud that filled shell)

             Figure 1a.) Pseudoaviculopectin princeps (Hall)   Figure 1b.) Turbonopsis shumardii DeVerneuil

     What would it have been like to be a shell collector during the Devonian period, between 408 and 360 million years ago? Today, collectors can find shells in a variety of shapes, sizes and colors washed up on a beach. Those are shells of mollusks, usually gastropods (snails) and bivalves (clams).

     The fossil-bearing rocks at the Falls of the Ohio indicate that a Devonian beachcomber would have found mollusks and other types of shells washed up. The most common shell-bearing creature in the Devonian at the Falls is almost never found washed up on a beach today - that is the brachiopod (pronounced brak - e - o -pod). Snails, clams and the occasional cephalopod would have been found, too. However, their shells were usually composed of a form of calcium carbonate called aragonite. Sometime after burial, aragonite would dissolve away from percolating ground water. Brachiopod shells were composed of calcite, which were more often preserved intact.              

     As a result, most mollusks are not found as clearly preserved shells. A cast is a replica of the original, a mold is a negative of the original. Internal molds show the shape of the interior, formed when the empty shell is filled with sediment. Casts showing the exterior of the shell are much less common at the Falls of the Ohio (figure 1). Molds -- the opposite of a cast -- are formed when a fossil has been removed leaving a fossil-shaped impression in the rock.

     There is no character common to all mollusks. Snails and clams are the best-known examples of this phylum. They can be found in the Ohio River today just as their distant ancestors inhabited the shallow sea floor preserved at the Falls from about 390 million years ago. No doubt mollusks without shells, like the sea slug, existed in the ocean so long ago. Creatures without shells were (and are) unlikely to become fossils.

     Most Snails (Class Gastropoda) have the body contained in an asymmetric coiled shell. The tip of the shell is usually pointed away from the head. Sole-like foot is used for creeping. The head is distinct and can move independently from the body.

     Snail shells come in a variety of shapes. The most commonly observed in the Devonian are exemplified in these four forms:

                                  Turbinate                                                    Planispiral

             Fig. 2) Turbinopsis shumardi (DeVerneuil)               Fig. 3) Pleuronotus decewi (Billings)

  (Illustrations from Henry Nettelroth's 1889 Kentucky Fossil Shells, Courtesy Kentucky Geological Survey)

                                  Turretiform                                             Trochiform  

          Fig. 4) Palaeozygopleura hamiltoniea (Hall)           Fig. 5) Bembexia sulcomarginata (Hall)

       (Illustrations from Edward M. Kindle's 1901, The Devonian Fossils and Stratigraphy of Indiana)

     One snail that deserves a closer look is Platyceras (fig. 6). Eighteen species may be attributed to the genus Platyceras and Orthonychia (a similar snail). It may be split into more species than might have actually existed. Platycerid snails were commonly coprophagous, living off the waste products of other organisms, particularly crinoids. Their shape may be been influenced by the shape of their host organism. A few varieties may exceed four-inches (10 cm) in length, too large to feed off the waste of crinoids. These may have fed on algae or organic debris in the sediment.

Fig. 6) Platyceras bucculentum Hall

   Bellerophonts require a special note. There is some discussion whether or not they are gastropods, monoplacophorans (tergomyans) or something else. They are spiraled in a flat plane and unlike most snails, exhibit bilateral symmetry (fig. 7).

 Fig. 7) Retispira leda (Hall) 

   Most Clams (Class Bivalva) have the body contained between two, right and left, symmetrical valves that form the shell. Some clams have one valve that is more concave that it's opposite. The shell is hinged with "teeth" and "sockets" and may be opened or closed with special muscles and ligaments. Clams do not have a head. They feed, respire and reproduce through two siphons. One draws in and the other exhales water. Most live buried in sediment (fig. 8), although some (i.e. scallops) live on the surface and can move by ejecting water from the siphon (fig. 9). Some mussels attach themselves to rocks and other hard surfaces by byssal threads, becoming completely immobile.

Fig. 8 Paracyclas rugosa (Conrad)                 Fig. 9     Leptodesma spinigerum Conrad

Rostroconchs (Class Rostroconchia) have bivalved shells that were confused for years as belonging to arthropods like trilobites (fig. 10). This class of mollusks has been extinct since the end of the Permian. There are no modern descendants. Most lived buried in sediment, similar to the life style of clams.

Fig. 10     Hippocardia cuneus (Conrad)

References

Kindle, E. M., 1901. The Devonian Fossils and Stratigraphy of Indiana, Indiana Dept. of Natural Resources, p. 529 - 775, 31 plates.

Nettelroth, H., 1889, Kentucky Fossil Shells, Kentucky Geological Survey, 237 p., 36 plates.

Species Lists

               Gastropods       Bivalves     Rostroconchs & Cephalopods

Created May 6, 2010, Updated July 28, 2010