Mollusks

What would it have been like to be a shell collector during the Devonian period, between 408 and 360 million years ago? Today collector's find shells in a variety of shapes, sizes and colors washed up on a beach. Those are shells of mollusks, usually gastropods (snails) and pelecypods (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 mineral known as 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. Internal casts 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.

What are mollusks?

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 387 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.

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

One snail that deserves a closer look is Platyceras (figure 6). It may be split into more species than might have actually existed. Twenty species may be attributed to the genus Platyceras and Orthonychia (a similar snail). Platycerid snails were commonly coprophagous, living off the waste products of other organisms, particularly crinoids. The shape of their shell may be influenced by the shape of their host organism. A few varieties may exceed four-inches (8 cm) in length, too large to feed off the waste of crinoids. These may have fed on algae or organic debris in the sediment.

Snail shells come in a variety of shapes.
Turbinate: Turbinopsis shumardi (DeVerneuil) (figure 1)
Planispiral: Pleuronotus decewi (Billings) (figure 2)
Turretiform: Palaeozygopleura hamiltoniea (Hall) (figure 3)
Trochiform: Bembexia sulcomarginata (Hall) (figure 4)

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 (see figure 5).

Clams (Class Pelecypoda) have the body contained between two symmetical shells. 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 (figure 7), although some (i.e. scallops) live on the surface and can move by ejecting water from the siphon (figure 8). Some mussels attach themselves to rocks and other hard surfaces by byssal threads, becoming completely immobile.

Rostroconchs (Class Rostroconchia) have bivalved shells that were confused for years as belonging to arthropods like trilobites (figure 9). This class of mollusks have 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.

Bellerophonts Old Name
Bellerophon pelops (Hall) = Acanthonema?	Ptomalis patulus (Hall) Bellerophon
Retispira leda (Hall) Bellerophon (figure 5)	Tropidodiscus curvilineatus (Conrad) Bellerophon

Gastropods
*Aclinsia barnetti Kindle	*Aclinsia barnetti var. elongata Kindle
Bembexia arbella (Nettelroth) Pleurotomaria	Bembexia procteri (Nettelroth) Pleurotomaria
Bembexia sulcomarginata (Conrad) Pleurotomaria (figure 4)	Bucania devonica Hall & Whitfield
*Capulus cassiensis Kindle	"Cyclonema" crenulata Meek
"Cyclonema" multilira Hall	Elasmonema bellatula (Hall) Callonema, Isonema, etc.
Elasmonema clarki (Nettelroth) Callonema	Elasmonema imatator (Hall & Whitfield) Pleurotomaria, Callonema
Isonema humulis (Meek) Naticopsis	Mourlonia lucina (Hall) Pleurotomaria
Mourlonia lucina var. perfasciata (Hall) Pleurotomaria	Murchisonia desiderata Hall
Naticonema lineata (Conrad) Platystoma	Naticonema turbinata (Hall) Platystoma
Naticopsis levis Meek	Orthonychia conicum (Hall) Platyceras
Orthonychia fluctuosum (Ulrich) Platyceras	Palaeozgyopleura hamiltoniae (Hall) Loxonema
Palaeozgyopleura laeviusculum (Hall) Loxonema	Palaeozgyopleura rectistriatum (Hall) Loxonema
*Palaeozgyopleura? teres (Hall) Loxonema?	Platyceras? arctiostoma Ulrich
*Platyceras blatchleyi Kindle (= P. dumosum?)	Platyceras bucculentum Hall
Platyceras carinatum Hall	Platyceras compressum Nettelroth
Platyceras dumosum Conrad and varieties	Platyceras echinatum Hall (= P. dumosum?)
Platyceras erectum Hall	Platyceras fornicatum Hall
Platyceras indianensis Miller & Gurley	Platyceras lineare Kindle
Platyceras milleri Nettelroth	Platyceras multispinosum Meek
Platyceras rictum Hall	Platyceras subcirculare Kindle
Platyceras symmeticum Hall	Platyceras thetis Hall
Platyceras ventricostum Conrad	Pleuronotus decewi (Bilings) Euomphalus
Polyphemopsis louisvillae Hall & Whitfield = Holopea?	Soleniscus carinatus (Nettelroth) Macrocheilus
*Soleniscus hebe (Hall) Macrocheilus	*Straparollus exiguus Kindle Euomphalus
*Straparollus planodiscus Hall Euomphalus	Straparollus sampsoni (Nettelroth) Euomphalus
Trochonema emacerata Hall & Whitfield	*Trochonema meekanum (Meek)
Trochonema rectilatera Hall & Whitfield	Trochonema yandellana Hall & Whitfield
Turbinopsis shumardi (DeVerneuil) Turbo

Clams
Actinodesma erectum (Hall) Glyptodesma	Actinodesma occidentale (Hall) Glyptodesma
Actinopteria boydi Conrad	-Cardiopsis crassicostata Hall & Whitfield
Clinopistha antiqua Meek	Clinopistha striata Nettelroth
Clinopistha subnasuta Hall & Whitfield	Cornellites fasculata (Goldfuss) = Cornellites flabella Meek
Cornellites grandis (Hall) Pterinea	Cypricardinea cataracta Conrad
-Cypricardinea cylindrica Hall	Cypricardinea indenta Conrad
Eoschizodus contractus (Hall) Schizodus	Glyptodesma cancellata Nettelroth = Limoptera?
Goniophoria hamiltonensis Hall	Goniophoria truncata Hall
-Grammysoidea arcutata (Hall) Grammysia	Grammysoidea secunda var. gibbosa (Hall) Grammysia
Grammysoidea subarcutata (Hall) Grammysia	Leptodesma spinigerus Conrad = Leptodesma rogersi Hall
Limoptera cancellata Hall	Modiomorpha concentrica Conrad = M. charlestownensis Nettelroth?
Modiomorpha myteloides Conrad = M. affinis Hall, M. alta Conrad	-Modiomorpha recta Hall
Nuculoidea hanoverensis (Kindle) Nucula	Nuculoidea corbuliformis (Hall) Nucula
Nuculoidea herzeri (Nettelroth) Nucula	*Nuculoidea lirata (Conrad) = N. lamellata (Hall) Nucula
Nuculoidea neda (Hall & Whitfield) Nucula	Nuculoidea niotica (Hall & Whitfield) Nucula
Paracyclas ohioense Meek = P. elliptica Hall, P. elongata Nettelroth	Paracyclas rugosa Conrad = P. liriata Conrad (figure 7)
Pseudaviculopecten crassicostata (Hall & Whitfield) Aviculopecten	Pseudaviculopecten exactus (Hall) Aviculopecten
Pseudaviculopecten fasciculatus (Hall) Aviculopecten	Pseudaviculopecten princeps (Conrad) Aviculopecten (figure 8)
Pseudaviculopecten terminalis (Hall) Pterinopecten	Pterinopecten nodosus Hall
Pterinopecten reflexus Hall	*Pterinopecten undosus Hall
Ptychodesma knappianum Hall	*Sanguinolites? sanduskiensis Meek
Solemya vetusta Hall	Yoldia? valvulus Hall & Whitfield

Rostroconchs
Bigalea Conocardium	Hippocardium cuneus (Hall) Conocardium (figure 9)

* Reported from the Jeffersonville Limestone in central Indiana, but might occur in Falls area.
- Inadequately described from the Falls area.

Suggested Reading

The Audubon Society Field Guide to North American Fossils, I. Thompson. Published by Alfred A. Knopf, 1982. Good general fossil book.

The Devonian Fossils and Stratigraphy of Indiana, E. M. Kindle, Indiana Dept. of Natural Resources, 1901. Out of print. Identifies most Devonian brachiopods found in Indiana. May be found in college, university and large public libraries.

Fossil Invertebrates, edited by R.S. Boardman, A.H. Cheetham, and A.J. Rowell. Published by Blackwell Scientific Publications, 1987. Chapter 16. College level text.

Kentucky Fossil Shells, Henry Nettelroth. Kentucky Geological Survey, 1889. Out of print. The first comprehensive, well illustrated book of fossil brachiopods and mollusks from Kentucky. This is a rare book, but may be found in large public or university libraries in the region.