MISCELLANEOUS MOLLUSCA
There are several classes of Mollusca that have modest to poor fossil records because of either low overall diversity or poor preservation potential. We will examine them first.
There are three recognized subphyla of molluscs: Amphineura, Cyrtosoma, and Diasoma. The former ("dubious nerve") comprises three classes, none of which is common as fossils. In fact, two of them (Caudofoveata and Aplacophora) have no shell and are unknown or exceedingly poorly represented as fossils. The third (Polyplacophora -- "multiple shell") are modestly diverse in the modern world and have a better fossil record, though the nature of the shell means that whole fossils are pretty rare. The common name for members of this class is "chiton". The pair of photos below shows a Recent example (Katherina, GSW Biol. Coll.) in dorsal (left) and ventral (right) views, with the anterior end toward the top of each photo. In dorsal view the eight separate plates that are typical (and give the class its name) are apparent near the center where they are not covered by the embedding connective tissue. On the right, the soft parts are apparent, with the mouth at the front of the foot (near the top of the photo), the anus near the back of the foot, and the paired rows of gills along the side of the foot. The gut passes through the body in a virtually straight line in the Amphineura.
A second subphylum of molluscs is the Cyrtosoma ("curved body"), which includes the snails and cephalopods and well as a couple of rarer classes.
The simplest shelled molluscs in the Cyrtosoma are in the class Monoplacophora, and these are thought to be close in for to the ancestors of all the common molluscan classes. The name means "single shell piece" or some such thing, and refers to the low, conical, limpet-like dorsal covering of the typical forms. The class was thought to have been extinct at least since the Devonian until 1952, when Neopilina was dredged alive from deep water off Central America. Several additional deepwater species have subsequently been discovered. The diagram below shows the basic structure of Neopilina. Fossils that are unquestionably monoplacophorans all have shells that are rather conical like that of Neopilina. Note that the soft anatomy is very similar to that of the Amphineura, like the chiton shown above.
Some specialists believe that the planispirally coiled shells of the bellerophontids (photo at right below Carter Coll. Gb1) also belong in this class because there are scars on the interior that suggest a series of segments/gills as in the Monoplacophora and not in the Gastropoda (where they have traditionally been placed). If this is correct then the range of the class extends up to the Triassic. The photo is of a Pennsylvanian species from northern West Virginia.
The two major classes of cyrtosomes will be treated on separate pages.
The final subphylum of molluscs is the Diasoma ("double body") which have bilateral symmetry arising from paired body parts. There are three classes, of which the bivalves are the most common and best known. They will be treated on a separate page, but two other classes have modest fossil records and will be treated here.
The oldest diasomes are in the class Rostroconchia and look very much like bivalves. However, despite superficial appearances what seem to be paired valves of the shell are not hinged as in clams, but rather are rigidly joined along what appears to be the hingeline. The bivalve shell is thought to have evolved from this state by the development of a hinge. In addition, the oldest known rostroconchs are similar in shape and size to laterally compressed monoplacophorans of about the same age (Cambrian) and it is reasonable to suppose that they evolved from them. This set of observations is actually a pretty good case of finding "missing links" or transitional forms in an evolutionary series. The photo below (from Pojeta, J., 1980, Tulane Stud. in Geol. and Pal., v. 16) shows the probable transitional forms. A, B, and C are the monoplacophoran and D, E, and F the rostroconch. Notice the gapped ends of the latter.
The other minor group of fossil diasomes is the Scaphopoda. These critters have very elongate conical shells which are buried wide-end down in the sediment of marine environments. The shell form gives them their common name: "tusk shells". The photos below show some Recent and fossil examples. On the left is the Dentalium thalloides from the Middle Eocene of southern Alabama (Carter Coll. Gc2), in the middle is D. cf. porterensis from the Late Eocene of southern Washington (Carter Coll. Gc12), and on the right is D. eboreum from the Recent of northeastern Florida (Carter Coll. RGc13). Such a simple shell form doesn't allow much shape variation, so diversity is very low, and very few higher taxa have been named.
Scaphopods are thought to have evolved from elongate rostroconchs in the Ordovician, when the oldest of the former and the most elongate of the latter are known. The photograph below is also from Pojeta, J., 1980, Tulane Stud. in Geol. and Pal., v. 16 and shows the appropriate fossils of each. The upper picture is the scaphopod (Rhytiodentalium) and the lower two the rostroconch (Pinnocaris).
Two other groups are worth mentioning here because they have at times in the past been considered molluscs of some sort and still have not been convincingly assigned anywhere else. The first are the Hyolitha which had a two part shell: a larger conical part called the conch and a smaller cover called the operculum. Some have prominent spine-like structures flanking the aperture called "helens". The photo below shows an example from the Middle Cambrian of northwestern Georgia (Carter Coll. Ge1). The conch and operculum are both apparent and the small marks at the corners of the conch may represent parts of the helens. The taxon is restricted to the Paleozoic, so details of the soft-part morphology are not well known. Nobody doubts that they are related to molluscs, but how closely is widely debated. Some place the hyoliths in a separate phylum, some as a class within the Mollusca, and others as a subclass within the Monoplacophora.
The other group whose affinities are uncertain are the tentaculitids. They also have very small conical shells and are also restricted to the Paleozoic (Ordovician - Devonian). These have been considered to be hyolithids, shelled annelid worms, a separate phylum, a separate class of molluscs, or even specialized cephalopods. The shells do have internal septa like nautiloids and specimens from the Devonian Hunsruck Slate lagerstatte in Germany appear to have tentacles in x-ray photographs (hence the name). The photos below show two typical species from North America: Tentaculites richmondensis from the Late Ordovician of Indiana on the left (Carter Coll. Gf2) and T. gyracanthus from the Early Devonian of West Virginia on the right (Carter Coll. Gf3).
