MOLLUSCA

MOLLUSCS Molluscs have a fleshy mantle encasing the visceral mass and a muscular foot. Unlike gastropods and cephalopods, bivalves secrete two shells rather than one. Bivalves have lost the radula, which is presumed to be a primitive feature among all mollusks.

Class BIVALVIA Bivalves, sometimes called Pelecypods (meaning axe foot) are a very diverse and abundant group of molluscs which inhabit a variety of marine and non- marine environments. Their long geologic history and variety of forms have made them the popular subjects of many evolutionary and functional morphological studies.

BIVALVES The shell of bivalve molluscs is characterized by two calcareous halves, called valves, which can be composed of either calcite and/or aragonite. Some groups, such as the oysters, are exclusively calcitic, while others, such as the pterioids, have an aragonitic inner layer.

Feeding Most bivalves are filter feeders, trapping suspended food particles as water passes through their gills. Only two groups, the nuculoids and cryptodonts, actively feed on organic material within the sediment and are thus true deposit feeders.

Relations to Substrate Infaunal –Burrowing: Shells are usually equivalved and isomyrian (or anisomyrian) with a distinct pallial line. They include: the nuculoid burrowing deposit feeders, the shallow burrowing non-siphonate forms lacking a pallial sinus, and deep burrowing siphonate forms identified by a distinct pallial sinus. –Boring: Shells are usually thick, equivalved, and cylindrical in cross section. Some forms are moderately ornamented with ridges and stout spines whereas others such as the "ship worms" are tubular in form. Semi Infaunal –Byssally attached (endobyssate). Similar to many epifaunal byssate forms (see below), yet maximum shell width (inflation) is at mid-line of shell cross-section. Some forms can be elongated and fan-like with a reduced anterior area. Examples include pen shells, and the mussel- like modiolids, and some ark shells. The depth to which the bivalves are partially buried can often be deduced by looking for encrusting organisms that may have attached themselves above the sediment- water interface.

Epifaunal Byssally attached (epibyssate). –Shells can be either equivalved or inequivalved depending on their orientation to substrate during life. Usually, all epibyssate forms have a reduced anterior region. Some groups, such as the blue mussels, are similar to endobyssate forms except the maximum inflation is below the mid-line of the valves cross-section. Other forms may have a byssal notch and/or a well defined auricle, or, as in the case of some arks, have a gape along the ventral margin. Reclining. –Shells are commonly inequivalved with a larger lower (usually the left) valve which is more inflated or convex while the upper valve may be planar. Some also exhibit spines, especially on the lower valve, to aid in stabilization in soft substrates in a manner similar to some brachiopods. Many have a small attachment area at beak where earliest growth stages were cemented. The giant clam Tridacna, who has photosymbionts similar to hermatypic scleractinian corals, is a recliner even though it had a functional byssus during its earliest juvenile stages. Swimming. –Shells are usually equilateral but not equivalved. The lower (usually the left) valve is usually slightly larger. Swimming forms are typified by having a greater umbonal angle (greater than 105°) than similar-looking epibyssate forms. Furthermore, swimming forms typically have a single (monomyrian), large, centrally located adductor muscle. Cementing. –Shells are commonly inequivalved with the lower (usually left) valve assuming the form of the object to which it is cementing, a condition called xenomorphism. In such cases, both valves are usually highly variable in shape, as in the common oysters and other forms as well. Some groups such as the Cretaceous rudists could reach very large sizes and were able to form reefs mimicking corals in both morphology and ecology.

Class GASTROPODA Gastropods, including such common forms such as snails, slugs, and whelks, occupy both marine and non-marine environments. Although many gastropods are herbivorous grazers, several groups are active carnivores able to drill through the skeleton of the luckless victim.

GASTROPODA Gastropods are radulate organisms with a torted body (e.g., the body is rotated 180 o so that the anus is above the head). Gastropods typically have a helical coiled univalved shell whose opening (aperture) may be closed by an operculum.

Class CEPHALOPODA Cephalopods are a class of mobile mollusks, most of which are nektic or nekto-benthic. Cephalopods have a bilaterally symmetrical body, a prominent head, and a modified foot in the form of tentacles. Although during the Paleozoic and Mesozoic, cephalopods achieved great diversity and abundance in marine habitats, only two genera possessing skeletons are known today. Superficially the shell or conch of cephalopods resemble gastropods; however, most cephalopods coil in a plane, whereas gastropods are helicoiled. Cephalopods with an external conch, the coiled shell is chambered.

Shape of External Shells Several different shapes are common among fossil and extinct cephalopods. These include orthoconic or straight, brevicone, evolute planispiral, involute planispiral, or heteromorphic.

straight brevicone evolute planispiral involute planispiral

Subclass NAUTILOIDEA Nautiliods are an important group of cephalopods with an external shell. However, unlike the ammonoids, the nautiloids have living representatives in the genus Nautilus. Nautiloid shells are external and are characterized by either straight or slightly wavy sutures. Nautiloid shells are either orthoconic, or they are coiled, such as Nautilus; The siphuncle may be small or large, but is typically centrally located.

Subclass AMMONOIDEA Very important extinct group of cephalopods which includes all forms with an external shell with fluted septa. Most are planispiral, but some may be heteromorphic The siphuncles are generally small and ventral in position. Division within the ammonoids is based upon the grades of suture fluting. There are three grades

SUTURES Goniatite suture. Saddles and lobes are present. The goniatite suture is characterized by undivided rounded saddles and undivided angular lobes. Ammonoids with this type of suture are called goniatites. Ceratite suture. Saddles are undivided whereas the lobes are divided. Ammonoids with this type of suture are called ceratites. Ammonite suture. Both the saddles and lobes are divided. Ammonoids with this type of suture are called ammonites. Although many of the ammonites are coiled, there are many genera such as Baculites, which is heteromorphic and encompasses a variety of coiling shapes.

Goniatite suture Ceratite suture Ammonite suture