BIVALVIA


INTRODUCTION

This laboratory discusses two classes of molluscs: the bivalves and rostroconchs; of which the bivalves have the most robust fossil record. Although these two groups have species that are far too long-ranging for precise stratigraphic correlation and zonation, their morphologic attributes are very diagnostic of their diverse life-habits and paleoecology. As such, this laboratory deals with a minimum of classification (only to class level) and concentrates more on paleoecology.

Like other molluscs, these two classes have a fleshy mantle encasing the visceral mass and a muscular foot. Unlike gastropods and cephalopods, bivalves secrete two shells rather than one. Additionally, bivalves have lost the radula, which is presumed to be a primitive feature among all mollusks. The Rostroconchs are similar in many regards to bivalves, especially in their hypothesized soft parts, a significant difference is that the shell is pseudobivalved.


CLASSIFICATION & GEOLOGIC RANGES

Phylum Mollusca

Class Rostroconchia (Cambrian-Permian)

Class Bivalvia (Cambrian-Recent)

Subclass Paleotaxodonta

Subclass Isofilibranchia

Subclass Pteriomorphia

Subclass Herteroconchia

Subclass Anomaldesmata

 


Class ROSTROCONCHIA

Rostrochonchs are a relatively minor Paleozoic group which may have an important phylogenetic position within the phylum Mollusca. Some authors suggest that rostroconchs evolved from an early monoplacophoran ancestor and gave rise to both bivalves and scaphopods, leaving the cephalopods and gastropods as descendants from a separate monoplacophoran stock.

Although they may have an internal anatomy similar to bivalves, rostroconchs are characterized by a single, pseudobivalved shell which encloses the mantle and muscular foot. The anterior part of shell has a gape from which the foot could probably emerge and an elongated tube on the posterior end called the rostrum which may have aided in water filtration. Although only one example is provided from the laboratory, please examine the illustration below.


Figure 1 - General Rostrochonch Morphology

From McRoberts (1998)


Class BIVALVIA

Bivalves, sometimes called Pelecypods (meaning axe foot) or, in older literature, referred to as Lamellibranchs, 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.

For those who work on modern bivalves, often the characters such as the gill structure and even color patterns have taken prominence in classification. However, for those working on only the preserved hard parts of fossils, usually features such as differences in teeth provide the classification scheme. Unfortunately, because bivalves have many morphologic features with adaptive value, many of these features have arisen more than once. As a result, it is often difficult to erect a classification that reflects an evolutionary history. The classification scheme given above (which you are not responsible for) is derived in part from your text and is an attempt to incorporate both hard and soft part morphology.

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. The outermost layer of the bivalve shell, called the periostracum, is composed of a horny organic substance which in the example is the darker material only on the external part of the shell.

The beak is located in the dorsal region of bivalves. The byssus or foot protrudes from the anterior of the shell, while the posterior the shell is the region of siphon protrusion (at least in some bivalves). The plane between the two valves (or commissural plane) is the plane of symmetry which separates the left from right valve. Note there is not an upper and lower valve (as in brachiopods) because most bivalves are oriented with their commissure plane vertical. Although this symmetry is retained in most bivalves, the symmetry is secondarily lost in others. Note that in one of the specimens, there is close to a plane of symmetry within the valves. This asymmetry corresponds to the living habit of the beasts, and those who have lost their original bilateral symmetry between the valves commonly live with their plane of commisure not perpendicular to the sediment surface.

Left or right valves can be determined by viewing the posterior end (as shown in the figure below).


Figure 2 - Determination of Left and Right Valve

 


The two halves of the bivalve shell are usually joined at the dorsal margin by a ligament which acts as a spring. The ligament may be internal, as in oysters, mussels, or scallops. Some internal ones consist of horizontal or vertical bands which sit in grooves. Other ligaments may be external. The hinge margin may also be occupied by a series of teeth and sockets collectively referred to as dentition (see below). The opening and closing of the shell are controlled by adductor muscles (which oppose the force of the ligament) that often leave physical scars on the valve interior (see below). Other muscles that leave scars include the pallial muscles which attach the mantle to the shell, in addition to smaller ones that control the siphons, foot, and/or byssus. General terms that you will need to know are given in the next section.


Continue on to

MORPHOLOGY


Return to
Topic List