Page 347 - Introduction to Paleobiology and The Fossil Record

334 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

burrowing and boring strategies together with hinge ligament is constrained between the
migrations to freshwater habitats were sec- dorsal parts of the shells; when the adductors
ondary innovations. There are over 4500 relax, the ligament expands and the shells
genera of living bivalves, with fewer than half spring open. The scars of these shell-closing
of that number described from the fossil muscles may be seen usually as clear rough-
record. In view of the wide range of life strate- ened and depressed areas inside both
gies and their relationships to particular sedi- valves.

ments, the bivalves are good facies fossils. Classification of the bivalves is based pri-
Although non-marine bivalves have been used marily on gill structure (Fig. 13.6a). Dentition
extensively, in the absence of other groups, to is of secondary importance (Fig. 13.6b). Teeth
zone parts of the Upper Carboniferous and by may be all along the hinge line or separated
Charles Lyell in his classic work in the 1820s into discrete cardinal (subumbonal) and
and 1830s to subdivide the Tertiary (the lateral (both anterior and posterior of the
increasing proportion of living forms in fossil hinge line) teeth. The three most important
faunas through the Tertiary was used to sub- tooth arrangements are: (i) taxodont – numer-
divide the system; see p. 29), their biostrati- ous subequal teeth arranged in a subparallel
graphic precision is limited. pattern; (ii) actinodont – teeth radiating out
from beneath the umbo; or (iii) heterodont – a
mixture of cardinal (beneath umbo) and
Basic morphology
lateral teeth. Various other terms have been
Bivalves are twin-valved shellfi sh superfi cially employed in the past when the teeth are thick-
resembling the brachiopods and common in ened, modified or reduced, but are now less

modern seas (Fig. 13.5). In contrast to the commonly used.
Brachiopoda, bivalve shells are always com- In most cases the umbones of the valves
posed of calcium carbonate, usually arago- point or face obliquely anteriorly, the pallial
nite, and many have a plane of symmetry sinus (if present) is situated posteriorly and
parallel to the commissure separating the left the posterior adductor is usually the larger of
and right valves from each other, i.e. the two the two scars. In some forms the anterior
valves are virtually mirror images of each adductor is lost, together with the foot. When
other. Bivalves have sometimes been termed the valves are held with the commissure
lamellibranchs or pelecypods, but they between the two valves vertically, the anterior
were first named Bivalvia by Linnaeus in end pointing away from the observer and the

1758. umbones at the top, then the right and left
In the bivalves the molluskan head is lost, valves are in the correct orientation.
only the anterior mouth indicates its position.
Sensory organs are concentrated instead on
the mantle margins and include eye-spots,
chemoreceptors and statocysts. The bivalve Main bivalve groups
exoskeleton has two lateral valves, left and The Bivalvia are classifi ed by zoologists mainly
right, essentially mirror images of each other, on the basis of soft-part morphology such as
united dorsally along the hinge line by an features of the digestive system and the gills;
elastic ligament and usually interlocking teeth paleontologists have usually attempted to use
and sockets; the valves open ventrally. The details of the hinge structures. There are seven
valves are secreted by mantle lobes. The basic features that are of use for classifi cation
attachment of the mantle is marked by at various levels within the Bivalvia: gill
the pallial line, which may be indented poste- structure (subclass and infrasubclass levels),
riorly with the extension of the siphons. The dentition (all levels), ligament insertion (infra-
earliest-formed parts of each shell, the beaks subclass down to ordinal levels), adductor
or umbones, may be separated by the cardinal muscle scars (superfamily to orders), pallial
area supporting the dorsal ligament. When line (family level and below), shell shape (all
the valves are closed, a pair of adductor levels), and shell fabric (infrasubclass down to
muscles, situated anteriorly and posteriorly, is superfamily level). Two subclasses are recog-
in contraction. While the shells are closed, the nized: (i) the Protobranchia with simple pro-