Page 351 - Introduction to Paleobiology and The Fossil Record

338 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

1. Infaunal shallow burrowers equivalved, adductor muscles of equal
sizes and commonly with strong external
ornament.
Glycimeris

2. Infaunal deep burrowers elongated valves, often lacking teeth and
with permanent gape and a marked pallial
Mya sinus.
3. Epifaunal with byssus elongate valves with flat ventral surface
and reduction of both the anterior part of
Mytilus the valve and the anterior muscle scar.
Attached by thread-like byssus.
4. Epifaunal with cementation markedly differently shaped valves,
sometimes with crenulated commissures;
large single adductor muscle.
Ostrea

5. Unattached recumbents markedly differently shaped valves
sometimes with spines for anchorage or
to prevent submergence in soft sediment.
Gryphaea
6. Swimmers valves dissimilar in shape and size with
very large, single adductor muscle and
commonly with hinge line extended as
Pecten ears.
7. Borers and cavity dwellers elongate, cylindrical shells with strong,
sharp external ornament; cavity dwellers
commonly grow in dimly lit conditions
Teredo following the contours of the cavity.
Figure 13.8 Morphology and adaptations of the main ecological groups of bivalve mollusk.

equipped to handle life deep in the sediments swimming and unique to the mollusks. During
of nearshore and intertidal zones where they development, the head and foot remain fi xed
diversifi ed. but all the visceral mass, the mantle and the
larval shell are, in effect, rotated through
180˚. The process of torsion is characteristic
CLASS GASTROPODA
of the Gastropoda, although in some groups
The gastropods, the “belly-footed” mollusks, there may be secondary reversal. The coiling
are the most varied and abundant of the mol- of the gastropod shell is unrelated to the rota-
luskan classes today. The group includes the tion of the soft parts. Following torsion, the
snails and slugs, forms both with and without mantle cavity and anus are open anteriorly
a calcareous shell. During a history spanning and the shell is coiled posteriorly in an endo-
the entire Phanerozoic, gastropods evolved gastric position, in contrast to the exogastric
creeping, floating and swimming strategies style of the “monoplacophoran” grade shell.

together with grazing, predatory and parasitic The gastropod shell is usually aragonitic,
trophic styles. usually conical with closure posteriorly at the
Most gastropods are characterized by pointed apex, and open ventrally at the aper-
torsion in which the mantle cavity containing ture. Each revolution of the shell or whorl
the gills and anus, excretory and reproductive meets adjacent whorls along a suture, and the
openings comes to lie above the head (Fig. whorls together comprise the spire. Tight
13.11). The advantages of this arrangement coiling about the vertical axis generates a
are unclear. In fact, torsion seems to be dis- central pillar or columella. The aperture is
tinctly disadvantageous because it involves commonly oval or subcircular and is circum-
the loss of one of the gills and/or development scribed by an outer and inner lip. The head
of a peristomal slit allowing separation of emerges at the anterior margin of the aperture,
inhalant and exhalant currents. The fi rst larval where the aperture may be notched or extended
stage, the trochophore, is usually fi xed. as a siphonal canal supporting inhalant fl ow
However, the second, veliger, phase is free- through the siphons. Material is ejected