Page 350 - Introduction to Paleobiology and The Fossil Record

SPIRALIANS 2: MOLLUSKS 337
prismato-nacreous shells, taxodont dentition,
equivalved shells and protobranch gills. Most
are detritus-feeding infaunal marine animals,
such as Nucula, and most abundant today in
deeper-water environments. Ctenodonta has
a typical taxodont dentition, an elliptical shell
and an external ligament; it is principally
Ordovician in age.
Members of the infrasubclass Solemyoida
are specialized, infaunal burrowers with an
anteriorly elongate shell. Most have symbiotic
autochemotrophic bacteria allowing them to
live in fetid muds, ranging in age from Early
Ordovician to Recent.
Autolamellibranchs
Autolamellibranchs were derived from the
protobranchs by the earliest Ordovician, pos-
sibly via a group of nuculoids that developed
hinge-teeth allowing greater opening of the
valves. This is necessary to avoid sediment
inadvertently trapped by the gills during the
food-gathering process.
The pteriomorphs are mainly marine, fi xed
benthos, attached by a byssus, or pad of sticky
threads, modified from the foot, or they may
be cemented. They are an important part of
bivalve faunas from the earliest Ordovician
and most had an outer mineralized shell layer
of calcite; the gills are of fi libranch grade. The
group includes the mussels Modiolus and
Mytilus and the ark shells Arca and Anadara,
the scallops Chlamys and Pecten, and the
oysters Crassostrea and Ostrea.
The heteroconchs are a mixed bag of mainly
suspension feeders, important in bivalve
faunas from the earliest Ordovician and radi-
ating during the Mesozoic when mantle fusion
and the development of long siphons pro-
moted a deep-infaunal life mode. They are
and were very successful burrowers. Gill
grades are mainly eulamellibranch and many
have crossed-lamellar or complex crossed-
lamellar shell microstructures. This group
includes the typical clams such as the giant
clam Tridacna, the horse-hoof clam Hippopus
and the surf-clam Donax, together with the
razor shells Ensis and Tagelus, the ship-worm
Teredo and the cockle Cerastoderma.
The anomalodesmatans are predominantly
suspension-feeding marine forms with pris-
mato-nacreous shells and reduced dentitions,
such as Pholadomya. They have eulamelli-
branch or septibranch gill grades. These too
are found from the earliest Ordovician but
only form a minor part of bivalve faunas.
Lifestyles and morphology
There are seven main bivalve forms that relate
to their modes of life (Stanley 1970): infaunal
shallow burrowing, infaunal deep burrowing,
epifaunal attached by a byssus, epifaunal with
cementation, free lying, swimming, and borers
and cavity dwellers. Specifi c assemblages of
morphological features are associated with
each life mode; these are summarized in
Fig. 13.8. Steven Stanley’s studies have been
adapted by a number of authors for similar
bivalve-dominated communities throughout
the Phanerozoic (Fig. 13.9). Most bizarre
were the rudists that built extensive reefs in
the Cretaceous (Box 13.5).
Bivalve evolution
The earliest known bivalves have been
reported from the basal Cambrian. Two Early
Cambrian genera are the praenuculid Pojetaia
from Australia and China and Fordilla from
Denmark, North America and Siberia. Both
genera have two valves separated by a working
hinge with a ligament, together with muscles
and teeth. These probably came about 10 myr
after the oldest rostroconch, Heraultipegma,
and so the bivalves might just have evolved
from rostroconchs (see p. 357) or something
like them. The class evolved rapidly in the
Early Ordovician to include basal forms of all
bivalve infrasubclasses. Not only were tax-
odont, actinodont and heterodont dentitions
established, but a variety of feeding types had
also developed following the Tremadocian
and Floian radiation.
Following this major diversifi cation, the
group stabilized during the remaining part of
the Paleozoic, although some groups evolved
extensive siphons that aided deep-burrowing
life modes. This adaptation, together with the
mobility provided by the bivalve foot, were
important advantages over most brachiopods,
which simultaneously pursued a fi xed epifau-
nal existence. The earliest autolamellibran-
chiate forms are known from the Early
Tremadocian. The early Mesozoic radiation
of the group featured siphonate forms
with desmodont and heterodont dentitions,