Page 370 - Introduction to Paleobiology and The Fossil Record

SPIRALIANS 2: MOLLUSKS 357
shell aperture
gonad
kidney
anus
water–sediment interface
gut mantle cavity
mantle
head
foot
(a)
gape
rostrum
anterior posterior
(b)
Figure 13.25 (a) Scaphopod morphology and
(b) rostroconch morphology.
benthos of the Paleozoic, although many more
localized, often nearshore, assemblages were
dominated by mollusks.
During the Paleozoic, bivalves were
common in nearshore environments, often
associated with lingulide brachiopods,
although the class also inhabited a range of
deeper-water clastic environments; and by the
Late Paleozoic bivalves had invaded a variety
of carbonate environments. However, at the
end of the Paleozoic, the appearance of more
typical bivalves in shallow-water belts may
have displaced the Paleozoic associations
seaward. During the Late Mesozoic and Ceno-
zoic the significant radiation of infaunal
taxa may have been a response to increased
predation.
The majority of Paleozoic gastropods were
Eogastropoda that commonly dominated
shallow-water marine environments and some
carbonate reef settings. The Mesozoic, however,
was dominated by the Mesogastropoda, which
grazed on algal-coated hard substrates. The
Cenozoic, marks the acme of the group with
the radiation of the siphonal carnivorous
Eras Periods
Quaternary
Pliocene
Neogene Miocene
Cenozoic
Tertiary Oligocene
Eocene
Paleogene Paleocene
Cretaceous
Mesozoic Jurassic
Triassic
Permian
Upper Carboniferous
Devonian
Paleozoic
Silurian
Lower Ordovician
Cambrian
Monoplacophora Diplacophora Helcionellida Bivalvia Rostroconchia Tergomyonia Cephalopoda Polyplacophora Gastropoda Scaphopoda Aplacophora
Figure 13.26 Stratigraphic range of the main
mollusk groups.
neogastropods, and with a further diversifi ca-
tion of mesogastropods (Fig. 13.26).
The cephalopods evolved through the
development of a chambered shell with a sip-
huncle, which gave them considerable control
over attitude and buoyancy; this system was
refi ned in the nautiloid groups. The evolution
of complex folded sutures in the ammonoids,
the exploitation of a pelagic larval stage and
a marginal position for the siphuncle appar-
ently set the agenda for the further radiation
of the group during the Mesozoic.
Throughout the Phanerozoic, the fl eshy
mollusks provided a source of nutrition for
many groups of predators. The evolution of
the phylum was probably in part infl uenced
by the development of predator–prey rela-
tionships and minimization of predator
success. Thick armored shells were developed
in some groups while the evolution of deep-
infaunal life modes was also part of a defen-
sive strategy. Predation and the development
of avoidance strategies, together with the
so-called arms race, had an important infl u-
ence on molluskan evolution. Predators