Page 340 - Introduction to Paleobiology and The Fossil Record

SPIRALIANS 2: MOLLUSKS 327
This famous tongue twister was fi rst recited
over 200 years ago in England, and it is very
likely based on the exploits of Mary Anning,
the most famous fossil collector of her time.
She is best known for her spectacular discov-
eries of marine reptiles in the Lower Jurassic
rocks of her native Lyme Regis in southern
England; but she made most of her regular
income from selling fossil ammonites and
other mollusk fossils to visitors. Most of us
have found seashells while playing or walking
on the beach and have been amazed by their
colors, shapes and ornaments. Not only are
clams, oysters and scallops good to eat, but
their shells, throughout historic times, have
featured as ornaments, tools and even cur-
rency. The Mollusca is the second largest
animal phylum after the Arthropoda, with
records of over 130,000 living species and
a history extending back into the
Precambrian.
MOLLUSKS: INTRODUCTION
The Phylum Mollusca includes the slugs,
snails, squids, cuttlefish and octopuses in
addition to all manner of marine shellfi sh
such as clams, mussels and oysters (Box 13.1).
Although some mollusks are the size of sand
grains, the giant squid Architeuthis can grow
to over 20 m in length, the largest and possi- identity of the first mollusks, and new fi nds
bly the most frightening genus of all living
invertebrates. Mollusks are probably the most
common marine animals today, occupying a
very wide range of habitats, from the abyssal
depths of the oceans across the continental
shelves and intertidal mudfl ats to forests,
lakes and rivers. Mollusks are usually unseg-
mented, soft-bodied animals with a body plan
based on four features:
1 The head contains the sensory organs, and
a rasping feeding organ, the radula, com-
posed of chitin and designed to scrape and
in some cases drill.
2 The foot is primitively a sole-like structure
on which the animal crawls, but is consid-
erably modified in many mollusks.
3 The visceral mass of the digestive, excre-
tory, reproductive and circulatory organs
is enclosed in the celomic cavity.
4 The mantle is a sheet of tissue lying dor-
sally over the visceral mass that is respon- the Late Precambrian. Signifi cantly, the artic-
sible for secreting the shell.
Molluskan shells are secreted as calcium
carbonate, mainly aragonite, with an organic
matrix and an outer organic layer. In the case
of the bivalves, a range of shell fabrics have
evolved from simple prismatic structures,
through nacreous and prismatic, to crossed-
lamellar aragonitic and prismatic and foliated
calcite fabrics. Shell structure has been used
in the higher classification of the group (as in
the brachiopods). Crossed-lamellar structures
evolved independently in some gastropods.
Beneath the mantle, the mantle cavity lies
behind the visceral mass and is the respiratory
chamber that houses the molluskan gills
(ctenidia); the openings of the excretory and
reproductive ducts and the anus open into the
mantle cavity and their products are carried
out on the exhalant current.
From simple beginnings as a limpet-like
crawler back in the Precambrian, mollusks
have evolved a spectacular range of shapes
and sizes, and their hard, calcareous shells are
readily fossilized.
A simple method of visualizing molluskan
evolution is to consider the hypothetical
ancestor, or archemollusk, with a minimal
molluskan morphology; this approach has
been modified and merged with a recent
cladogram for the phylum (Fig. 13.1). There
is still a great deal of uncertainty about the
constantly change the picture (Boxes 13.2,
13.3). The most recent common ancestor of
the mollusks probably had seven- to eight-
fold serial repetition, the presence of valves
and a foot, and had a crawling mode of life
(Sigwart & Sutton 2007).
EARLY MOLLUSKS
The Early Cambrian was a time of experimen-
tation, with a variety of short-lived, often
bizarre, molluskan groups, such as the helcio-
nelloids, dominating many faunas (Peel 1991).
Most workers now agree that the fi rst mol-
lusks were descended from forms like living
flatworms – probably spiculate animals with
radula and gills situated posteriorly. These
mollusks were similar to modern soft-bodied
aplacophorans, a group of shell-less mollusks.
The aplacophorans and the shelled mollusks
shared a common ancestor probably during
ulated remains of a halkieriid mollusk from