Early Mollusks

The Early Cambrian was a time of experimentation, 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 first 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 the Late Precambrian. Significantly, the articulated remains of a halkieriid mollusk from

Box 13.1 Classification of Mollusca

Class CAUDOFOVEATA

• Worm-like, shell-less mollusks living inverted in burrows in the seabed

Class APLACOPHORA

• Worm-like, spiculate mollusks

• Possibly Carboniferous (or older) to Recent

Class MONOPLACOPHORA

• Limpet-like, cap-shaped shells with segmented soft parts

• Cambrian (Lower) to Recent

Class DIPLACOPHORA

• Anterior and posterior shell separated by elongate zone of scale-like sclerites

Class POLYPLACOPHORA

• Segmented shell usually with eight plates, large muscular foot and a series of gill pairs

• Cambrian (Upper) to Recent

Class TERGOMYA

• Exogastrically coiled, univalved, bilaterally symmetric, often planispirally coiled or cap-shaped mollusks

• Cambrian (Middle) to Recent

Class HELCIONELLOIDA

• Endogastrically coiled, univalved, untorted mollusks

• Cambrian (Lower) to Devonian (Pragian)

Class GASTROPODA

• Univalved, shell usually coiled, having head with eyes and other sense organs, muscular foot for locomotion. Internal organs rotated through 180° during torsion early in ontogeny

• Cambrian (Upper) to Recent

Class BIVALVIA

• Twin-valved, joined along dorsal hinge line commonly with teeth and ligament; lacking head but with well-developed muscular foot and often elaborate gill systems

• Cambrian (Lower) to Recent

Class ROSTROCONCHIA

• Superficially similar to bivalves but with shells fused along dorsal midline

• Cambrian (Lower) to Permian (Kazanian)

Class SCAPHOPODA

• Long, cylindrical shell, open at both ends

• Devonian to Recent

Class CEPHALOPODA

• Most advanced mollusks with head and well-developed sensory organs together with tentacles

• Cambrian (Upper) to Recent

Shell

Mouth with radula

Visceral mass (guts, reproductive organs, etc.)

Visceral mass (guts, reproductive organs, etc.)

Shell

Mouth with radula

Gills HAM

Anus

Mantle cavity

Foot

Shell

Mouth with radula

Visceral mass (guts, reproductive organs, etc.)

Visceral mass (guts, reproductive organs, etc.)

Shell

Mouth with radula

Gills
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