Page 341 - Introduction to Paleobiology and The Fossil Record

328 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 13.1 Classifi cation of Mollusca

Class CAUDOFOVEATA

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

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
• Cambrian (Lower)

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)