Article: Shell structure, growth, and functional morphology of an elongate Cretaceous oyster
Konbostrea (gen. nov.) is an aberrant oyster of the Crassostrea group, characterized by its dorsoventrally elongated stick-like shell. It is found in life position, perpendicular to the bedding in brackish-water muddy deposits of the Upper Turonian to Upper Coniacian, in Sakhalin and north Japan. A narrow body space is restricted to the ventral end of the shell. The ligamental area is very small, separated from the body space, and the ligament is considered to have been active only during the younger stages of growth. The adult animal most probably took advantage of the elasticity of the thin, flat, right valve to open its shell. Having reached its adult width the shell grew only in a ventral direction, at an apparently constant rate, without notable increase in the size of the body cavity. The shell is composed of an outer foliated layer which defines the structural framework of the shell, and inner chalky deposits which fill in most of the inner space. All these characteristics are thought to result from adaptation to keep up with rapid sedimentation on a soft muddy bottom. Konbostrea constantly grew upward, accumulating chalky deposits to maintain the body above the rising sediment surface.