Morphological and structural adaptations to soft substrates in the Early Jurassic monomyarians Lithiotis and Cochlearites

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Abstract
L. problematica and C. loppianus are sessile monomyarian bivalves known from the Early Jurassic of the Tethyan region, where they are found standing vertically in the calcareous mud of lagoonal facies. Their shells are characterized by a long cardinal area, ventrally displaced body space, thick attached valve, and a thin, flat free valve of equal length. A functional ligament is present only in juvenile stages of Cochlearites. To grow straight, the ventral end of the shell had to gape, so in both species it was probably the elasticity of the thin free valve that caused the shell to open. In the adult stage the shell grew only towards the center with an apparently constant growth rate; the soft body size remained unchanged. The outer shell is composed of a compact aragonitic layer; the inner part is filled with loose chalky deposits which probably supported the soft body and lifted it upwards.

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