Preserved ligament in the Jurassic bivalve Lithiotis: adaptive and evolutionary significance

An exceptionally well-preserved specimen of Lithiotis problematica from the Lower Jurassic of NE Italy possesses multiple resilia, composed of thin fibres and joining both valves, housed in ligamental grooves. The resilia were modified to allow small changes in the reciprocal distance of the valves. This increased the area in which the extremely thin free valve flexed to allow shell closure, and reduced the risk of breakage by fatigue of the valve and/or ligament. The fact that the valves remained connected by ligament for an extended height of the ligamental area implies that (1) the free valve closed by flexing, not by articulating, (2) it did not migrate in the ventral direction as often observed in cemented bivalves, and (3) it was approximately as long as the attached one.