Fibrous ligament and aragonitic shell material have been found preserved in two specimens of radiolitid rudists [cf.Radiolites angeoides(De Lapeirouse)] from the Lower to Middle Coniacian of Tirol, Austria. The invaginated ligament apparently comprised two torn and overgrown non-functional strings, one in each valve, connected medially by a thin strip of active ligament. The original dimensions of the active ligament have been estimated, for shells of 4 to 5 cm commissural diameter, at 0.58 mm2cross-sectional area and 1.3 mm length, with the shell closed. Using data on the adductor muscle of Recent oysters and the results of compression tests carried out on samples of oyster fibrous ligament, it is estimated that these radiolitid shells opened less than 1 mm in life; their gaping was highly restricted. This is taken to support the proposal that the expanded mantle margins (principally that of the right, attached valve) were responsible for food particle entrapment, rather than the gills as in most other bivalves. It is suggested that a tentaculate mantle flange was extruded through the narrow gap between the valves during feeding. The pair of pronounced radial bands on the posterior flanks of the shell are interpreted as marking pseudofaecal and faecal ejection sites. Finally, it is speculated that slime (possibly poisonous or foul-tasting) on the mantle tentacles inhibited predators.
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