Article: The microstructure and mineralogy of the shell of a Jurassic mytilid (Bivalvia)

Well-preserved shells of Praemytilus strathairdensis (Anderson and Cox) from the Great Estuarine Series (Middle Jurassic) of the Isle of Eigg, Scotland, are described with special reference to their microstructure, mineralogy, and chemical composition. Both the main calcareous shell layers are nacreous, with the crystal structure apparently identical to modern nacre. Myostracal layers are composed of prismatic aragonite; calcified traces of the periostracum have been observed. The inner layer shows sub-layers defined by variations in thickness of the nacreous laminae. These probably represent seasonal growth. Mineralogically, the shells are wholly aragonite, with a high strontium content compared to modern bivalves. The organic matrix is in part physically preserved but with altered ultrastructure; it is reduced in amount from the probable original value and the carbon-nitrogen ratio is much higher. The species is one of the earliest representatives of Mytilus sensu lato; the structural data agree with this. The aragonitic composition suggests warm temperatures, by comparison with modern Mytilidae; the growth pattern suggests the existence of seasons.