Comparison of the diagenesis of the mineral part and of the soluble organic matrices in aragonitic tests of nautiloids and ammonites

The compositions of both organic and mineral parts of shells of Nautilus mi cromphalus, a Cenozoic nautiloid and Jurassic ammonites are compared. The m icrostructures of the fossil shells show minor diagenetic changes, despite their aragonitic mineralogy. The bulk: analysis of the organic matrices. mo lecular weights, acidity and amino-acid contents have been studied. All the fossil shells are compositionally altered relative to modern counterparts. with thc organic components both extensively lost and chemically modified. Alterations are variable within a single fossil site. implications of thes e diagenetic changes for palaeoenvironmental reconstructions are examined.