vendredi 27 février 2015

Mollusk shells

1- Bivalves

Microstructures:  

Foliated, prismatic, nacre and crossed lamellar structures are known in bivalve shells.

 Composition: 

 


FTIR spectra:organic comopnents of mollusk shells (YD287) and thin layer chromatography (lipidic components) (YD287) 


SEM images and TOF-SIMS chemical maps of the prism- nacre transition in Pinctada
From left to right: Ca, glycine, alanine and proline maps
(YD293)


2-Gastropods

Microstructures:  

The cross lamellar layer is the main structure of gastropod shells (a, b), but columnar nacre is also present (c). Calcitic and aragonitic prisms sometimes co-exist in a single shell (d, e).

Composition:  


Cross lamellar layers are aragonitic, but the organic contents vary in different taxa.


In Mollusc shells, Sr contents of aragonitic layers are low.




Infra-red maps show the differences in mineralogy (b) and in organic contents (c, amide I) in the two layers of Concholepas. PR: calcitic prismatic layer, CL: aragonitic cross lamellar layer, IL: intermediate layer 


Various alterations in the cross lamellar layers of gastropods collected in the Pleistocene of Greece.
 

3- Cephalopods

Tetrabranchiata

Microstructures:  

Nacre and prisms (and spherolitic prismatic) layers are known in Cephalopods
  Nacreous layer in a modern Nautilus


From left to right: nacreous layer in a fossil nautilid (Cenozoic), nacreous layer in a triassic orthocone, aragonitic cameral deposit in a triassic orthocone (YD19)




Nacreous layers in ammonites from Lukow (Poland, Callovian)


Protoconch of cretaceous ammonoids (YD1, YD6, YD7)

Composition:   

Organic matrices extracted from modern Nautiloid shells are composed of proteins and sugars.The insoluble matrices of Nautilus macromphalus shell are more "chitinous" than the soluble matrices extracted from the same species, as shown by these two graphics :

 The composition of fossil shells is more or less modified. Aragonitic shells are not "perfectly" preserved, even if the microstructure is still visible.

 
Amino acid composition of the nacreous layer of the soluble organic matrices of the modern Nautilus and an ammonite from Lukow.

Dibranchiata 

Microstructures: 

Spirula: protoconch

 
Sepia: dorsal shield and ventral part
 

Nacre in Spirula and Sepia