Organic periostracum preserved in Cretaceous ammonoids from the Andean Neuquén Basin

Exceptionally preserved 135-million-year-old ammonoids from the Neuquén Basin at the Andean foothills revealed a fossilised structure never recorded before. Ammonoids are cephalopods that inhabited the oceans for about 400 million years until they became extinct 66 million years ago. Their shells are composed of aragonitic layers bounded externally by an organic periostracum. The latter plays an essential role in initiating shell biomineralisation and protecting minerals from dissolution and abrasion. Here we describe a preserved periostracum in Cretaceous ammonoids, an extremely fragile yet flexible layer, with an approximate thickness of 2 µm and an internal horizontal lamination. The external surface appears mostly smooth, while the internal surface displays a reticulated appearance, interpreted as the casts of aragonite prisms of the calcareous shell. Our results reveal that the ammonoid periostracum contains proteins, polysaccharides, and lipids, consistent with the composition of the periostracum in modern-day molluscs. This study sheds light on a previously little-known organic structure in ammonoids. Its morphological and chemical characteristics allow us to establish that it is a highly conservative structure among molluscs. Furthermore, we show that such a delicate organic structure can be preserved for 135 million years in favourable environmental conditions, opening up the possibility of future discoveries.