The coarse wrinkle layer of Palaeozoic ammonoids: new evidence from the Early Carboniferous of Morocco

The wrinkle layer is a dorsal shell structure occurring in a number of ammonoids, but its function is still debated. Here, we describe, from Moroccan material of the Early Carboniferous species Maxigoniatites saourensis (Pareyn, 1961 ), the most conspicuous wrinkle layer known within the Ammonoidea. This additional shell layer occurs in the ventrolateral portion of the adult body chamber and forms continuous lamellae, which range about two millimetres into the lumen of the body chamber. Possible functions are discussed and the most likely interpretation for the structure is ‘fabricational noise’, which is related to the coarsening of the shell ornament of the terminal body chamber.