Here is another type of pyritized Ammonite that can be found in the Toarcian aged rocks (Jurassic) around Millau, Aveyron Dept., France. It's labelled as an Amaltheus sp. and is a small fossil. The whorls overlap about 25% of each previous volution. The external ornamentation is preserved well with the strong ribs that do not continue onto the keel. The keel is smooth except for a raised portion that has slight notches regularly space so that it looks like a braid from the side. Not enough of the internal sutures are visible to tell for sure, but this specimen looks very similar to one that I saw on the Steinkern.de forum, in this post, that was identified as Amaltheus margaritatus cf. forma subnodosus.
Thanks to Gery for sending me this fossil!