West of Dallas in Lower Cretaceous rock are Eopachydiscus Ammonites, over 100 million years old and also quite large. Lake Texhoma and Lake Whitney are famous for large Ammonites. Ammonites are some of the best known and most found invertebrate fossils in Cretaceous period rock. Ammonites differ from Nautiluses in having complicated lines across their surface to mark chamber wall locations. Nautilus has nearly straight lines.
Nautiluses still exist.