A key for the description of Palaeozoic ammonoids

Palaeozoic ammonoids have been described for almost 200 years. Since the first nomination of the Carboniferous ammonoid “Ammonites Listeri” by Sowerby (1812), styles and contents of descriptions have significantly been modified and completed. The numbers of conch characteristics taken into account have increased considerably, and external as well as internal characters were evaluated to form a balance. While the early authors mainly described more or less mature stages of single individuals, later authors also included the ontogeny of specimens and or the variability within populations. Although there is a sort of consensus among ammonoid workers about the use of the various conch characters and their importance for ammonoid description and classification, there is no agreement on how species should be characterised in terms of diagnoses and illustrations. It was particularly Ruzhencev (e.g. 1956, 1960), who in a series of monographs, developed a standard for ammonoid descriptions accompanied by illustrations of representative specimens on plates, well-drawn suture lines of mature and often of juvenile stages, and occasionally also conch cross sections. His diagnoses and descriptions of species followed a strict scheme, in which all the important characters were outlined. Based on the study of Permian forms, Kutygin (1998) has proposed a scheme for the description of ammonoid conchs. In a bivariate diagram he showed the conch width index and umbilical width index and separated between 35 possible morphs. He named these morphs either according to their shape (e.g. ‘sferokon’, ‘kadikon’) or after ammonoid genera (e.g. ‘oppellikon’, ‘daktilikon’); name pairs such as ‘ofikon’ (for very slender forms) and ‘subofikon’ (for moderately slender forms) lead to a reduction of descriptive terms. The description key outlined here can be seen as a further development of the schemes proposed by Ruzhencev and Kutygin. This key is specifically coined for the Early Carboniferous ammonoids from Algeria, which will be described in monographs of this volume (Bockwinkel et al. 2010; Ebbighausen et al. 2010; Korn et al. 2010a, 2010b, 2010c); it was developed mainly for Carboniferous ammonoids of the predominant suborder Goniatitina, which has a wide morphological range but very similar suture lines.