of orthocerids is still jumbled and requires revision based on features of the embryonic shell. Since cephalopod fossils from the Cambrian and Early Ordovician Periods are rare and usually incomplete, we do not know the ancestral states of many characters, making it difficult to assess which derived characters unite the neocephalopods and which other nautiloid groups belong among them. Nevertheless, recent descriptions of shells with preserved apical ends have aided in the classification of major taxa. For instance, ascocerids and pseudorthocerids have a cicatrix and so do not belong among the neocephalopods. Conversely, true orthocerids and lituitids lack a cicatrix and have a round protoconch, which suggest neocephalopod affinities. In contrast, some workers have suggested that crown cephalopods did not evolve until approximately the Silurian Period. If that is correct, then all early Paleozoic cephalopods must be stem cephalopods and not neocephalopods. Footnotes References Berthold, Thomas, & Engeser, Theo. 1987. Phylogenetic analysis and systematization of the Cephalopoda (Mollusca). Verhandlungen Naturwissenschaftlichen Vereins in Hamburg. (NF) 29: 187-220. Engeser, Theo. 1996. The Position of the Ammonoidea within the Cephalopoda. In: Ammonoid Paleobiology, Vol. 13 of Topics in Geobiology, ed. by Neil Landman et al., Plenum Press, New York. Chapter 1, pp. 3–19. Lehmann, U., & Hillmer, G. 1980. Wirbellose Tierre der Vorzeit Leitfaden der Systematischen Paläontologie. Ferdinand Enke Verlag, Stuttgart, Germany, 1–340. Lehmann, U., & Hillmer, G. 1983. Fossil Invertebrates. Transl. by Janine Lettau from Lehmann & Hillmer 1980. Cambridge University Press, New York. External links The Palcephalopoda/Neocephalopoda Hypothesis, from Dr. Theo Engeser's Fossil Nautiloidea Page Cephalopod taxonomy