Plain-Language Summary:
Anionoceras is a genus of straight-shelled nautiloids (cephalopods similar to the modern Nautilus) whose classification has been controversial over the past several years. This article traces the history of Anionoceras classification and reviews the morphological features that have been used to infer relationships between this genus and other, morphologically similar, nautiloid genera. The author concludes that Anionoceras is sufficiently distinct from a morphological point-of-view to warrant placement within its own family (the Arionocertidae). Patterns of morphological variation that may reflect sexual differences between individual specimens are also discussed.
Maurizio Gnoli, (Dipartimento Scienze della Terra, Modena University)