Article: An Early Cretaceous orthocerid cephalopod from north-western Caucasus
An orthocerid cephalopod is reported for the first time from the Lower Cretaceous deposits of the Caucasus. Discovery of this mollusc, Zhuravlevia insperata gen. et sp. nov., in such young beds shows that evolution of the orthocerid branch within the cephalopods was not terminated in the Late Triassic, as was formerly believed, but continued for a further 90-95 Ma, at least until the late Aptian (Clansenian). Z. insperata was found in association with numerous and diverse Aptian ammonites, rare belemnites, and a single phragmocone-bearing coleoid Naefia, characterized by the absence of the guard, all comprising the cephalopod fauna of the shallow Aptian sea in the Hokodz River Basin. SEM studies of Z. insperata and Naefia recovered from the same Aptian concretions have shown that the former possesses lamellar nacreous ultrastructure of the septa common for cephalopods with an external shell, while the latter possesses granular nacreous ultrastructure of the septa common for cephalopods with an internal shell. The ultrastructure of the septal neck of Z. insperata is closely similar to that of previously studied Carboniferous orthocerids from the Buckhorn Asphalt, hitherto the only orthocerids studied with well preserved primary ultrastructure.