The ammonoid genus Spiroceras (Spiroceratidae, Ammonoidea) from the Upper Bajocian of the Northern Caucasus

Resumo

Ammonites of the genus Spiroceras are described from the Upper Bajocian of the Kyafar River Basin (Karachay-Cherkessia), including macroconchs with preserved apertures and (for the first time in the entire history of the study of Middle Jurassic heteromorphs) microconchs with lateral lappets. The microconchs of S. bispinatum (Baugier et Sauzé), mainly represented by cyrtocones with one ventral row of nodes, correspond to microconchs with a gyroconic shell and a single row of nodes. Macroconchs and microconchs of S. annulatum (Deshayes), the ribs of which lack prominent nodes or spines, are found together. The assemblage also contains small-sized slightly uncoiled shells of Spiroceras aff. S. fourneti Roman et Pétouraud with widely spaced ribs and two prominent rows of large nodes, presumably macro- and microconchs. The ornamentation of this species resembles that of the monomorph Bajocia rarinoda Sturani from the upper part of the Lower Bajocian; perhaps the latter was ancestral to the Middle Jurassic heteromorph. The above species of Spiroceras, as well as S. obliquecostatum (Quenstedt), are figured.