A new morphotype of lower jaw associated with Calliphylloceras (Cephalopoda: Ammonoidea) from the Middle Jurassic of the Northern Caucasus

The body chamber of an ammonoid of the genus Calliphylloceras (Suborder Phylloceratina) containing its lower jaw is reported from the Upper Bajocian (Strenoceras niortense Zone) of the Northern Caucasus. The arrangement, state of preservation, and size of the lower jaw suggest preservation in situ. The general morphology of the lower jaw strongly resembles the chitinous inner lamella of the aptychus-type lower jaws, because it is divided into two almost symmetric parts by a median harmonic midline joint termed symphysis. However, outer bivalved calcitic plates (apytchus sensu stricto) cannot be recognized in the present lower jaw. Early Jurassic Phylloceratina are known to have had lower jaws of the anaptychus- type, whereas Late Cretaceous taxa had lower jaws of the rhynchaptychus-type. It appears that in the Middle Jurassic a third morphotype of lower jaws was present in Phylloceratina, for which we introduce the new term 'phyllaptychus'.