Mironenko Aleksandr, Rogov Mikhail (2018) Ammonoid lower jaws of rhynchaptychus type from the Cretaceous of Crimea. Cretaceous Research, 91, 350-361


Ammonoid lower jaws of rhynchaptychus type from the Cretaceous of Crimea.

The jaws of Cretaceous nautiloids and some ammonoids (Phylloceratina and Lytoceratina) contain calcareous elements, namely rhyncholites in the upper jaws and conchorhynchs in the lower. Until now, only several types of numerous rhyncholites have been described from the Cretaceous deposits of Crimea, but lower jaw elements have never been reported from this region. Here we present the first finds of ammonoid lower jaws from the Cenomanian of Crimea that comprise well-preserved calcitic elements. The shape of these conchorhynchs and the ratio of their size to dimensions of the jaw vary in the different specimens. This difference may indicate a variety of food strategies amongst ammonoids with rhynchaptychus-type jaws, whereas all of them were likely durophagous. This assumption is confirmed by longitudinal scratches on the dorsal surface of these conchorhynchs. Amongst dozens of Crimean rhyncholites, specimens which belong to the form genus Tillicheilus have been described previously. Tillicheilus differs fundamentally from other rhyncholites by its shape. A comparison of the conchorhynchs from jaws with Tillicheilus rhyncholites has shown that these calcitic elements are identical, i.e., Tillicheilus constitutes lower jaw elements (conchorhynchs), rather than rhyncholites as interpreted earlier. Finds from Crimea significantly expand the stratigraphical and geographical distribution of Cretaceous rhynchaptychus-type ammonoid jaws.


The article is devoted to the unique findings of the ammonoid lower jaws of the rhynchaptychus type in the Cenomanian of Crimea and to the study of the ammonoid conchorhynchs (lower jaw elements). Until now, hundreds of rhyncholites (calcitic elements of the upper jaw) and not one conchorhynch has been described from Crimea. The article shows that one of the formal genera of the rhyncholites, known from Crimea, Tillicheilus, is actually an ammonoid conchorhynch. Such possibility was previously assumed by several researchers, but now it is confirmed by findings of whole lower jaws containing Tillicheilus.


Ammonoidea, Cenomanian, Tillicheilus, Jaws, Conchorhynchs, Rhyncholites, Rhynchaptychi