Ammonoid lower jaws of rhynchaptychus type from the Cretaceous of Crimea. (November 2018)

• Record Type: Journal Article
• Authors: Mironenko, Aleksandr A.; Rogov, Mikhail A.
• Is Part Of: Cretaceous research. Volume 91(2018)
• Page Start: 350
• Page End: 361
• Publication Date: 2018-11
• Subjects: Ammonoidea -- Cenomanian -- Tillicheilus -- Jaws -- Conchorhynchs -- Rhyncholites -- Rhynchaptychi; Geology, Stratigraphic -- Cretaceous -- Periodicals; 551.77
• DOI: 10.1016/j.cretres.2018.07.005
• Languages: English
• ISSNs: 0195-6671

Abstract

Abstract: 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.