Middle Jurassic ammonoid jaws (anaptychi and rhynchaptychi) from Dagestan, North Caucasus, Russia

The Middle and Upper Jurassic stage of evolution of the anaptychus-type ammonoid jaw apparatus is relatively poorly known due to a small number of findings and uncertainty of their taxonomic position. All previously found anaptychi of this age are preserved either in flattened and dissolved shells or separately from ammonoid conchs. Rhynchaptychus-type jaws were still hitherto unknown from Jurassic deposits. In this paper we describe three-dimensionally preserved ammonoid lower jaws from the Bajocian/Bathonian boundary (Middle Jurassic) beds of Dagestan, Russia. These findings demonstrate a wide variety of their shape and structure. One specimen, consisting only of organic matter which is considered as anaptychus sensu stricto, is located in situ in the body chamber of Lytoceras (Dinolytoceras) zhivagoi (Besnosov). Three specimens which likely belonged to any Phylloceratiadae (Adabofoloceras, Holcophylloceras, or Pseudophylloceras which are presented in the ammonoid assemblage) contain prominent calcareous conchorhynchs, and the outer organic lamellae of these jaws were initially covered with a thin calcareous layer. The last lower jaw, probably from Nannolytoceras, has also a small calcareous conchorhynch in its tip despite a lack of coating. These findings are the first direct evidence of the existence of rhynchaptychus-type lower jaws in the Middle Jurassic. A variety in the shape and structure of the studied lower jaws indicates a variation in the mode of life and feeding behavior of Middle Jurassic ammonoids.