On New Records of Cephalopod Jaws from the Upper Bajocian (Middle Jurassic) of the Northern Caucasus (Russia, Karachay-C
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ew Records of Cephalopod Jaws from the Upper Bajocian (Middle Jurassic) of the Northern Caucasus (Russia, Karachay-Cherkessia) A. A. Mironenkoa, * and V. V. Mittab, c, ** a
b
Geological Institute, Russian Academy of Sciences, Moscow, 119071 Russia Borissiak Paleontological Institute, Russian Academy of Sciences, Moscow, 117647 Russia c Cherepovets State University, Cherepovets, 162600 Russia *e-mail: [email protected] **e-mail: [email protected] Received July 19, 2019; revised August 1, 2019; accepted August 1, 2019
Abstract—New finds of cephalopod jaw apparatuses from the upper part of the Niortense Zone and the lower part of the Parkinsoni Zone (Upper Bajocian) from the interfluve of the Kuban and Urup rivers (Northern Caucasus) are described. Two isolated valves of aptychi which are considered to be ammonoid lower jaws are assigned to the superfamilies Haploceratoidea and Stephanoceratoidea. Two upper jaws likely belonged to ammonites with aptychus-type jaw apparatus (Ammonitida). The seven anaptychus-type jaws are assigned to the orders Phylloceratida and Lytoceratida. A well-preserved upper jaw of coleoid affinity is described for the first time from the Mesozoic of the Northern Caucasus. Keywords: Ammonoidea, Coleoidea, jaws, aptychi, anaptychi, Middle Jurassic, Upper Bajocian, Northern Caucasus DOI: 10.1134/S0031030120050093
INTRODUCTION Cephalopods, unlike other mollusks, have a welldeveloped jaw apparatus, consisting of two jaws (upper and lower) and a radula between them. The jaws of most modern cephalopods are composed exclusively of organic matter, and only survivors of the Nautiloidea subclass have calcite elements at their jaw tips (Saunders et al., 1978; Tanabe et al., 2015a). However, in the past, the diversity of cephalopod jaws was significantly higher, primarily due to representatives of the subclass Ammonoidea, in which from four to six different types of jaw apparatuses are distinguished (Tanabe et al., 2015a; Mitta and Schweigert, 2016). The Jurassic period is particularly important for understanding the evolution of the ammonoid jaw apparatus. At the end of the Early Jurassic (Toarcian), representatives of the suborder Ammonitina acquired an aptychus-type jaw apparatus, in which the lower jaw consisted of two halves, often covered with a calcite layer and interconnected by a flexible ligament (Engeser and Keupp, 2002; Tanabe et al., 2015a). It is likely that in the early Jurassic the rhynchaptychustype jaw apparatus appeared, in which the tips of the jaws (like those of nautilids) were reinforced with calcite elements. Findings of isolated calcite elements of
the jaw of cephalopods (rhyncholites) are known starting from the Upper Triassic, but they most likely belonged to nautilids (Riegraf and Luterbacher, 1989; Riegraf and Schmitt-Riegraf, 1995). The oldest rhyncholites, presumably belonging to ammonoids, are known starting from the Pliensbachian (Riegraf and Luterbacher, 1989), and the earliest rhynchaptychus jaws are found in the Middle Jurassic (Bajocian) of the Northern Caucasus (Mir
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