Cephalopod jaws from the Middle Jurassic of central Russia

Abstract

Three-dimensionally preserved ammonoid and coleoid jaw elements are described from the Lower Callovian of the Unzha-river region. Isolated praestriaptychi are attributed to the kosmoceratid ammonites Kepplerites/Sigaloceras based on correlation between their shape with the outlines of aperture of ammonite conchs found in the same strata. The width/length-ratios of the aptychi cluster into four morphotypes representing macro-and microconchs of two species. Only one incomplete praestriaptychus valve may represent a perisphinctid. The praestriaptychi of Kepplerites/ Sigaloceras exhibit a thin lamellar calcite coating composed of rosette-like calcification centres. Two complete jaw apparatuses of the aptychophoran kosmoceratid Kepplerites show reduced rostra of their upper jaws of the same length as the lower jaws. The reduced or complete absence of a rostrum suggests that the aptychus bearing ammonits did not eat large prey, because they could not hold and cup up it. Therefore, we suggest that the Middle Jurassic aptychophoran ammonoids fed on plankton or other minute organisms. We find the function of aptychi as an operculum more important than as a specialized feeding apparatus. In contrast to the simplified morphology of upper ammonoid jaws, the associated upper jaws of coleoids, probably belonging to belemnites, are characterized by prominent rostra. A small isolated shovel-like jaw element may represent the lower jaw of a vampyromorph coleoid.

Ammonoid Intraspecific Variability

“…Intraspecific variation also occurs in the shape or position of the suture line (e.g., Yacobucci and Manship 2011) or in dextral or sinistral coiling of the conch (e.g., Matsunaga et al 2008) as seen in extant gastropods. Ammonoids might also have differed intraspecifically in colour patterns (e.g., Mapes and Sneck 1987;Bardhan et al 1993; see also Mapes and Larson 2015), buccal mass (Davis et al 1996;Keupp 2000;Keupp and Mitta 2013;compare Kruta et al 2015) or other characteristics which are rarely or not preserved at all such as soft-tissues (Klug et al 2012) or other peculiar structures (Landman et al 2012). We will herein focus on intraspecific variation in shell shape, ornamentation and size, as well as spacing and shape of the septa (suture lines), for which more data are available.…”

Soft-tissue preservation in the Middle Jurassic ammonite Cadoceras from Central Russia

“…2). The specimen comes from a layer of calcareous sandstone nodules, often phosphatized, with inclusions of pyrite (see Keupp and Mitta 2013 for taphonomy and geological setting). The diameter of the specimen is about 10 cm.…”

Ammonoid Paleopathology

“…There is a long controversial discussion about the function of the double-valved, calcareous aptychi interpreted as lower jaw, lid, or a combination of both (e.g., Schindewolf 1958;Lehmann 1972Lehmann , 1990Morton 1981;Lehmann and Kulicki 1990;Seilacher 1993;Keupp and Veit 1996;Keupp 2003;Schweigert 2009;Kruta et al 2011;Tanabe 2011;Keupp and Mitta 2013). Regenerated injuries causing Fig.…”