Cephalopod jaws from the Middle Jurassic of central Russia

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.