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Middle Jurassic (Upper Bathonian and Lower Callovian) jaws of Kosmoceratid ammonites of Central Russia

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Abstract

Elements of the jaw apparatuses of the ammonite genus Kepplerites (Ammonoidea: Stephanoceratoidea, Kosmoceratidae, Keppleritinae) are described from two Upper Bathonian and one Lower Callovian localities of the Russian Platform. The lower jaws (aptychi), based on their size and shape can be assigned to two groups and certainly belonged to the co-occurring macroconchs K. (Kepplerites) and theirs microconchs K. (Toricellites). It is established that the presence or absence of tuberculate ornamentation on the surface calcite layer in the studied kosmoceratid aptychi (and accordingly the assignment of the aptychi of kosmoceratids to Granulaptychus-type or Praestriaptychus-type) is a result of burial and fossilization in different settings. Most likely all Kosmoceratidae had lower jaws of the Granulaptychus-type, apparently like the related subfamily Garantianinae (family Stephanoceratidae). For the first time, upper jaws of cephalopods supposedly also belonging to the ammonites of genus Kepplerites are described from the Bathonian Stage.

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... The structure of the outer lamella (hood) of the upper jaws of aptychophoran ammonites is poorly studied, they are often described as reduced (Keupp and Mitta, 2013;Tanabe et al., 2015;Keupp et al., 2016). However, the outer lamellae of the Middle and Late Jurassic (late Bathonian, early Callovian and late Volgian) upper jaws, some of which were preserved in situ within the ammonite body chambers (Mironenko, 2014;Mitta and Mironenko, 2021), consist of two plates with a central tube enclosed between them. Consequently, relatively square jaw elements with a central tube ( Fig. 5AeC) are, most likely, the outer parts of the upper jaws of Valanginian ammonites. ...
... The upper jaws of polyptychitids are similar to previously described upper jaws of Jurassic and Cretaceous ammonites with a jaw apparatus of the aptychus-type (see Tanabe et al., 2015). The structure of the outer plate of polyptychitid upper jaws is practically identical to that in Middle Jurassic kosmoceratids (Keupp and Mitta, 2013;Mitta and Mironenko, 2021), which had aptychi of a completely different shape and with a calcitic layer, covered with tubercles. This indicates conservatism in the structure of the upper jaw in ammonites with an aptychus-type jaw apparatus. ...
... internal lamellae: D. right lateral wall of lamella, PIN 5322/041; E. left lateral wall, PIN 5322/030; F. paired lateral walls of the inner lamella viewed from the anterolateral side, PIN 5322/024. Scale bars equal 5 mm, r e rostrum, lw e lateral wall of the inner lamella;bt e basal part of the tube-like structure (is not fully preserved but seeMitta and Mironenko, 2021: ...
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... The first formal genus -Lamellaptychus -appeared in the family Oppeliidae and was typical for all representatives of superfamily Haploceratoidea. Granulaptychus are known in Bathonian and Callovian Garantianinae and Kosmoceratidae and most likely belonged to all stephanoceratid ammonites (Mitta and Mironenko, 2021). Praestriaptychus was one of the most widespread formal genera, characteristic of many (but not all) ammonites of superfamily Perisphinctoidea. ...
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A specimen of the Callovian ammonite genus Kosmoceras which exhibits its Aptychus (Granulaptychus calloviensis TRAUTH, very close to the type species of Granulaptychus) still in situ is reported from the Ornatenton Formation of Swabia. The much younger "Granulaptychi" belonging to Late Jurassic perisphinctids are included in a new taxon Strigogranulaptychus n. g. (type species: Granulaptychus planulati QUENSTEDT in TRAUTH). Praestriaptychus gerzensis TRAUTH from the Bajocian is designated as type species of Praestriaptychus TRAUTH.
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Nine proposals of aptychus (sensu stricto) function have been published (in historical order): operculum, micromorphic males, lower mandible, protection of gonades, ballast for lowering of aperture, flushing of benthic prey, filtering microfauna, pump for jet propulsion, and active stabilizer against rocking produced by the pulsating jet during forward foraging and backward swimming. Some ammonites bear thick, laevaptychus- and lamellaptychus-type aptychi (aspidoceratids and haploceratoids) that may have improved lowering of the aperture as part of a mobile cephalic complex, enabling many of these functions. Aptychi were multifunctional, most commonly combining feeding (jaw, flushing, filtering) with protection (operculum), and/or with propulsion (ballast, pump, diving and stabilizing plane). Multifunctionality would have been a strong constraint in ontogeny and evolution as shown by the limited diversity of aptychi with respect to the wide variety of shell morphologies known in the Mesozoic Ammonitina. Calcification of aptychi in the Jurassic Ammonitina is known from the Early Toarcian Hildoceras which is also the first ammonite with males bearing well-formed lateral peristomatic projections or lappets. Calcification allowed aptychi to be involved in functions, which would have improved, in different degrees and combinations, feeding, propulsion and protection. It is herein suggested that multifunctional calcareous aptychi allowed the gradual development of a wide variety of new life-styles. These new life-styles would have led to the origin and early evolution of haploceratids and stephanoceratids producing the wide diversification of the Ammonitina observed from the Early Aalenian.
Article
The different forms of the aptychi (opercula, homologous with lower jaws) of the Ammonoidea are used for the first time in a phylogenetic analysis of part of the classic Ammonoidea phylogeny. The results indicate that the aptychi-possessing ammonoids form a monophylum for which we propose the informal name Aptychophora nov. Among the Jurassic ammonoids, it is possible to recognize several monophyletic groups. In part, our results support existing superfamilies (e.g. Hildocerataceae, Haplocerataceae) by new synapomorphies. However, the Perisphinctaceae can now be much more clearly differentiated than in the previously established phylogenetic tree. The Upper Cretaceous ammonoid superfamilies cannot be derived from the Haplocerataceae, but are descendants of a ‘primitive’ perisphinctacean possessing a praestriaptychus. Nor can they be derived from the ‘higher’ perisphinctaceans (family Perisphinctidae) because that clade is characterized by granulaptychi. The consequence of these results is that the quadrilobate primary suture of the ‘Ancyloceratina’ must have evolved more than once by reduction from an ancestral quinquelobate primary suture. The Ancyloceratidae have praestraptychi or aptychi types which can be derived from praestriaptychi, whereas the Crioceratitinae have longitudinally striated anaptychi.
Article
The microstructure of aptychi (bivalved calcareous coverings on lower jaws) of three genera of Late Cretaceous Ancyloceratina, Baculites, Polyptychoceras and Jeletzkytes is described for the first time on the basis of well-preserved and in situ material from the Western Interior of the USA and Hokkaido, Japan. Optical and scanning electron microscope observations of aptychi on polished median and cross-sections reveal some variation in their relative size, shape and microstructure among the three genera. The aptychus of Baculites is composed of two calcitic layers: one with tilted lamellae and the other one with horizontal lamellae, whereas those of Polyptychoceras and Jeletzkytes consist of a thin layer with horizontal lamellae. Comparison with aptychi (e.g. Laevaptychus) of Jurassic Ammonitina shows that the aptychi of Ancyloceratina differ from those of Jurassic Ammonitina in the smaller number of layers and the absence of a sponge-like structure. We propose for the first time growth models for a sponge-like aptychus of Jurassic Ammonitina and the lamellar aptychus of Cretaceous Ancyloceratina. The remarkable microstructural variation of aptychi observed in Mesozoic Ammonoidea is probably related to the diversity of their modes of feeding and the secondary function of the lower jaws as opercula.
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Monographie der Otoitidae (Ammonoidea): Otoites, Trilobiticeras, Itinsaites, Epalxites, Germanites, Masckeites (Pseudotoites, Polyplectites), Normannites
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