Aptychus de Oxyparoniceras (Oxyparoniceras) buckmani (BONARELLI, 1895) (Paroniceratinae, Ammonoidea) del Toarciense de Ariño (Cordillera Ibérica, España)

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En este trabajo se describe y analiza una pieza de cornaptychus hallada en el interior de un ejemplar de Oxyparoniceras (Oxyparoniceras) buckmani (BONARELLI), procedente del Toarciense Superior de Ariño (Teruel, España). Procede de un nivel que forma parte de una secuencia de profundización en la que está asociado a Osperlioceras sp., Pseudogrammoceras fallaciosum (BAYLE), Pseudolillia emiliana (REYNES), Geczyceras goyi MARTÍNEZ y Hammatoceras semilunatum (QUENSTEDT), situados inmediatamente por encima de los últimos niveles con Hammatoceras insigne (SCHLÜBLER in ZIETEN). Esta asociación es característica de la Zona Dispansum, Subzona Insigne. Las características morfológicas del aptychus permiten caracterizarlo como cornaptychus, y la notable equivalencia entre su contorno y la forma de la sección del ejemplar de Oxyparoniceras (Oxyparoniceras) buckmani (BONARELLI) hace posible que este cornaptychus funcionara como opérculo, sin desestimarla posibilidad de que actuara también como aparato mandibular. Por otra parte su disposición dentro de la cámara de habitación, permite suponer que la pieza no se encuentra en posición anatómica, sino que ha sufrido un desplazamiento hacia el interior de dicha cámara de habitación. [ABSTRACT] A piece of cornaptychus (modified from LEHMANN, 1981, after TRAUTH, 1930) found inside a specimen of Oxyparoniceras (Oxyparoniceras) buckmani BONARELLI, collected in the Upper Toarcian deposits of Ariño (Teruel province, NE Spain) is described and analyzed. The particular record of identifiable aptychi associated with ammonite specimens, is quite rare in the Jurassic of the Iberian Peninsula and of interest for paleobiologic and taxonomic purposes. The Toarcian deposits of the Ariño region have been previously studied. The abundance and good state of preservation of the fossils, probably due to favourable conditions of fossilization, as well as the great thickness reached by the Toarcian deposits in this section, can be highlighted. The piece of cornaptychus is located in the body chamber of a specimen of Oxyparoniceras (Oxyparoniceras) buckmani BONARELLI, which was collected in Upper Toarcian deposits (Turmiel Fm.). These are organized in deepening upwards sequences, which were deposited during a transgressive episode that reached its maximum during the Insigne Zone. The studied specimen (recorded in the level 58 of this section) is registered in the Dispansum Zone (Insigne Subzone), in the same sedimentary sequence that Pseudogrammoceras fallaciosum (BAYLE), Pseudolillia emiliana (REYNES), Geczyceras goyi MARTÍNEZ and Osperlioceras sp., above the last record of Hammatoceras insigne (SCHLÜBLER in ZIETEN) (of level 57) and below Gruneria gruneri (DUMORTIER) of level 64. The genus Oxyparoniceras GUEX (= Jacobella JEANNET) probably derives from Paroniceras BONARELLI. It shows a wide; distribution in Western Europe and North Africa and has also been mentioned in North America (JAKOBS et al., 1994). On the other hand, as has already been published, the genus is recorded on the whole Mediterranean area and occasionally in Central and Western Europe (France, Germany and Southern England) from the upper part of Thouarsense Zone to Dispansum Zone (Insigne Subzone). This genus occurs at the Dispansum Zone in the Iberian and Vasco-Cantabrian ranges, but the species Oxyparoniceras (Oxyparoniceras) buckmani BONARELLI is limited to the Insigne Subzone, with an uncommon and sporadic record. The Department of Paleontology, Complutense University of Madrid (Spain) is the depository of the specimen examined (3AR-58/1) and coming from West of Ariño (province of Teruel, NE Spain). The specimen shows involute coiling, ogival section, convergent flanks and sharp keel. Narrow umbilical region with rounded border and almost vertical wall. A very subtle thin radial striation is observed. Suture is simple, but due to poor preservation it cannot be described in detail. The two valves of the cornaptychus are located inside body chamber. Each valve lies on its flank, near the ventral region. The apex is oriented towards the aperture of the shell and the ventral angle faces the internal portion of the body chamber. The exposed valve shows a very sharp ventral angle, a rounded apex and a semicircular outline on its umbilical projection. The symphysis is slightly sinuous and 11.57 mm long. The crest -the “adharmonic ridge” of Arkell-, with a maximum width of 1.58 mm measured in the ventral angle. A faint keel 8.71 mm long delimits two parts of the valve: the anterior one bulges out, while the posterior one is slightly depressed. The ornamentation, mainly comarginal, stands out better in the postero-lateral region of the valve and crosses the crest. This postero-lateralarea also displays a very fine radial striation. According to the latest interpretations, these pieces could have an opercular – mandibular double function or, in other way, an exclusively opercular function as an evolutionary result. The specimen studied here is found articulated, but not in anatomical position because the valves are located in the ventral region and very close to the septum of the body chamber, after a slight backwards displacement. We should stress the close correspondence between the size and shape of the body chamber of this shell of Oxyparoniceras (Oxyparoniceras) buckmani (BONARELLI) and the aptychus found inside it.
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