Reeent data from Chile, Cuba, southern USA, and Iberia show that the Oxfordian Plieatilis p. p.,Transversarium p. p., and Bimammatum Zones are eorrelated reliably in these areas. Signifieant for eorrelations are Dichotomosphinc tes, close forms to Gemmellarites, Gregoryceras, and the assoeiation of Euaspido ceras with Mirosphinctes andJor Praeataxioceras. Above the lower Bimammatum Zone, Cubaspidoceras, Metahaploceras and probably Geyssantia are of speeial interest. The general absenee of ammonites belonging to the Bifureatus Zone in Chile, Mexieo and southern USA, as well as their endemism in Cuba, eannot yet be eonclusively interpreted. Zusammenfassung: Neue Daten aus Chile, Cuba, den südliehen USA und Iberia gestatten die genaue Korrelation von Plieatilis p. p.,-Transversarium p. p.-und Bimammatum-Ammonitenzonen in diesen Gebieten. Wiehtig fUr die Korrelation sind Dichotomosphinctes-Formen (die Gemmellarites nahestehen), Gregoryceras und die Vergesellsehaftung von Euaspidoceras mit Mirosphinctes und/oder Praea taxioceras. In Ammonitengesellsehaften, die jünger als das frühe Bimammatum Chron sine!, haben wahrseheinlieh Cubaspidoceras Metahaploceras und Geyssantia besondere Bedeutung. Das generelle Fehlen von Ammoniten im Bifureatus-Chron in Chile, Mexiko und den südliehen USA und deren endemisehes Vorkommen in Cuba, laf3t sieh noeh nieht entseheidend erklaren.
Hybonoticeras mundulum (Oppel) (m) s.s. is reported from the Mexican Altiplano on the basis of material collected bed‐by‐bed in sections in the States of Durango and Zacatecas. The known range of this species in the Mexican Altiplano is interpreted to be uppermost Kimmeridgian (upper to uppermost Beckeri Zone) to lowermost Tithonian (basal to lower Hybonotum Zone). The Mexican specimens studied are the most complete and valuable collection of H. mundulum (Oppel) s.s. known from a given area, and reveal that Mexican populations show phenotypic features different from European ones. Vicariant events accord with data available about the areal and biostratigraphic distribution of this species, as well as with the combination of allocyclic and autocyclic factors influencing Mexican seas, the breaking of populations rather than colonization events, and the impoverished ammonite assemblages showing endemic traits within the stratigraphic interval studied.
Lower Kimmeridgian to Lower Tithonian (Upper Jurassic) sections studied at Sierra de Palates (Durango) and Sierra de Catorce (San Luis Potosi), Mexico, show low-energy deposits in which the composition of fossil macroinvertebrate assemblages, including megabenthos, reflects biostratinomic control. Monotonous siltstones provide continuous records of ammonite assemblages and reflect dominant deposition of shells in living areas; meanwhile, discontinuos records were forced by episodic post-mortem transportation of shells, which was especially accentuated under storm influence. Rhythmic marly-silty limestones and mark illustrate a fossil record probably determined by minor transgressive-regressive pulses. The major changes in lithofacies are reflected by condensed silty and phosphatic mudstones deposited during significant floodings affecting areas under dominant terrigenous sedimentation. These changes determined more or less significant variations in the composition of fossil assemblages according to their relation to changing ecological conditions. However, shifting ecospaces exhibit no direct relationship to changes in lithofacies. Post-mortem transportation, operating in relation to both marine floodings and changes in the pattern of upper-water currents, was the main biostratinomic factor affecting the areal distribution of ammonite populations. Shell transportation and sedimentation rate controlled preservation and ultimately influenced diversity in recorded ammonite assemblages. The post-mortem behaviour (interpreted from shell structure and preservation), and therefore distribution, of ammonite shells points to shallow-water environments during the Kimmeridgian -Early Tithonian in areas (such as SE Durango and San Luis Potosi) close to the changing boundary between dominant carbonate and terrigenous sedimentation. No reworking affecting ammonite biostratigraphy has been identified in the sections studied. OTaphonorny, ammonites, Upper Jurassic, Mexico.Federico Oloriz Ifoloriz@goliat. ugr.es],
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