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The present work carries out a biostratigraphic analysis of the Valanginian ammonite record from the Cañón de la Vaca section in the Sierra de Parras, Coahuila, Mexico. The collection of ammonites exhibits an abundance of specimens, representing a biodiverse range of taxa, and numerous occurrences throughout the biostratigraphic section. This record in the Taraises Formation allowed a solid identification and diagnosis of the ' Thurmanniceras ' pertransiens , Neocomites neocomiensiformis and Karakaschiceras inostranzewi zones, along with the Neocomites premolicus , Neocomites neocomiensiformis , Busnardoites campylotoxus , Karakaschiceras inostranzewi and Neocomites platycostatus subzones. These biostratigraphic units corresponds to the lower Valanginian of the Standard Mediterranean Ammonite Zonation. Among the collected ammonites, numerous specimens of significant biostratigraphic importance remain scarcely known or lack a proper taxonomic identification. This situation has been already observed in other sections, such as Peña del Águila, Zacatecas, Mexico, and in the Betic Cordillera, southeast Spain. Fortunately, these unknown taxa can be compared with other sections in Mexico and with lower Valanginian ammonites from Spain, facilitating a more comprehensive biostratigraphic analysis. The present biostratigraphic analysis is expected to contribute to an increased understanding of lower Valanginian strata, thereby enabling future taxonomic and biostratigraphic research on the lower Valanginian in Mexico and globally. • The Cañón de la Vaca section includes an excellent lower Valanginian ammonite record. • This study identifies a complete lower Valanginian ammonite succession. • It includes the majority of the biostratigraphic units from the lower Valanginian. • This work includes the first report of ' Thurmanniceras ' pertransiens in Mexico. • Lower Valanginian ammonite assemblages of Mexico and Spain are very similar. [ABSTRACT FROM AUTHOR]

Abstract: It is now generally accepted that the Oceanic Anoxic Event 1a [OAE 1a] correlates with the lower part of the Leupoldina cabri planktonic foraminiferal Zone. Its calibration against the standard ammonite scale, however, seems to be more problematic. This is due, in part, to the fact that ammonites are scarce and/or of little diagnostic value from a biochronological viewpoint in the lower Aptian pelagic successions where the black shale horizons are better developed. We have been able to characterize OAE 1a geochemically in the relatively shallow water deposits of the eastern Iberian Chain (Maestrat Basin, eastern Spain), where ammonite faunas are rich. The interval corresponding to this event is dominated by the genera Roloboceras and Megatyloceras, accompanied by Deshayesites forbesi and Deshayesites gr. euglyphus/spathi. This assemblage is characteristic of the middle/upper part of the Deshayesites weissi Zone. The first occurrence of the species Deshayesites deshayesi (d''Orbigny), which marks the base of the overlying zone, takes place in our sections some metres above the OAE 1a interval. In the historical stratotype region of Cassis-La Bédoule (southern Provence Basin, southeastern France), the OAE 1a interval is also characterized by the presence of Roloboceras and Megatyloceras. Nevertheless, it has usually been correlated with the D. deshayesi Zone. In our opinion, this discrepancy is due to divergences in the taxonomic assignments of the deshayesitids present in these beds. In fact, the specimens attributed by French authors [Ropolo, P., Conte, G., Gonnet, R., Masse, J.P., Moullade, M., 2000. Les faunes d''Ammonites du Barrémien supérieur/Aptien inférieur (Bédoulien) dans la région stratotypique de Cassis-La Bédoule (SE France): état des connaissances et propositions pour une zonation par Ammonites du Bédoulien-type. Géologie Méditerranéenne 25, 167–175; Ropolo, P., Moullade, M., Gonnet, R., Conte, G., Tronchetti, G., 2006. The Deshayesitidae Stoyanov, 1949 (Ammonoidea) of the Aptian historical stratotype region at Cassis-La Bédoule (SE France), Carnets de Géologie / Notebooks on Geology Memoir 2006/01, 1–46.] to D. deshayesi and D. dechyi can be reinterpreted as belonging to D. forbesi. Following this reinterpretation, the Roloboceras beds (equivalent of OAE 1a) of Cassis-La Bédoule would also correspond to the D. weissi Zone. This age is additionally corroborated by data from southern England [Casey, R., 1961a. The stratigraphical palaeontology of the Lower Greensand. Palaeontology 3, 487–621; Casey, R., 1961b. A Monograph of the Ammonoidea of the Lower Greensand, part III. Palaeontographical Society, London, pp. 119–216], and by our recent observations in Le Teil (Ardèche Platform, southeastern France), where the Roloboceras faunas are also associated with Deshayesites consobrinus and Deshayesites gr. euglyphus, taxa that are characteristic of the D. weissi Zone. [Copyright &y& Elsevier]

Here we present aptychi and one perisphinctid ammonite from a 2.23 km-thick sedimentary section along the Aztlán Creek, Hidalgo state (East Central Mexico). The sedimentary sequence is predominantly build-up by basinal carbonates and is assigned to the Tepexic/Cahuasas (partim) formations, which are of Bathonian to Callovian in age and were deposited in the proto-Atlantic Basin at the western Tethys realm, Caribbean sub province. These strata yielded fourteen poorly preserved aptychi, of which two could be identified. One of them is the largest aptychi ever formally reported for the Jurassic and is identified as cf. Laevaptyhcus sp. This specimen reaches 182 mm in the harmonic margin. The finding represents the first report of Middle Jurassic aptychi from Mexico. The perisphinctid ammonoid is assigned to the genus Procerites sp. and is Bathonian in age. • Two Middle Jurassic aptychi specimens and one assemblaged perisphinctid ammonite were studied. • One of the aptychi is the largest ever formally reported for the Jurassic aptychi record worldwide. • These findings represent the first report of Middle Jurassic aptychi from Mexico. [ABSTRACT FROM AUTHOR]