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A total of 113 species belonging to 54 genera are recovered from a study of the Maastrichtian and Danian deposits in the Cerro Azul section of the Neuquén Basin, west‐central Argentina. Of these, 9 new genera, 38 new species and 28 species in open nomenclature are recorded. These form the basis of the present study and are listed below. Genera: Phelocyprideis gen. nov., Aleisocythereis gen. nov., Apatoleberis gen. nov., Mimicocythereis gen. nov., Castillocythereis gen. nov., Hysterocythereis gen. nov., Orthrocosta gen. nov., Petalocythereis gen. nov. and Sthenarocythereis gen. nov. Species: Cytherella saraballentae sp. nov., Cytherella semicatillus sp. nov., Paracypris bertelsae sp. nov., Paracypris imaguncula sp. nov., Argilloecia abnormalis sp. nov., Argilloecia concludus sp. nov., Argilloecia hydrodynamicus sp. nov., Bythoceratina cheleutos sp. nov., Phelocyprideis acardomesido sp. nov., Eucythere dinetos sp. nov., Krithe crepidus sp. nov., Cytheropteron hyperdictyon sp. nov., Cytheropteron bidentinos sp. nov., Cytheropteron translimitares sp. nov., Aversovalva glochinos sp. nov., Eucytherura stibaros sp. nov., Hemingwayella verrucosus sp. nov., Heinia prostratopleuricos sp. nov., Loxoconcha (s.l.) posterocosta sp. nov., Keijia circulodictyon sp. nov., Keijia kratistos sp. nov., Paramunseyella epaphroditus sp. nov., Munseyella costaevermiculatus sp. nov., Ameghinocythere archaios sp. nov., Aleisocythereis polikothonus sp. nov., Castillocythereis multicastrum sp. nov., Castillocythereis albertoriccardii sp. nov., Cythereis stratios sp. nov., Cythereis clibanarius sp. nov., Cythereis trajectiones sp. nov., Henryhowella (Wichmannella) praealtus sp. nov., Hysterocythereis paredros sp. nov., Hysterocythereis coinotes sp. nov., Hysterocythereis diversotuberculatus sp. nov., Orthrocosta decores sp. nov., Orthrocosta atopos sp. nov., Orthrocosta phantasia sp. nov. and Sthenarocythereis erymnos sp. nov. The nature of the fauna and aspects of its evolution are discussed.

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Two tuffs in the Lower Cretaceous Agrio Formation, Neuquén Basin, provided U-Pb zircon radioisotopic ages of 129.09 ± 0.16 Ma and 127.42 ± 0.15 Ma. Both horizons are well constrained biostratigraphically by ammonites and nannofossils and can be correlated with the 'standard' sequence of the Mediterranean Province. The lower horizon is very close to the base of the Upper Hauterivian and the upper horizon to the Hauterivian/Barremian boundary, indicating that the former lies at c. 129.5 Ma and the latter at c. 127 Ma. These new radioisotopic ages fill a gap of over 8 million years in the numerical calibration of the current global Early Cretaceous geological time scale.

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Abstract. The numerical age of the Jurassic–Cretaceous boundary has been controversial and difficult to determine. In this study, we present high-precision U–Pb geochronological data around the Jurassic–Cretaceous boundary in two distinct sections from different sedimentary basins: the Las Loicas, Neuquén Basin, Argentina, and the Mazatepec, Sierra Madre Oriental, Mexico. These two sections contain primary and secondary fossiliferous markers for the boundary as well as interbedded volcanic ash horizons, allowing researchers to obtain new radioisotopic dates in the late Tithonian and early Berriasian. We also present the first age determinations in the early Tithonian and tentatively propose a minimum duration for the stage as a cross-check for our ages in the early Berriasian. Given our radioisotopic ages in the early Tithonian to early Berriasian, we discuss implications for the numerical age of the boundary.