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International audienceField sampling data in the Vocontian Basin (southeastern France) and Mallorca (Spain), at the transition between the lower and upper Barremian (upper Moutoniceras moutonianum Zone and lower Toxancyloceras vandenheckei Zone), provides new information about the early representatives of the ammonite family Ancyloceratidae. Several successive species of the genera Moutoniceras and Toxancyloceras are identified. A review of the history of the acquisition of the Astier collection by the Natural History Museum of London, which contains the holotype of the type-species T. vandenheckei, clarifies some misinterpretations which this species previously suffered. T. vandenheckei and M. eigenheeri are revised, and a new species is described: T. canuti sp. nov. With respect to an evolutionary perspective under biostratigraphic control, their phylogeny is considered (M. eigenheeri -> T. canuti sp. nov. -> T. vandenheckei). The hypothesis of the origin of the genus Toxancyloceras within the Moutoniceras is strengthened; this link is consistent both stratigraphically and morphologically. Moutoniceras appears to be the oldest known representative of the Ancyloceratidae, which is rooted in the early Barremian. The ontogenetic and evolutionary patterns of the phyletic lineage Moutoniceras/Toxancyloceras are twofold: the first concerns the ornamental changes (itself determined by three imbricated patterns) and the second involves the adult size. Both patterns determine two evolutionary phases through time: (1) the giant Moutoniceras and the progressive disappearance of the tubercles through heterochrony (paedomorphosis), and (2) the drastic size reduction and the reappearance of the tubercles from the “small” Moutoniceras to the Toxancyloceras (through heterochrony, with peramorphosis and a combination of pseudo-dwarfism, acceleration and graduaptation). The oscillation in disappearance and reappearance of the tubercles demonstrates a possible case of evolutionary reversibility where heterochrony helped by the progenesis impact, favours character repeatability in the evolutionary patterns. The results for the genera Moutoniceras and Toxancyloceras have significant biostratigraphic implications for the Tethyan Barremian. The status of T. vandenheckei as a zonal and subzonal index species (basal upper Barremian) is reinforced, and three new ammonite horizons are defined: the Moutoniceras eigenheeri, Toxancyloceras canuti and T. vandenheckei horizons. The stratigraphic distribution of all their index species is very restricted, indicating a well established evolutionary context

Abstract:The Late Barremian Hemihoplitidae (Ancyloceratina, Ammonoidea) are widely known in the northern Tethyan Margin and the Essaouira-Agadir Basin (Morocco). Their rapid evolution and diversification make them one of the key groups for that period, but their origin remains poorly known and several competing hypotheses have been published. These hypotheses are tested here with cladistic analysis in order to reject those receiving the least support and discuss those well supported. The analysis discards the Crioceratitidae, Emericiceratidae (Emericiceras and Honnoratia) and Toxancyloceras as stem-groups of the Hemihoplitidae (Gassendiceras). The Toxancyloceras appear instead to be a sister-taxon of the Moutoniceras, so we propose the latter to be classified with the Ancyloceratidae rather than with the Heteroceratidae. The best supported hypothesis assumes that the Hemihoplitidae first appeared suddenly in the Essaouira-Agadir Basin at the end of the Early Barremian from small populations of Boreal Paracrioceras. These latter could have migrated southward episodically before invading the northern Tethyan margin at the beginning of the Late Barremian. As a consequence, the Paracrioceratidae fam. nov. is proposed to include the Boreal groups Fissicostaticeras / Paracrioceras / Parancyloceras, and Gassendiceras essaouirae sp. nov. is proposed as a new name for the Moroccan endemic "Barrancyloceras" maghrebiense sensu COMPANY et al., 2008, non IMMEL, 1978 Key Words: Ammonites; Hemihoplitidae; Barremian; cladistics; allopatry; founder effect; palaeobiogeography. Résumé : L'origine des Hemihoplitidae téthysiens testée par la cladistique (Ancyloceratina, Ammonoidea, Crétacé inférieur) : un événement migratoire ?.-Les Hemihoplitidae du Barré-mien supérieur (Ancyloceratina, Ammonoidea) sont connus sur la plus grande partie de la marge Nord Téthysienne et dans le Bassin de Essaouira-Agadir (Maroc). Leur évolution et leur diversification rapide en font un des groupes clés pour cette période, mais leur origine reste peu connue et plusieurs hypothèses contradictoires ont été développées dans la littérature. Ces hypothèses sont testées ici par une analyse cladistique afin d'écarter les moins étayées d'entre elles et de discuter les meilleures. L'analyse écarte les Crioceratitidae, les Emericiceratidae (Emericiceras et Honnoratia) et les Toxancyloceras en tant que groupe-souche pour les (Gassendiceras). D'autre part, les Toxancyloceras apparaissent être taxon-frêre des Moutoniceras, aussi est-il proposé de classer ces derniers parmi les Ancyloceratidae plutôt que dans les Heteroceratidae. L'hypothèse la mieux soutenue suggère que les Hemihoplitidae seraient initialement apparus soudainement à la fin du Barrémien inférieur dans le Bassin d'EssaouiraAgadir à partir de petites populations de Paracrioceras boréaux. Ces derniers auraient pu migrer épisodiquement vers le Sud avant d'envahir la marge Nord-Téthysienne au début du Barrémien supérieur. En conclusion, la Famille des Paracrioceratidae fam. nov. est proposée pour individ...