Schneidia zacatense sp. nov. – First population level study of Ataxioceratinae from the Lower Kimmeridgian in northern Mexico — Biostratigraphic and palaeobiogeographic significance

• First population level study of Lower Kimmeridgian ataxioceratin in the Americas.
• Ribbing patterns and multivariate analysis of Schneidia zacatense sp. nov.
• Incursion epibole, ecological epibole and local extinction events.

Lower Kimmeridgian ataxioceratin ammonites have been reported from Mexico for a long-time. This research shows the first analysis made on a population level study based on the material collected in a bed-by-bed sampling in the Cañón de Las Bocas section, Zacatecas, north–central Mexico. Aside from obtaining ribbing curves and coiling degree values, as well as the application of image analysis for the precise identification of preserved ribbing style details, principal component and cluster analyses has been applied to evaluate morpho-grouping of those individuals providing precise values of selected variables among preserved shell features of the 323 ammonite remains collected in situ. The coiling degree and the ribbing pattern are the most useful traits for phenotype characterization and interpretation at the genus and species levels. A more intricate combination of measured phenotype features is recognized at intra-species level. The new species level taxon Schneidia zacatense sp. nov. is erected and compared with European species of the same genus. The biochronostratigraphic interpretation points to a latest Platynota Chron age, which in turn could represent a local expression of a slightly wider time range –i.e., potential, local fluctuations of the stratigraphic range assumed in Mexico for the Schneidia Assemblage cannot be ruled out at the present state of knowledge. The palaeobiogeographic analysis is approached taking into account the eco-sedimentary context derived from a particular phase in the geodynamic evolution of the Mexico-Caribbean area during Late Oxfordian to Early Kimmeridgian times. Based on the composition of the macroinvertebrate fossil assemblage analysed and its stratigraphic range, the interpreted biogeographic dynamics points to an incursion epibole, followed by ecological epibole and local extinctions events. Two alternative hypotheses are evaluated for interpreting Panthalassic against west-Tethyan origins for the colonizers. A biogeographic source based on ammonite populations providing propagules and related to the Hispanic Corridor is favoured.

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