Variations morphologiques des Toxaster (Echinoida : Spatangoida) en regard des fluctuations spatiales (Arc de Castellane, SE France) et temporelles (Valanginien-Hauterivien) du milieu sédimentaire : expression d'un potentiel adaptatif restreint

Abstract

The impact of sedimentary conditions on the settlement, growth, and morphology of primitive spatangoid sea urchins of the genus Toxaster from the Lower Cretaceous (Valanginian and Hauterivian of the Provencal platform) is analyzed. The analyses concern spatial and temporal patterns of variation. The spatial approach is based on six isochronous populations of Toxaster granosus kiliani (424 specimens in total) dated from the Verrucosum biozone, and distributed along a proximo-distal gradient. The temporal approach concerns a reference section (Carajuan) ranging from the Lower Valanginian to the Upper Hauterivian, and involving the species T. granosus kiliani and T. gibbus. Sedimentary environments have been characterized by a factorial analysis of five characteristics of facies. Sea urchin morphology has been described with seven measures allowing the computation of two angles (α and β), and of six shape indices analyzed through a principal components analyses. The spatial approach shows that the adult mean size (> 20 mm) increases distally, and that morphology changes significantly across localities. However, only variations in α and β are sequentially organized along the proximo-distal gradient (they correspond to a forward migration of the peristome and to an expansion of the plastronal area). The stratigraphic survey of facies variation along the 369 beds analyzed in Carajuan shows a clear change in sedimentary conditions: from unstable with bioclastic events (Valanginian), to homogeneous with a low content in bioclasts (Hauterivian). Most Toxaster specimens are concentrated in three stratigraphic units (Campylotoxus and Verrucosum biozones for T. granosus kiliani; Sayni biozone for T. gibbus), their abundance being related to the macrofaunal presence and diversity and to burrowing activity. The adult mean size is only significantly related to facies variation, for the T. granosus kiliani from the Verrucosum biozone, increasing with textural gradient and bioclastic content. Shape – facies relationships have been identified for T. granosus kiliani in both Campylotoxus and Verrucosum intervals. However, the morphological variables and facies characteristics involved differ between the two biozones. The synthesis of all significant relationships between size or shape and facies characteristics demonstrates that the response of Toxaster to environmental changes was limited, and varied in space as well as for the different periods of time considered. The only recurring impact of sedimentary changes involved sea urchin abundance, that is, their ability to settle. The low adaptive potential of Toxaster is discussed in the context of the early radiation of the Spatangoida.