Affinity, morphology, and diagenesis of Mesozoic conical nannofossils from the family Eoconusphaeraceae

Abstract Mesozoic calcareous nannofossils of the family Eoconusphaeraceae have traditionally been interpreted as nannoliths of uncertain biological affinity. However, their morphology, crystallographic organization, and occurrence in articulated coccospheres strongly suggest that they represent coccoliths produced by haptophyte algae. This study re-examines the genera Eoconusphaera Jafar, 1983 and Conusphaera Trejo, 1969 using light and scanning electron microscopy from an Upper Triassic section of the Northern Calcareous Alps and published Jurassic material. Particular attention is given to coccolith ultrastructure, coccosphere organization, and the effects of diagenetic alteration on the morphology of the genus Eoconusphaera. Newly documented articulated coccospheres of Eoconusphaera zlambachensis (Moshkovitz, 1982) Jafar, 1983 demonstrate that this species formed monomorphic coccospheres composed of conical coccoliths arranged around a central cell cavity. Morphological observations further indicate a protolith-type rim structure and biologically controlled crystallographic organization comparable to those observed in other early coccolithophores. Variations in preservation reveal five successive stages of dissolution-related alteration that progressively obscure primary ultrastructural features. The results highlight that the Eoconusphaeraceae represent true coccoliths rather than nannoliths and support their interpretation as early calcihaptophytes. Similar morphologies observed in Triassic and Jurassic conical coccoliths might have resulted from repeated morphological evolution, although the possibility of phylogenetic relationships between some taxa remains open.