Abstract:
Chiasmolithus is abundant in Southern Ocean Ocean Drilling Program Sites 689, 738 and 748, and consequently provides an opportunity to study, in detail, evolutionary lineages and investigate the biostratigraphic distribution of some of the species. In particular, in the Middle to Late Eocene, a new species, Chiasmolithus eoaltus, has been recognised and distinguished from Chiasmolithus altus. Quantitative analysis highlights a gap between the last occurrence of C. eoaltus in the Upper Eocene and the first occurrence of C. altus in the Lower Oligocene. The morphological similarity is probably the result of convergent evolution. Additionally, the evolutionary steps that mark the phylogenesis of Chiasmolithus species during the Middle Eocene to Late Oligocene were investigated. Biostratigraphic and morphometric data permit reconstruction of a hypothetical phylogenetic tree, delineated by speciation events: we hypothesise that Chiasmolithus solitus gave rise to C. oamaruensis and C. eoaltus between 40.69 and 38.34 Ma (Reticulofenestra reticulata Nannofossil Zone), followed by the evolution of C. altus from C. oamaruensis between 33.97 and 32.49 Ma (Blackites spinosus Nannofossil Zone). A correlation between palaeoclimatic interpretations and the Chiasmolithus evolutionary tree underline that sea-surface temperature changes played a key role in the nannofossil evolutionary process, and suggest that phyletic gradualism, a recognised theoretical approach to evolutionary process, resulted from intraspecific variability subordinated to natural selection. This study represents, beyond the description of a new nannofossil species, confirmation of the neo-Darwinian concept of phyletic gradualism.