Exceptionally well-preserved Cretaceousmicrofossils reveal new biomineralization styles

摘要

Calcareous microplankton shells form the dominant components of ancient and modernpelagic sea-floor carbonates and are widely used in palaeoenvironmental reconstructions.The efficacy of these applications, however, is dependent upon minimal geochemicalalteration during diagenesis, but these modifying processes are poorly understood. Here wereport on new biomineralization architectures of previously unsuspected complexity incalcareous cell-wall coverings of extinct dinoflagellates (pithonellids) from a Tanzanianmicrofossil-lagersta¨tte. These Cretaceous ‘calcispheres’ have previously been consideredbiomineralogically unremarkable but our new observations show that the true nature of thesetests has been masked by recrystallization. The pristine Tanzanian fossils are formed fromfibre-like crystallites and show archeopyles and exquisitely constructed opercula, demonstratingthe dinoflagellate affinity of pithonellids, which has long been uncertain. The interwovenfibre-like structures provide strength and flexibility enhancing the protective functionof these tests. The low-density wall fabrics may represent specific adaptation for oceanicencystment life cycles, preventing the cells from rapid sinking.