Deciphering <i>Spirotubus</i> : new observations and palaeobiological interpretations of an enigmatic Devonian microfossil from Poland

Enigmatic calcareous tubiform microfossils from the Middle to Upper Devonian, featuring tapered ends and regular spiral-rhomboid ornamentation, have been described as the ‘genus’ Spirotubus. These microfossils from southern Poland form gently curved tubes with no clear distinction between proximal and distal ends and ‘monocrystalline’ wall structure. Of particular interest is the presence of a morphological gradation of their surfaces, from a spiral pattern, characterized by furrowed rhomboid zonal plates, to almost smooth surfaces with incipient ornamentation. The Spirotubus-producing organisms thrived in restricted marine lagoons, where biota were prone to hypercalcification, influenced probably by seasonally fluctuating CaCO3 saturation level. Two general alternative hypotheses (taphonomy versus biocalcification) are proposed for the origin of the tubes. The living organic-walled foraminifer Tinogullmia hyalina, with a strikingly similar test form, as well as the agglutinated foraminifera similar to Phainogullmia, provide intriguing modern ‘anchor points’ for a taphonomic model of two-stage post-mortem calcification. Alternatively, based on extant ostracods, it is speculated that Spirotubus may represent a highly calcified spirally coiled sperm or rather similarly shaped soft-bodied organism. The formation of the most advanced tube morphotype remains a puzzle in these taphonomic hypotheses and suggests an unusual mode of hypercalcification. On the other hand, the structural and highly oriented organization of the Spirotubus wall tube is typical of many calcite biominerals, which require a complex crystallization process under the control of a living organism. The inferred model suggests calcification of variable intensity in vivo, but has no direct relationship to any living or extinct helicoid organism. The study highlights still mysterious ‘microcosmic’ world and specific fossilization processes in the non-actualistic Devonian calcium carbonate supersaturated (‘hyper-calcitic’) seas.