Calcitarcha indet.

Calcitarchs, formal group name Calcitarcha, are hollow, essentially spherical, calcareous walled microfossils of unknown taxonomic affinity, ranging in size from around 60–500 microns, although some smaller forms are also known. The term was erected by G.J.M. Versteegh et al. in 2009 as a parallel to the organic walled microfossil group acritarchs which are similarly of unknown taxonomic affinity, and likewise most likely represent a collection of forms of variable origin united only by general morphological appearance plus the chemical composition of their walls. Most of the taxa included have also been referred to as calcispheres, however that term is more loosely defined and also contains some taxa that can be linked more definitively with "parent" taxonomic groups and have accordingly been moved elsewhere. Calcitarchs occur mainly in Palaeozoic and Mesozoic rocks, with a small number extending to the Cenozoic. As defined by Versteegh et al., the group includes the pithonellids (pithonelloids), which according to some workers are considered calcareous cysts of dinoflagellates, but which in Versteegh et al.'s opinion lack sufficient dinoflagellate-like characters to be definitively assigned there. Per the 2009 proposal of the group, and in parallel with the situation proposed for acritarchs, nomenclature for its members is governed by the botanical nomenclatural Code (ICNafp).