Shimanskya
Shimanskya is a late Carboniferous fossil tentatively interpreted as an early spirulid. This identification was based on: Doguzhaeva "et al." also identify these features in living Spirula, and the fossil 'Spirulida' Naefia, "Groenlandibelus" and "Adygeya"—though see these respective articles for discussion as to whether or not these extinct genera are themselves Spiruliids. Some authors are happy to accept this designation. But others have argued that none of the characters observed in "Shimanskya" is clearly diagnostic of the Spirulids. For example, a nacreous layer may have been lost more than once in cephalopod evolution. Others view the microstructural evidence as  ambiguous. Interpreting "Shimanskya" as a spirulid creates a large gap in the fossil record of the lineage. Moreover, some molecular clock results predict that spirulids evolved much later than the Carboniferous, leading some to suggest that "Shimanskya" ought to be assigned to the coleoid stem group. Other clock analyses, however, are consistent with its position in the spirulid lineage.