''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
The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleo ...
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
In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor ...
. Other clock analyses, however, are consistent with its position in the spirulid lineage.
[Strugnell, J., Jackson, J., Drummond, A. J., & Cooper, A. (2006). Divergence time estimates for major cephalopod groups: evidence from multiple genes. Cladistics, 22(1), 89–96. doi:10.1111/j.1096-0031.2006.00086.x]
References
{{Taxonbar, from=Q7496834
Coleoidea
Cephalopod genera