''Shimanskya'' is a late

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...

fossil tentatively interpreted as an early

spirulid Spirulida is an order of cephalopods comprising one extant species (''Spirula spirula'') and several extinct taxa. Fossil record * Oldest representative: Carboniferous, though contested: see ''Shimanskya'' * Oldest uncontested representat ...

. This identification was based on: Doguzhaeva ''et al.'' also identify these features in living Spirula, and the fossil 'Spirulida'

Naefia Groenlandibelidae is a family of coleoid cephalopods believed to belong to the spirulids. Morphologically, its taxa seem to have some belemnoid characteristics, suggesting a possible intermediate relationship. Genera ''Groenlandibelus '' J ...

, ''

Groenlandibelus Groenlandibelidae is a family of coleoid cephalopods believed to belong to the spirulids. Morphologically, its taxa seem to have some belemnoid characteristics, suggesting a possible intermediate relationship. Genera ''Groenlandibelus '' Je ...

'' and ''

Adygeya ''Adygeya'' is a genus of cephalopods assigned to the Spirulida Spirulida is an order of cephalopods comprising one extant species ('' Spirula spirula'') and several extinct taxa. Fossil record * Oldest representative: Carboniferous, thou ...

''—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 nucleoti ...

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