Shimanskya

{{Short description|Genus of molluscs}}

{{Speciesbox

| fossil_range = {{geological range| Gzhelian }}

| grandparent_authority = Doguzhaeva, Mapes & Mutvei, 1999

| genus = Shimanskya

| parent_authority = Doguzhaeva, Mapes & Mutvei, 1999

| species = postremus

| authority = (Miller, 1930)

| synonyms =

• Bactrites postremus
  Miller, 1930

}}

Shimanskya is a late Carboniferous fossil tentatively interpreted as an early spirulid.{{Cite journal | last4 = Cooper | first3 = A. J. | first4 = A. | journal = Cladistics | pages = 89–96 | volume = 22 | last3 = Drummond | first2 = J. | year = 2006 | title = Divergence time estimates for major cephalopod groups: evidence from multiple genes | last1 = Strugnell | first1 = J. | last2 = Jackson | doi = 10.1111/j.1096-0031.2006.00086.x}}

This identification was based on:

{{blockquote|the well-developed phragmocone [which] possesses comparatively long camerae and [a] comparatively wide marginal siphuncle, the [absence of the] rostrum (at adult stages at least), and the [construction of the] shell wall, which is as thin as septa, has no nacreous layer and is subdivided into the inner and outer plates|source=Doguzhaeva et al. 1999Doguzhaeva, L. A., Mapes, R. H., & Mutvei, H. (1999). A Late Carboniferous spirulid coleoid from the southern mid-continent (USA): shell wall ultrastructure and evolutionary implications. In F. Olóriz & F. J. Rodríguez-Tovar (Eds.), Advancing Research on Living and Fossil Cephalopods (pp. 47–57). New York: Kluwer Academic Publishers.}}

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.Kröger, B. (2003). The size of the siphuncle in cephalopod evolution. Senckenbergiana Lethaea, 83, 39–52.Warnke, K., & Keupp, H. (2005). ~Spirula~—A window to the embryonic development of ammonoids? Morphological and molecular indications for a palaeontological hypothesis. Facies, 51(1–4), 60–65. doi:10.1007/s10347-005-0054-9

But others have argued that none of the characters observed in Shimanskya is clearly diagnostic of the Spirulids.{{Cite journal

| title = Reflections on the phylogenetic position of Spirula (Cephalopoda): preliminary evidence from the 18S ribosomal RNA gene

| author1 = Warnke, K

| author2 = Plötner, J

| author3 = Santana, JI

| author4 = Rueda, MJ

| author5 = Llinas, O

| journal = Berliner Paläobiologische Abhandlungen

| volume = 3

| pages = 253–260

| year = 2003

| url = http://www.geo.fu-berlin.de/geol/fachrichtungen/pal/eigenproduktion/Band_03/22.pdf

}}

For example, a nacreous layer may have been lost more than once in cephalopod evolution.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

Others view the microstructural evidence as ambiguous.Hewitt, R. A., & Jagt, J. W. M. (1999). Maastrichtian Ceratisepia and Mesozoic cuttlebone homeomorphs. Acta Palaeontologica Polonica, 44(3), 305–326.

Interpreting Shimanskya as a spirulid creates a large gap in the fossil record of the lineage.Hewitt, R. A., & Jagt, J. W. M. (1999). Maastrichtian Ceratisepia and Mesozoic cuttlebone homeomorphs. Acta Palaeontologica Polonica, 44(3), 305–326. 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.Warnke, K. M., Meyer, A., Ebner, B., & Lieb, B. (2011). Assessing divergence time of Spirulida and Sepiida (Cephalopoda) based on hemocyanin sequences. Molecular Phylogenetics and Evolution, 58(2), 390–394. doi:10.1016/j.ympev.2010.11.024Kröger, B., Vinther, J., & Fuchs, D. (2011). Cephalopod origin and evolution: a congruent picture emerging from fossils, development and molecules. BioEssays, 33(8), 602–613. doi:10.1002/bies.201100001 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