Shimanskya Explained

Shimanskya is a late Carboniferous fossil tentatively interpreted as an early spirulid.^[1]

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.^{[2] [3]}

But others have argued that none of the characters observed in Shimanskya is clearly diagnostic of the Spirulids.^[4]

For example, a nacreous layer may have been lost more than once in cephalopod evolution.^[5]

Others view the microstructural evidence as ambiguous.^[6]

Interpreting Shimanskya as a spirulid creates a large gap in the fossil record of the lineage.^[7] 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.^{[8] [9]} Other clock analyses, however, are consistent with its position in the spirulid lineage.^[10]

Notes and References

1. Cooper . A. J. . A. . Cladistics . 89–96 . 22 . Drummond . J. . 2006 . Divergence time estimates for major cephalopod groups: evidence from multiple genes . Strugnell . J. . Jackson . 10.1111/j.1096-0031.2006.00086.x.
2. Kröger, B. (2003). The size of the siphuncle in cephalopod evolution. Senckenbergiana Lethaea, 83, 39–52.
3. 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
4. Reflections on the phylogenetic position of Spirula (Cephalopoda): preliminary evidence from the 18S ribosomal RNA gene . Warnke, K . Plötner, J . Santana, JI . Rueda, MJ . Llinas, O . Berliner Paläobiologische Abhandlungen . 3 . 253–260 . 2003 .
5. 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
6. Hewitt, R. A., & Jagt, J. W. M. (1999). Maastrichtian Ceratisepia and Mesozoic cuttlebone homeomorphs. Acta Palaeontologica Polonica, 44(3), 305–326.
7. Hewitt, R. A., & Jagt, J. W. M. (1999). Maastrichtian Ceratisepia and Mesozoic cuttlebone homeomorphs. Acta Palaeontologica Polonica, 44(3), 305–326.
8. 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.2010.11.024
9. Kröger, 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
10. 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