Watsonella

Watsonella is a genus of 'mollusc' known from early (Terreneuvian) Cambrian strata.

Watsonella
Temporal range: Cambrian Stage 2, c. 530–525 Ma
↓
Scientific classification
Kingdom: Animalia
Phylum: Mollusca (?)
Genus: †Watsonella
Grabau, 1900
Species
  • W. crosbyi
  • ... and others?

The genus is closely related to Anabarella, with which it bears many morphological similarities,[1] including a laminar internal shell microstructure said to connect it with the early bivalves Fordilla and Pojetaia.[2]

Biostratigraphic significance

Watsonella has been proposed as an index fossil of the Cambrian, defining a W. crosbyi zone. Notwithstanding the weakness of a first appearance datum as a definition for the base of a period,[3] the species has been proposed as a marker for the base of the presently unratified second stage of the Terreneuvian (i.e. Cambrian Stage 2). However, the species has now been found late in the FOrtunian, drawing back its first occurrence.[4] But that said its occurrence in Australia seems to begin rather near the base of Stage 2.[5]

Affinity

Widely thought to be a mollusc, the connection between the two 'valves' of Watsonella recalls the configuration of Rostroconchs (a group thought to be paraphyletic to bivalves and scaphopods). Similarities to the shells of certain bivalves seem to place Watsonella closer to the bivalve crown group than typical rostroconchs, though there is not yet enough evidence for an unequivocal classification.[6]

Evidence for a molluscan affinity: (though for a sceptical view see [7])

  • Marginal accretion [8]
  • No rostrum or pegma, so not a rostroconchs after all[9]
  • ?Aragonitic shell, perhaps, but not preserved. Two microstructural layers: lamello-fibrillar within spherulitic prismatic[10]—see [11]
  • Untorted, endogastric [12]
  • "Stem bivalve" – based on microstructure,[13] timing, and the fact that it looks like it has two valves[14]
  • Cf. Anabarella, which also has a similar microstructure [15]

References
  1. Kouchinsky, A. V. (1999). "Shell microstructures of the Early Cambrian Anabarella and Watsonella as new evidence on the origin of the Rostroconchia". Lethaia. 32 (2): 173–180. doi:10.1111/j.1502-3931.1999.tb00537.x.
  2. Vendrasco, M.J.; Checa, A.G.; Kouchinsky, A.V. (2011). "Shell microstructure of the early bivalve Pojetaia and the independent origin of nacre within the Mollusca". Palaeontology. 54 (4): 825. doi:10.1111/j.1475-4983.2011.01056.x.
  3. 1. Landing, E., Geyer, G., Brasier, M.D., and Bowring, S.A. (2013). Cambrian evolutionary radiation: context, correlation, and chronostratigraphy—overcoming deficiencies of the first appearance datum (FAD) concept. Earth-Science Rev. 123, 133–172.
  4. Landing, E., and Kouchinsky, A. V. (2016). Correlation of the Cambrian Evolutionary Radiation: geochronology, evolutionary stasis of earliest Cambrian (Terreneuvian) small shelly fossil (SSF) taxa, and chronostratigraphic significance. Geol. Mag. 153, 750–756.
  5. Jacquet SM, Brougham T, Skovsted CB, et al. ~Watsonella crosbyi~ from the lower Cambrian (Terreneuvian, Stage 2) Normanville Group in South Australia. Geol Mag. October 2016. doi:10.1017/S0016756816000704.
  6. 1. Jacquet SM, Brougham T, Skovsted CB, et al. ~Watsonella crosbyi from the lower Cambrian (Terreneuvian, Stage 2) Normanville Group in South Australia. Geol Mag. October 2016. doi:10.1017/S0016756816000704.
  7. Budd, G. E. & Jensen, S. A critical reappraisal of the fossil record of the bilaterian phyla. Biol. Rev. 75, 253–295 (2000).
  8. Li, G.-X., Zhao, X., Gubanov, A., Zhu, M.-Y. & Na, L. Early Cambrian mollusc ~Watsonella crosbyi~: a potential GSSP index fossil for the base of the Cambrian Stage 2. Acta Geol. Sin. 85, 309–319 (2011).
  9. Li, G.-X., Zhao, X., Gubanov, A., Zhu, M.-Y. & Na, L. Early Cambrian mollusc ~Watsonella crosbyi~: a potential GSSP index fossil for the base of the Cambrian Stage 2. Acta Geol. Sin. 85, 309–319 (2011).
  10. Li, G.-X., Zhao, X., Gubanov, A., Zhu, M.-Y. & Na, L. Early Cambrian mollusc ~Watsonella crosbyi~: a potential GSSP index fossil for the base of the Cambrian Stage 2. Acta Geol. Sin. 85, 309–319 (2011).
  11. Vendrasco, M. J., Checa, A. G. & Kouchinsky, A. V. Shell microstructure of the early bivalve Pojetaia and the independent origin of nacre within the mollusca. Palaeontology 54, 825–850 (2011).
  12. Li, G.-X., Zhao, X., Gubanov, A., Zhu, M.-Y. & Na, L. Early Cambrian mollusc ~Watsonella crosbyi~: a potential GSSP index fossil for the base of the Cambrian Stage 2. Acta Geol. Sin. 85, 309–319 (2011).
  13. Vendrasco, M. J., Checa, A. G. & Kouchinsky, A. V. Shell microstructure of the early bivalve Pojetaia and the independent origin of nacre within the mollusca. Palaeontology 54, 825–850 (2011).
  14. Jacquet, S. M. et al. ~Watsonella crosbyi~ from the lower Cambrian (Terreneuvian, Stage 2) Normanville Group in South Australia. Geol. Mag. (2016). doi:10.1017/S0016756816000704
  15. Vendrasco, M. J., Checa, A. G. & Kouchinsky, A. V. Shell microstructure of the early bivalve Pojetaia and the independent origin of nacre within the mollusca. Palaeontology 54, 825–850 (2011).
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