Watsonella
Temporal range: Cambrian Stage 2, c.
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Mollusca (?)
Genus: †Watsonella
Grabau, 1900
Type species
Watsonella crosbyi
Grabau, 1900
Accepted species
  • Watsonella crosbyi Grabau, 1900
Synonyms
  • Heraultia Cobbold, 1935
  • Heraultipegma Pojecta and Runnegar, 1976

Watsonella is an extinct genus of mollusc known from early (Terreneuvian) Cambrian strata. It has been hypothesized to be close to the origin of bivalves.

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

Taxonomy

Watsonella was described by Amadeus William Grabau in 1900, with the type species Watsonella crosbyi. The genus name honors Thomas A. Watson of Weymouth, Massachusetts, whose fossil collecting activities supplied many of the Cambrian fossils studied by Grabau, and the species name honors Professor W. O. Crosby. Grabau initially interpreted Watsonella as a heteropod gastropod similar to the modern genus Carinaria.[1] In 1935, E. S. Cobbold named the genus Heraultia based on specimens from France, with the type species Heraultia varensalensis. He tentatively interpreted it as a notostracan crustacean. He noted "striking" similarity between it and Watsonella, but concluded that it was "impossible to bring them together" without more information on the affinities of Watsonella.[2] Because the name Heraultia had already been used for a genus of fly, in 1976 John Pojeta and Bruce Runnegar proposed the name Heraultipegma as a replacement. Pojeta and Runnegar reinterpreted Heraultipegma and Watsonella as the earliest known rostroconchs.[3] In 1988, Martin Kerber hesitantly synonymized Heraultipegma and Watsonella, and Ed Landing affirmed their synonymy in 1989.[4]

Description

Watsonella was a small, laterally-compressed animal, generally less than 5 millimetres (0.20 in) long.[4] It had a single "pseudobivalved" shell.[4] It is not known with certainty which end of the shell is the front.[5]

The external surface of the shell was ornamented with comarginal growth lines and ribs.[5]

Unlike rostroconchs, Watsonella lacked a pegma, although the curvature of the shell may have served a functionally similar role.[6] Although Pojeta and Runnegar[3] and Landing[4] interpreted a pegma as present, several studies have rejected this interpretation.[6] Li and colleagues interpreted the putative pegma as an artifact of the curvature of the shell.[7]

The shell of Watsonella was probably originally aragonitic in composition.[4] It was composed of two layers with distinct microstructure. The outer layer was composed of regular, tightly-packed prismatic units that were oriented perpendicular to the shell wall. The inner layer was composed of flattened units called lamellae that produced a stepwise texture.[8] Unlike many modern mollusks, the shell was not nacreous.[9]

Paleobiology

Watsonella was probably a burrowing animal. Because of the limited flexibility of its hingeless shell, its burrowing ability would have been less efficient than modern burrowing clams.[4]

Evolutionary relationships

Anabarella was probably the ancestor of Watsonella.[9] Morphological intermediates linking Anabarella and Watsonella are known.[10] The shell microstructure is very similar between Anabarella, Watsonella, and the early bivalves Fordilla and Pojetaia.[9]

A phylogenetic analysis conducted in 2000 by a team of researchers led by J. G. Carter recovered Watsonella as more closely related to bivalves than rostroconchs.[11]

In 2023, a team of researchers led by Hao Song argued that Watsonella was too early to be a stem-group bivalve, based on molecular clock results that suggested that bivalves and scaphopods diverged from each other approximately 520 million years ago. They suggested that Watsonella should be reinterpreted as stem-group members of Diasoma, the clade uniting bivalves and scaphopods.[12]

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,[13] 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.[14] But that said its occurrence in Australia seems to begin rather near the base of Stage 2.[15]

References

  1. ↑ Grabau, Amadeus William (1900). "Palaeontology of the Cambrian Terranes of the Boston Basin". Occasional Papers of the Boston Society of Natural History. 4 (3).
  2. ↑ Cobbold, E.S. (1935). "Lower Cambrian faunas from Hérault, France". Annals and Magazine of Natural History. 16 (91): 25–48. doi:10.1080/00222933508655023. ISSN 0374-5481.
  3. 1 2 Pojeta, John; Runnegar, Bruce (1976). "The paleontology of rostroconch mollusks and the early history of the phylum Mollusca". Geological Survey Professional Paper. 968.
  4. 1 2 3 4 5 6 Landing, Ed (1989). "Paleoecology and distribution of the Early Cambrian rostroconch Watsonella crosbyi Grabau". Journal of Paleontology. 63 (5): 566–573. doi:10.1017/S0022336000041196. eISSN 1937-2337. ISSN 0022-3360.
  5. 1 2 Guo, Jun-Feng; Li, Guo-Xiang; Qiang, Ya-Qin; Song, Zu-Chen; Zhang, Zhi-Fei; Han, Jian; Wang, Wen-Zhe (2021). "Watsonella crosbyi from the lower Cambrian (Terreneuvian, Stage 2) Yanjiahe Formation in Three Gorges Area, South China". Palaeoworld. 30 (1): 1–19. doi:10.1016/j.palwor.2020.04.006. ISSN 1871-174X.
  6. 1 2 Peel, J.S. (2021-04-11). "Pseudomyona from the Cambrian of North Greenland (Laurentia) and the early evolution of bivalved molluscs". Bulletin of Geosciences: 195–215. doi:10.3140/bull.geosci.1827. eISSN 1802-8225. ISSN 1214-1119.
  7. ↑ Li, Guoxiang; Zhao, Xin; Gubanov, Alexander; Zhu, Maoyan; Na, Lin (2011-04-24). "Early Cambrian mollusc Watsonella crosbyi: A potential GSSP Index Fossil for the base of the Cambrian Stage 2". Acta Geologica Sinica - English Edition. 85 (2): 309–319. doi:10.1111/j.1755-6724.2011.00400.x. eISSN 1755-6724. ISSN 1000-9515.
  8. ↑ Kouchinsky, Artem 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. eISSN 1502-3931. ISSN 0024-1164.
  9. 1 2 3 Vendrasco, Michael J.; Checa, Antonio G.; Kouchinsky, Artem V. (2011). "Shell microstructure of the early bivalve Pojetaia and the independent origin of nacre within the mollusca". Palaeontology. 54 (4): 825–850. doi:10.1111/j.1475-4983.2011.01056.x. eISSN 1475-4983. ISSN 0031-0239.
  10. ↑ Gubanov, Alexander P.; Kouchinsky, Artem V.; Peel, John S. (1999). "The first evolutionary‐adaptive lineage within fossil molluscs". Lethaia. 32 (2): 155–157. doi:10.1111/j.1502-3931.1999.tb00534.x. eISSN 1502-3931. ISSN 0024-1164.
  11. ↑ Carter, J. G.; Campbell, D. C.; Campbell, M. R. (2000). "Cladistic perspectives on early bivalve evolution". Geological Society, London, Special Publications. 177 (1): 47–79. doi:10.1144/GSL.SP.2000.177.01.04. eISSN 2041-4927. ISSN 0305-8719.
  12. ↑ Song, Hao; Wang, Yunan; Shao, Haojing; Li, Zhuoqing; Hu, Pinli; Yap-Chiongco, Meghan K.; Shi, Pu; Zhang, Tao; Li, Cui; Wang, Yiguan; Ma, Peizhen; Vinther, Jakob; Wang, Haiyan; Kocot, Kevin M. (2023-10-03). "Scaphopoda is the sister taxon to Bivalvia: Evidence of ancient incomplete lineage sorting". Proceedings of the National Academy of Sciences. 120 (40): –2302361120. doi:10.1073/pnas.2302361120. eISSN 1091-6490. ISSN 0027-8424.
  13. ↑ 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.
  14. ↑ 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.
  15. ↑ Jacquet, Sarah M.; Brougham, Thomas; Skovsted, Christian B.; Jago, James B.; Laurie, John R.; Betts, Marissa J.; Topper, Timothy P.; Brock, Glenn A. (2017). "Watsonella crosbyi from the lower Cambrian (Terreneuvian, Stage 2) Normanville Group in South Australia". Geological Magazine. 154 (5): 1088–1104. doi:10.1017/S0016756816000704. eISSN 1469-5081. ISSN 0016-7568.
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