Tommotiid

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Tommotiids
Temporal range: Terreneuvian–Cambrian Series 21
Scientific classification e
Kingdom: Animalia
Superphylum: Lophotrochozoa
Informal group: Lophophorata
Paraphyletic group: "Tommitiida"

Tommotiids are Cambrian (Terreneuvian) shelly fossils of uncertain affinity.

Unlike most of the small shelly fauna, tommotiids are mineralised with calcium phosphate rather than calcium carbonate.2 Tommotiids have a wide range of sclerite shapes and internal structures, and may in fact represent a polyphyletic set of lineages, having independently developed phosphatic scleritomes rather than inheriting them from a common ancestor. Others consider them to be a sister group to the brachiopods.34 Indeed, Micrina and Paterimitra possess bivalved shells in their larval phases, which preserve characters that would position them in the Linguliformea and Rhynchonelliformea stem lineages respectively. This would indicate that the brachiopod shell represents the retention of a larval character.5

For a long part of their history, the tommotiids were only known from disarticulated shells - a complete organism had not been found. The 2008 discovery of Eccentrotheca offered the first insight into a complete organism, and permitted a reconstruction of the animal as a sessile, tube-like animal made up of a spiral of overlapping plates.6 Articulated specimens of Paterimitra, discovered a year later, suggest a similar form and lifestyle - it is possible that many tommotiids need redescribing as sessile tube-dwellers.7

These discoveries have produced an alternative model for the origin of the brachiopods; it suggested that they evolved by the reduction of sessile tube-like organisms, until only two shells were left. This contrasts with the brachiopod fold hypothesis which suggests that they formed by the folding of a halkieriid-like organism.6

Taxonomy

Taxon Relationship1
Kulparina Paterinid stem-group
Paterimitra Paterinid stem-group
Askepasma Paterinid stem- (or crown-?) group
Tannuolina Linguliform stem-group
Micrina Linguliform stem-group
Mickwitzia Linguliform stem- (or crown-?) group
Camenella Stem-group to {Brachiopods + Phoronids}
Dailyatia Stem-group to {Brachiopods + Phoronids}
Lapworthella Stem-group to {Brachiopods + Phoronids}
Eccentrotheca Stem-group to Phoronids? (or perhaps Brachiopods?)

References

  1. ^ a b Skovsted, C. B.; Brock, G. A.; Topper, T. P.; Paterson, J. R.; Holmer, L. E. (2011). "Scleritome construction, biofacies, biostratigraphy and systematics of the tommotiid Eccentrotheca helenia sp. nov. from the Early Cambrian of South Australia". Palaeontology 54 (2): 253–286. doi:10.1111/j.1475-4983.2010.01031.x. 
  2. ^ Bengtson, S. (2004). Early skeletal fossils (PDF). In Lipps, J.H., and Waggoner, B.M. "Neoproterozoic- Cambrian Biological Revolutions". Paleontological Society Papers 10: 67–78. Retrieved 2008-07-18. 
  3. ^ Holmer, L.E., Skovsted, C.B, Brock, G.A., Valentine, J.L. , and Paterson,J.R. (June 2008). "The Early Cambrian tommotiid Micrina, a sessile bivalved stem group brachiopod". Royal Society: Biology Letters 4 (6): 724–8. doi:10.1098/rsbl.2008.0277. PMC 2614141. PMID 18577500. Retrieved 2008-07-23.  There is a reconstruction of Micrina at "Catalogue of Organisms: Back to the Scleritome - Tommotiids Revealed!". Retrieved 2008-07-23. 
  4. ^ Skovsted, C.B, Brock, G.A., Paterson,J.R., Holmer, L.E., and Budd, G.E. (February 2008). "The scleritome of Eccentrotheca from the Lower Cambrian of South Australia: Lophophorate affinities and implications for tommotiid phylogeny". Geology 36 (2): 171–174. doi:10.1130/G24385A.1. Retrieved 2008-07-23. 
  5. ^ Holmer, L. E.; Skovsted, C. B.; Larsson, C.; Brock, G. A.; Zhang, Z. (2011). "First record of a bivalved larval shell in Early Cambrian tommotiids and its phylogenetic significance". Palaeontology 54 (2): no. doi:10.1111/j.1475-4983.2010.01030.x. 
  6. ^ a b Skovsted, C. B.; Brock, G. A.; Paterson, J. R.; Holmer, L. E.; Budd, G. E. (2008). "The scleritome of Eccentrotheca from the Lower Cambrian of South Australia: Lophophorate affinities and implications for tommotiid phylogeny". Geology 36 (2): 171. doi:10.1130/G24385A.1. 
  7. ^ Skovsted, C. B.; Holmer, E.; Larsson, M.; Hogstrom, E.; Brock, A.; Topper, P.; Balthasar, U.; Stolk, P.; Paterson, R. (May 2009). "The scleritome of Paterimitra: an Early Cambrian stem group brachiopod from South Australia". Proceedings of the Royal Society B: Biological Sciences 276 (1662): 1651–1656. doi:10.1098/rspb.2008.1655. ISSN 0962-8452. PMC 2660981. PMID 19203919. 









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