tommotiid

{{Short description|Extinct clade of lophophorates}}

{{Automatic taxobox

| name = Tommotiids

| fossil_range = {{fossil range|Terreneuvian|Drumian|latest=Recent|ref={{Cite journal | url=http://www.diva-portal.org/smash/record.jsf?pid=diva2%3A809326&dswid=-5399 |title = Discovery of the youngest known tommotiid from the middle Cambrian (Drumian) Nelson Limestone of Antarctica|journal = Diva|year = 2015|last1 = Skovsted|first1 = Christian|last2 = Bassett-Butt|first2 = Lewis}} }} (Tommotiida survives until present day through extant lophophorates)

| image = Wufengella_reconstruction.jpg

| image_caption = Intepretive drawing (top) and life restoration (bottom) of Wufengella a camenellan, and one of the few tommotiids known from articulated remains

| image2 = Eccentrotheca.jpg

| image2_caption = Life restoration of Eccentrotheca, a sessile tommotiid

| taxon = Tommotiida

| authority =

| subdivision_ranks =

| subdivision =

}}

Tommotiids are an extinct group of Cambrian invertebrates thought to be early total-group lophophorates (the group containing Bryozoa, Brachiopoda, and Phoronida), including members of the lophophorate stem group, as well as early diverging members of the crown group.{{cite journal | title= The Early Cambrian tommotiid Micrina, a sessile bivalved stem group brachiopod | vauthors=Holmer LE, Skovsted CB, Brock GA, Valentine JL, Paterson JR | journal= Biology Letters|date=June 2008 | doi=10.1098/rsbl.2008.0277 |doi-access=free | pmid= 18577500 | volume= 4 | issue= 6 | pages= 724–728 | pmc= 2614141}}{{cite web|first=Christopher|last=Taylor|date=2008-06-27|title=Back to the Scleritome - Tommotiids Revealed!|work=Catalogue of Organisms|url=http://catalogue-of-organisms.blogspot.com/2008/06/back-to-scleritome-tommotiids-revealed.html|accessdate=2008-07-23}}{{cite journal | title=The scleritome of Eccentrotheca from the Lower Cambrian of South Australia: Lophophorate affinities and implications for tommotiid phylogeny | vauthors=Skovsted CB, Brock GA, Paterson JR, Holmer LE, Budd GE | journal=Geology |date=February 2008 | volume=36 | issue=2 | pages=171–174 | doi=10.1130/G24385A.1 | bibcode=2008Geo....36..171S}}

The majority of tommotiids are mineralised with calcium phosphate rather than calcium carbonate,{{Cite book |contribution = Early skeletal fossils |author = Bengtson, S. |editor = Lipps, J.H. |editor2 = Waggoner, B.M. |title = Neoproterozoic-Cambrian Biological Revolutions |year = 2004 |series = The Paleontological Society Papers |volume = 10 |pages = 67–78 |url = http://www.cosmonova.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf |accessdate = 2008-07-18 |url-status = dead |archiveurl = https://web.archive.org/web/20081003122817/http://www.nrm.se/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf|archivedate=2008-10-03

|doi = 10.1017/S1089332600002345}} although silicified examples hint that some species bore carbonate or carbonaceous sclerites.{{Cite journal|vauthors=Skovsted CB|year=2016|title=A silicified tommotiid from the lower Cambrian of Greenland|journal=Bulletin of Geosciences|volume=91|issue=3|pages=553–559|issn=1214-1119|doi=10.3140/bull.geosci.1609|url=http://www.geology.cz/bulletin/fulltext/1609_Skovsted_161125.pdf|doi-access=free}}

Micrina and Paterimitra possess bivalved shells in their larval phases, which preserve characters that might position them in the Linguliformea and Rhynchonelliformea stem lineages respectively. This would indicate that the brachiopod shell represents the retention of a larval character.{{Cite journal|vauthors=Holmer LE, Skovsted CB, Larsson C, Brock GA, Zhang Z|title=First record of a bivalved larval shell in Early Cambrian tommotiids and its phylogenetic significance|doi= 10.1111/j.1475-4983.2010.01030.x|journal=Palaeontology|volume=54|issue=2|pages=235–239|year=2011|doi-access=free|bibcode=2011Palgy..54..235H }}

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. 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.{{Cite journal|vauthors=Skovsted CB, Holmer LE, Larsson CM, Högström AE, Brock GA, Topper TP, Balthasar U, Stolk SP, Paterson JR|title=The scleritome of Paterimitra: an Early Cambrian stem group brachiopod from South Australia|journal=Proceedings of the Royal Society B: Biological Sciences|volume=276|issue=1662|pages=1651–1656|date=May 2009|issn=0962-8452|pmid=19203919|pmc=2660981|doi= 10.1098/rspb.2008.1655}} Eccentrotheca and other similar sessile tommotiids were likely filter feeders, similar to modern lophophorates.

However, the discovery of the articulated camenellan Wufengella showed that it was a free-living worm-like animal, suggesting that it was not a crown-group lophophorate, as the last living common ancestor of all living lophophorates has been predicted to be sessile, as bryozoans, brachiopods and phoronids are. This indicates that tommotiids are paraphyletic, with some tommotiids more closely related to bryozoans, brachiopods and phoronids than to other tommotiids.{{Cite journal |last1=Guo |first1=Jin |last2=Parry |first2=Luke A. |last3=Vinther |first3=Jakob |last4=Edgecombe |first4=Gregory D. |last5=Wei |first5=Fan |last6=Zhao |first6=Jun |last7=Zhao |first7=Yang |last8=Béthoux |first8=Olivier |last9=Lei |first9=Xiangtong |last10=Chen |first10=Ailin |last11=Hou |first11=Xianguang |last12=Chen |first12=Taimin |last13=Cong |first13=Peiyun |date=September 2022 |title=A Cambrian tommotiid preserving soft tissues reveals the metameric ancestry of lophophorates |journal=Current Biology |volume=32 |issue=21 |language=en |pages=4769–4778.e2 |doi=10.1016/j.cub.2022.09.011|s2cid=252564106 |doi-access=free |bibcode=2022CBio...32E4769G }}

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.