Tommotiid

Tommotiids are an extinct group of <a href="/facts/Cambrian/jIfzKtod">Cambrian</a> invertebrates thought to be early <a href="/facts/Lophophorata/772EG9EP">lophophorates</a> (the group containing <a href="/facts/Bryozoa/obAgpA6M">Bryozoa</a>, <a href="/facts/Brachiopoda/XVBnEPj1">Brachiopoda</a>, and <a href="/facts/Phoronid/cDXn9pBV">Phoronida</a>).
The majority of tommotiids are mineralised with <a href="/facts/Calcium_phosphate/tn03xvdb">calcium phosphate</a> rather than <a href="/facts/Calcium_carbonate/UQ1wLFna">calcium carbonate</a>, although silicified examples hint that some species bore carbonate or carbonaceous <a href="/facts/Sclerite/KWTShcvT">sclerites</a>.
<a href="/facts/Micrina/Fn2RzvHq">Micrina</a> and Paterimitra possess bivalved shells in their larval phases, which preserve characters that might position them in the <a href="/facts/Linguliformea/Qk83qSHu">Linguliformea</a> and <a href="/facts/Rhynchonelliformea/su7aw3XK">Rhynchonelliformea</a> stem lineages respectively. This would indicate that the brachiopod shell represents the retention of a larval character.
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 <a href="/facts/Eccentrotheca/bw8wG5oH">Eccentrotheca</a> 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. Eccentrotheca and other similar sessile tommotiids were likely <a href="/facts/Filter_feeder/hmVLwWB5">filter feeders</a>, similar to modern lophophorates.
However, the discovery of the articulated <a href="/facts/Camenellan/cs9JvqCs">camenellan</a> <a href="/facts/Wufengella/oX17GfR1">Wufengella</a> showed that it was a free-living worm-like animal, suggesting that it was not a <a href="/facts/Crown_group/fHDhwp5x">crown-group</a> lophophorate, as the last living common ancestor of all living lophophorates has been predicted to be <a href="/facts/Sessility_(motility)/f0xV4VV1">sessile</a>, as bryozoans, brachiopods and phoronids are. This indicates that tommotiids are <a href="/facts/Paraphyletic/tmTWjACz">paraphyletic</a>, with some tommotiids more closely related to bryozoans, brachiopods and phoronids than to other tommotiids.
These discoveries have produced an alternative model for the origin of the <a href="/facts/Brachiopod/XVBnEPj1">brachiopods</a>; it suggested that they evolved by the reduction of sessile tube-like organisms, until only two shells were left. This contrasts with the <a href="/facts/Brachiopod_fold_hypothesis/INuwbF50">brachiopod fold hypothesis</a> which suggests that they formed by the folding of a <a href="/facts/Halkieriid/g6sHTj1V">halkieriid</a>-like organism.

Tommotiid open-in-new

Tommotiid