Tommotiid
Encyclopedia
Tommotiids are Cambrian
shelly fossils of uncertain affinity.
Unlike most of the small shelly fauna, tommotiids are mineralised with calcium phosphate
rather than calcium carbonate
. 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 brachiopod
s. 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.
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.
These discoveries have produced an alternative model for the origin of the brachiopod
s; 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.
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
shelly fossils of uncertain affinity.
Unlike most of the small shelly fauna, tommotiids are mineralised with calcium phosphate
Calcium phosphate
Calcium phosphate is the name given to a family of minerals containing calcium ions together with orthophosphates , metaphosphates or pyrophosphates and occasionally hydrogen or hydroxide ions ....
rather than calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
. 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 brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s. Indeed, Micrina and Paterimitra possess bivalved shells in their larval phases, which preserve characters that would position them in the Linguliformea
Linguliformea
Linguliformea is a subphylum of inarticulate brachiopods. These were the earliest of brachiopods, ranging from the Cambrian into the Holocene. They rapidly diversified during the Cambrian into the Ordovician, but most families went extinct by the end of the Devonian.The articulation in these...
and Rhynchonelliformea
Rhynchonelliformea
Rhynchonelliformea is the name now given to the articulate brachiopods, Class Articulata, revised as a subphylum. Articulate brachiopods are those with hard, articulated, shells with a simple set of opening and closing muscles....
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 Eccentrotheca
Eccentrotheca
Eccentrotheca is a genus of sclerite-bearing marine organism known from Cambrian deposits. Its sclerite form rings that are stacked to produce a widening-upwards conical scleritome. Individual plates have been homologized with the valves of brachiopods, and a relationship with the phoronids is...
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.
These discoveries have produced an alternative model for the origin of the brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s; 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.
Taxonomy
| Taxon | Relationship |
|---|---|
| Kulparina | Paterinid stem-group |
| Paterimitra | Paterinid stem-group |
| Askepasma | Paterinid stem- (or crown-?) group |
| Tannuolina | Linguliform stem-group |
| Micrina | Linguliform stem-group |
| Mickwitzia Mickwitzia Mickwitzia is a genus of stem group brachiopods with punctate shells from the Cambrian period. Mickwitziid shells are composed of apatite . Punctae or tubes penetrate through multiple shell wall layers, and individual punctae often develop a single, axial phosphatic tube... |
Linguliform stem- (or crown-?) group |
| Camenella | Stem-group to {Brachiopods + Phoronids} |
| Dailyatia | Stem-group to {Brachiopods + Phoronids} |
| Lapworthella | Stem-group to {Brachiopods + Phoronids} |
| Eccentrotheca Eccentrotheca Eccentrotheca is a genus of sclerite-bearing marine organism known from Cambrian deposits. Its sclerite form rings that are stacked to produce a widening-upwards conical scleritome. Individual plates have been homologized with the valves of brachiopods, and a relationship with the phoronids is... |
Stem-group to Phoronids? (or perhaps Brachiopods?) |