Tommotiid

Tommotiids Temporal range: Terreneuvian–Cambrian Series 2[1] PreЄ Є O S D C P T J K Pg N
Scientific classification
Kingdom:	Animalia
Superphylum:	Lophotrochozoa
Informal group:	Lophophorata
Informal group:	†"Tommotiida"

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.^[3][4] 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

References