Stenothecoida is a taxon of bivalved fossils from the Early to middle

Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordov ...

period. They somewhat resemble brachiopods or bivalve molluscs.

Affinity

Their affinity is uncertain. They were considered by E. L. Yochelson 1968 to belong to Mollusca, whereas Runnegar and Pojeta (1974) suggested that they might be 'bivalved monoplacophorans'. This said, the

morphology Morphology, from the Greek and meaning "study of shape", may refer to: Disciplines *Morphology (archaeology), study of the shapes or forms of artifacts *Morphology (astronomy), study of the shape of astronomical objects such as nebulae, galaxies, ...

of the

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses Science Biology * Seashell, a hard outer layer of a marine ani ...

does not resemble the shell of any other class of molluscs; they also look a little like brachiopods It's not clear whether their two valves are each other's mirror images, as in bivalve molluscs, though they aren't identical to one another It's more likely that the valves each have a single plane of symmetry (as in brachiopods), suggesting a brachiopod affiliation. There is also evidence for a pedicle, further supporting their brachiopod affinity. As with many other Cambrian taxa, it has been suggested that they arise through reduction of a ''

Halkieria The halkieriids are a group of fossil organisms from the Lower to Middle Cambrian. Their eponymous genus is ''Halkieria'' , which has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the smal ...

''-like ancestor.

Mineralogy

Thought to be low-Mg calcite.Kouchinsky, A. V. et al. Chronology of early Cambrian biomineralization. Geol. Mag. 149, 221–251 (2012).

Occurrence

The earliest occurrence dates to 525 Ma. These fossil shells have been found in

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, and the

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References

Start of article at Springer Reference

Paleobiology Database info on this class

Abstract of a Yochelson 2007 article on the proposed class
Prehistoric mollusc taxonomy Mollusc classes Prehistoric protostome classes {{paleo-mollusc-stub