Rank Class | Phylum Mollusca (?) | |
Similar Rostroconchia, Monoplacophora, Tusk shell | ||
Stenothecoida are bivalved fossils from the Early Cambrian period.
Contents
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 of the shell 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 others' 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.
As with many other Cambrian taxa, it has been suggested that they arise through reduction of a Halkieria-like ancestor.
Occurrence
The earliest occurrence dates to 525 Ma.
These fossil shells have been found in Canada, China, Greenland, Kazakhstan, Kyrgyzstan, and the USA.