Stenothecoida is a taxon of bivalved fossils from the Early to middle Cambrian period. They somewhat resemble brachiopods or bivalve molluscs.
Affinity
Their affinity is uncertain. They were considered by E. L. Yochelson 1968[citation needed] 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;[4] they also look a little like brachiopods[1] 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 [?].[1] It's more likely that the valves each have a single plane of symmetry (as in brachiopods), suggesting a brachiopod affiliation.[1] There is also evidence for a pedicle, further supporting their brachiopod affinity.[5]
As with many other Cambrian taxa,[6][7] it has been suggested that they arise through reduction of a Halkieria-like ancestor.[1]
^ abKouchinsky, A. V., Bengtson, S., Runnegar, B.N., Skovsted, C.B., Steiner, M., and Vendrasco, M.J. (2012). Chronology of early Cambrian biomineralization. Geol. Mag. 149, 221–251.
^1. Cohen, B.L., Holmer, L.E., and Lüter, C. (2003). The brachiopod fold: a neglected body plan hypothesis. Palaeontology 46, 59–65.
^Skovsted, C.B., Betts, M.J., Topper, T.P., and Brock, G.A. (2015). The early Cambrian tommotiid genus Dailyatia from South Australia. Mem. Assoc. Australas. Palaeontol. 48, 1–117.
^Kouchinsky, A. V. et al. Chronology of early Cambrian biomineralization. Geol. Mag. 149, 221–251 (2012).
