Brachiopod fold hypothesis

The origin of the brachiopods is uncertain; they either arose from reduction of a multi-plated tubular organism, or from the folding of a slug-like organism with a protective shell on either end. Since their Cambrian origin, the phylum rose to a Palaeozoic dominance, but dwindled during the Mesozoic.

The long-standing hypothesis of brachiopod origins, which has recently come under fire, suggests that the brachiopods arose by the folding of a Halkieria-like organism, which bore two protective shells at either end of a scaled body. The tannuolinids were thought to represent an intermediate form, although the fact that they do not, as thought, possess a scleritome means that this is now considered unlikely. Under this hypothesis, the Phoronid worms share a similar evolutionary history; molecular data also appear to indicate their membership of Brachiopoda.

Under the Brachiopod Fold Hypothesis, the "dorsal" and "ventral" valves would in fact represent an anterior and posterior shell. This would make the axes of symmetry consistent with that of other bilaterian phyla and appears to be consistent with the embryological development, in which the body axis folds to bring the shells from the dorsal surface to their mature position. Further support has been identified from the gene expression pattern during development, but on balance, developmental evidence speaks against the BFH.

More recent developmental studies have cast doubt on the BFH. Most significantly, the dorsal and ventral valves have significantly different origins; the dorsal (branchial) valve is secreted by dorsal epithelia, whereas the ventral (pedicle) valve corresponds to the cuticle of the pedicle, which becomes mineralized during development.

An alternative to the BFH suggests that brachiopods arose through the shortening of a tube-like organism consisting of many shell plates. It is possible that they arose from within the tommotiid group in this fashion. The more derived tommotiid Paterimitra has a pair of brachiopod-like shells at its rear, in just the arrangement one would expect of a brachiopod. This is supported by the similarities in mineralogy between the Tommotiids and the earliest brachiopods.

The earliest unequivocal brachiopod fossils appeared in the early Cambrian Period. The oldest known brachiopod is Aldanotreta sunnaginensis from the lowest Tommotian Stage, early Cambrian of the Siberia was confidently identified as a paterinid linguliforms.

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