Evolutionary history of brachiopods

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 Paleozoic dominance, but dwindled during the Mesozoic.

Brachiopod fold hypothesis
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.

Tommotiids
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.

Crown group
The earliest unequivocal brachiopods in the fossil record occur in the early Cambrian, with the hingeless, inarticulate forms appearing first, followed soon thereafter by the hinged, articulate forms. Possible brachiopods have also been found in much older upper Neoproterozoic strata, although their assignment remains uncertain.

Paleozoic dominance
Brachiopods are extremely common fossils throughout the Paleozoic. During the Ordovician and Silurian periods, brachiopods became adapted to life in most marine environments and became particularly numerous in shallow water habitats, in some cases forming whole banks in much the same way as bivalves (such as mussels) do today. In some places, large sections of limestone strata and reef deposits are composed largely of their shells.

The major shift came with the Permian extinction, as a result of the Mesozoic marine revolution. Before the extinction event, brachiopods were more numerous and diverse than bivalve mollusks. Afterwards, in the Mesozoic, their diversity and numbers were drastically reduced and they were largely replaced by bivalve molluscs. Molluscs continue to dominate today, and the remaining orders of brachiopods survive largely in fringe environments.

Mesozoic decline
Throughout their long geological history, the brachiopods have gone through several major proliferations and diversifications, and have also suffered from major extinctions as well.

It has been suggested that the slow decline of the brachiopods over the last 100 million years or so is a direct result of the rise in diversity of filter feeding bivalves, which have ousted the brachiopods from their former habitats; however, the bivalves have undergone a steady rise in diversity from the mid-Paleozoic onwards, and their abundance is unrelated to that of the brachiopods; further, many bivalves occupy niches (e.g. burrowing) which brachiopods never inhabited.

Alternative possibilities for their demise include the increasing disturbance of sediments by roving deposit feeders (including many burrowing bivalves); the increased intensity and variety of shell-crushing predation; or even chance demise - they were hard hit in the End-P extinction and may simply never have recovered.