Clams or bivalves are molluscs, the second-largest phylum of invertebrates with about 85,000 extant species. While brachiopods share some similarities with their molluscan friends they are in a phylum all their own.
Brachiopods are small marine shellfish that are not so common today but back in the Palaeozoic they were plentiful the world over. The two valves that make up a brachiopods shell are of different sizes and if you look closely you'll see that the hinge runs top and bottom -- versus left and right like a clam.
Brachiopods had evolved from an ancestor similar to Halkieria, a slug-like Cambrian animal with "chain mail" on its back and a shell at the front and rear end; it was thought that the ancestral brachiopod converted its shells into a pair of valves by folding the rear part of its body under its front.
New fossils found in the last few years have shown us that the "chain mail" of tommotiids formed the tube of a sessile animal; one tommotiid resembled phoronids, which are close relatives or a subgroup of brachiopods, while the other tommotiid bore two symmetrical plates that might be an early form of brachiopod valves. Lineages of brachiopods that have both fossil and extant taxa appeared in the early Cambrian, Ordovician, and Carboniferous periods, respectively.
Other lineages have arisen and then become extinct, sometimes during severe mass extinctions. At their peak in the Paleozoic era, the brachiopods were among the most abundant filter-feeders and reef-builders and occupied other ecological niches, including swimming in the jet-propulsion style of scallops. Brachiopod fossils have been useful indicators of climate changes during the Paleozoic. However, after the Permian–Triassic extinction event, brachiopods recovered only a third of their former diversity.
Brachiopods were especially vulnerable to the Permian–Triassic extinction, as they built calcareous hard parts — made of calcium carbonate — and had low metabolic rates and weak respiratory systems. It was often thought that brachiopods went into decline after the Permian–Triassic extinction, and were out-competed by bivalves, but both brachiopod and bivalve species increased from the Paleozoic to modern times, with bivalves increasing faster; after the Permian–Triassic extinction, brachiopods became for the first time less diverse than bivalves.
Their lineage dates back to the Cambrian with over 12,000 fossil species and 350 living relatives sorted between 6,000 genera. There are two major groups of brachiopods, articulate with toothed hinges and simple open and closing muscles to manipulate their shells and inarticulate brachiopods with untoothed hinges and a more complex set of muscles used to control the brachial supports used to open and close their shells.