#lingula

Brachiopods are a very common type of fossils in rocks from the Paleozoic (the oldest geologic era with complex multicellular life, from 540 to 252 million years ago). They look a lot like bivalves (clams, mussels, cockles, oysters, and other mollusks with two-part, hinged shells), although the two groups are not related -- brachiopods aren’t even mollusks themselves, meaning bivalves are more closely related to squid or snails than to these guys.

There’s some physical differences between the two groups, too -- most obviously, many (but not all) brachiopods have a stalk or pedicle attaching them to the seafloor, which bivalves lack.

Another and more universal difference is the lophohore, a filter feeding structure composed of tentacles in a circular pattern unique to brachiopods and some related groups, which mollusks also lack.

Brachiopods are traditionally classified into two groups based on the shapes of their shells (although there’s been some debate lately about whether that’s a valid taxonomic classification, which is not an argument I feel knowledgeable enough about to tackle here and now): articulate and inarticulate.

Articulate brachiopods have a toothlike joint in the hinge where the two halves of their shell meet.

Inarticulate brachiopods lack this structure (their shells are by and large very similar to what you’d find on, say, a clam) and as a result the muscles they use to open and close their shells are more complex than in articulate brachiopods.

The reason that brachiopod fossils are so common is that, during the Paleozoic, the brachiopods were a very successful group. They were extremely widespread and diverse, to the point that today you can find some variety or another of brachiopod in almost every Paleozoic marine deposit. They would have been everywhere in shallow sea lagoon habitats, with bivalves reduced to a much more marginal role than they have in modern seas. 

For all their previous success, the brachiopods went into a serious period of decline during the Mesozoic (252 to 65 million years ago, a.k.a. the “age of the dinosaurs”). There’s some debate why -- competition with modern bivalve mollusks is one possibility, although the bivalves began to diversify well before the brachiopods went into decline. Another culprit would be the end-Permian extinction event, also known by the rather colorful name of the Great Dying, a devastating mass extinction that hit marine life -- including the brachiopods -- particularly hard. In all likelihood, it is likely that it was a mixture of factors -- the Great Dying undeniably cut down rather drastically on brachiopod diversity, and competition with the diversifying bivalves would certainly not have done much to help.

Despite their decline, brachiopods still survive today, although they’re far less common than they were. One of the most well-known modern brachiopods would be Lingula, a genus of burrowing inarticulate brachiopods (pictured below).

Lingula is actually one of the oldest brachiopod genuses known -- Lingula fossils have been found dating back to the Cambrian Period, having lived alongside the earliest animals to ever exist. Lingula fossils from way back then are almost identical to living specimens, despite the five hundred or more million years that have passed since then. In an ironic twist of fate, these living fossils weathered through every mass extinction in Earth’s history and the decline of the brachiopods as a group to endure to the modern day, while their more derived and adapted relatives have long since died out.