Today, let’s talk about brachiopods. Possibly you’ve never heard about brachiopods – they are not widely distributed today, and when you do see their shells you might dismiss them as just another odd bivalve, or two-shelled mollusk like a clam. But brachiopods are not clams. They are their own phylum, not closely related to mollusks at all.
The main visible difference may seem subtle – brachiopods are symmetrical through a plane that divides the two shells vertically, while mollusks, if they have any symmetry at all, are symmetrical between the shells. There are other differences as well, especially in internal organs. Brachiopods have things called lophophores, tentacles that they extend and wave around to create a current from which they can filter food particles out of the ocean water where they live.
Brachiopods first appeared in the early Cambrian, part of the Cambrian explosion. They were incredibly prolific and successful during much of the Paleozoic Era, and in fact there are 12,000 fossil species known, in 5,000 genera, that is, groups of related species, in contrast to only 100 genera known today. Brachiopods suffered a lot in the Permian-Triassic extinction, at the end of the Paleozoic Era 250 million years ago, and their decreasing diversity after that time may also reflect the growing success and diversity of the bivalves that occupy some of the same ecological niches as brachs.
Although a few species reached nearly eight inches across, most brachiopods are an inch or two across, a perfect size for preservation and easy for collectors to find. There are two main types, articulate and inarticulate. Articulated brachs have two shells that are attached to each other along a hinge line, so the critter opens in a way similar to a clam. Inarticulate varieties have two shells that were not attached, but were held together by the animal’s muscular system. One of the most famous inarticulate brachs is one called Lingula – also called a living fossil, because the types that exist today are hardly changed from Lingulas that lived in the Cambrian period. They are also interesting because their shells are not made of calcium carbonate, like most clams, scallops, snails, and other shelly creatures, but they’re made of calcium phosphate, the mineral apatite, the same mineral that makes your bones and teeth.
I’ve never seen a modern brachiopod, alive or dead. But they were so prolific in Paleozoic seas that most any fossil collector will have some in his or her collection. We’ll talk about some of them later in the Paleozoic.
—Richard I. Gibson
Images from USGS.