Articulate brachiopods get their name from the toothed hinge of their shell that allows the two halves to close snugly together (articulate = “jointed”). The fossil record shows that the earliest brachiopods had a smooth hinge. The toothed hinge likely evolved, along with strong adductor muscles, as an adaptation to lock the shell tightly closed when necessary. The first articulate brachiopod fossils date to the Cambrian (about 500 million years ago), when animals with shells were first becoming abundant on Earth. Articulate brachiopods flourished in the extensive, shallow, warm seas. During the 250 million years that followed, articulate brachiopods became widespread, but with the composition of species changing as some went extinct and new forms arose. Few survived the massive extinction at the end of the Permian (about 250 million years ago). However one group of articulate brachiopods, the Rhynchonellata, is still represented by living species on Earth today.

Brachiopods are small, shelled organisms that look superficially like bivalves (such as clams). Despite their resemblance, brachiopods have different evolutionary origins. Also, if you carefully compare a clam and a brachiopod shell, you will notice that the plane of symmetry differs by 90 degrees; two clam shells are mirror images of each other, while the plane of symmetry in a brachiopod passes through the middle of each shell. While species of brachiopods number in the hundreds today, they numbered in the thousands during the Paleozoic. They originated on Earth more than 500 million years ago, and proliferated through the Paleozoic. Scientists use fossil brachiopods as indicators of prehistoric climate change because gradual shifts in climate affected the distribution of brachiopod species. At the end of the Permian, most brachiopod species were wiped out in a massive extinction event that affected many other organisms, too. While they made a slow recovery, brachiopods have never achieved the incredible diversity and abundance they had during the Paleozoic.