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.