Brachiopods are an ancient group of organisms, at least 600 million years old. They might just look like clams, but they are not even closely related. Instead of being horizontally symmetrical along their hinge, like clams and other bivalves, they are vertically symmetrical, cut down the middle of their shell.
While they may all look the same to us, during the Paleozoic era (roughly 250-500 million years ago), brachiopods ruled the sea. They were the most common and most diverse organisms around, each clinging to the seafloor with a muscular foot and even accumulating into ancient reefs. There are some 30,000 fossil brachiopod species known, but only around 385 are alive today. They are found in very cold water, in polar regions or in the deep sea, and are rarely seen.