Shortly before the Mesozoic Era, when dinosaurs evolved, brachiopods declined. Brachiopods weren t the only animals that declined at the end of the Permian Period, the last period in the Paleozoic Era. In fact, something caused a mass extinction about 230 million years ago. Although brachiopods didn t become extinct, they were greatly reduced in numbers. New species of bivalves, including clams, had evolved which could burrow into soft sediments.
Clams draw water into their shells through a siphon that they extend through the mud into the water.
Just imagine burying yourself in mud and drinking water through a straw. This ability to hide from predators probably increased their survival rate, and they took the place of many brachiopods. Because clams are not connected to the seafloor by stalks, they can move about. This may be another advantage over brachiopods. More than 200 species of brachiopods still live in the world s seas.