Page "Bivalvia" Paragraph 44

By the middle of the Paleozoic, around 400 million years ago, the brachiopods were among the most abundant filter feeders in the ocean and over 12, 000 fossil species are recognized.

By the Permian-Triassic extinction event 250 million years ago, bivalves were undergoing a huge radiation of diversity.

The bivalves were hard hit by this event but re-established themselves and thrived during the Triassic period that followed.

In contrast, the brachiopods lost 95 % of their species diversity.

Scientists have speculated that the ability of some bivalves to burrow and thus avoid predators was a major factor in their success.

Other new adaptations within various families allowed species to occupy previously unused evolutionary niches.

These included increasing relative buoyancy in soft sediments by developing spines on the shell, gaining the ability to swim, and in a few cases, adopting predatory habits.