Pteriomorphia includes many of the bivalves we know best today, such as mussels, oysters, and scallops. The first Pteriomorphia lived on Earth during the middle Ordovician (about 470 million years ago). They evolved as part of the huge diversification of bivalves that occurred during what is called the Cambrian Explosion. One of the oldest known bivalves, Fordilla trovensis, may be the ancestor of modern mussels. Various shell shapes emerged during the Cambrian Explosion, and internal body structures changed as well. One of the changes was enlargement of the gills, which are used by bivalves to get oxygen from their watery surroundings. Larger gills became useful for feeding by acting as strainers to trap food particles. Further changes eventually led to the intricate gills of modern Pteriomorphia that include a complex structure with lots of surface area for filtering and grooves for transporting food.

Bivalves first made their appearance on earth about 500 million years ago during what is called the Cambrian Explosion when there was a great diversification of life. The earliest bivalves burrowed in soft sediments on the bottom of the ocean. They lived just under the surface, not deep down like some modern bivalves. Over time, bivalves evolved structures allowing them to bury deeper such as snorkel-like tubes (siphons) to get water from above; and also evolved modified gills for filter feeding. Bottom-dwelling (benthic) bivalves became fossilized when sediment hardened around them. Fossil evidence is a shell itself or just an impression of the shell left in the sediment (an ichnofossil). Bivalves have evolved into many, distinct forms over time, and survived through major extinction events to become a very diverse group of organisms. Most modern forms are benthic, but others attach themselves to hard surfaces or even swim around a bit (e.g., scallops).