Heterodont bivalves first appeared on Earth in the early Triassic (about 250 million years ago). The first heterodont bivalves were tiny, as were many Triassic organisms, thus often overlooked by fossil hunters. Paleontologists continue to discover heterodont bivalve fossils that push back the date of their appearance. Like modern bivalves, ancient ones lived in shallow, tidal areas and cemented themselves to the bottom. Larger, heterodont bivalves called rudists became the dominant reef-builders of the Cretaceous, forming huge pileups of shells like corals do today. Rudists built reefs in warm seas near the equator, flourishing in a warm, salty seaway (the Tethys) that connected the main oceans of the time. Rudists went extinct in the large extinction event at the end of the Cretaceous (65 million years ago), but other heterodont bivalves survived to diversify into the various clams and cockles that we see today.

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).

Mollusks have inhabited the Earth for at least 540 million years, dating back to the early Cambrian period. The fossil record of mollusks consists mostly of shells. Rarely, it includes mouthparts (radulas) or the trapdoors (opercula) that cover shell openings. The rest of the body is soft and usually does not fossilize. Before the Cambrian, taxa such as Kimberella had many traits expected of early molluscs, but no shells or spines. Cambrian, worm-like animals with spiny scales, such as Wiwaxia, had mollusk-like mouthparts. The earliest agreed-upon mollusk looked like a snail with a single, cap-like shell that curved at the tip. More than 90,000 living species of mollusk have been described, plus another 70,000 species known only from fossils. Because many mollusks live in remote places such as in the bottom sediments of deep ocean, and new species are being discovered at a rapid rate, scientists think that many more species remain to be described.